Symposium EQ16—Infrared and Thermal Photonic Materials and Their Applications

The continuous growth of the field of photonics over the last several decades has led to a myriad of groundbreaking applications, from ultrafast quantum computation to long-distance data transmission to energy conversion. However, most of the current research in photonics is performed at room temperature in an ambient environment. Determining the optical behavior of materials in different environments and at higher temperatures is crucial to understanding their properties and thus the performance of devices in the real world. Similarly, certain materials with advantageous mechanical and thermal properties such as refractory metals remain minimally explored in the optical regime. Because of this, refractory metals have for the most part been overlooked for photonic applications, even though their refractive indices may prove highly useful to many situations, e.g. thermophotovoltaics (TPV). We resolve the effects of environment (air, N₂, and Ar) and temperature (25 – 600°C) on the complex refractive index of seven refractory metals (Cr, Hf, Mo, Ru, Ta, Ti, and W) as well as TiN by performing in situ ellipsometry, where all thin films are fabricated via sputtering. Our in situ ellipsometry experiments suggest a reaction at high temperatures between certain refractory metals and an N₂-rich environment. For example, we show a maximum change of 38% for the refractive index and 24% for the extinction coefficient in Mo, and a maximum change of 23% for the refractive index and 47% for the extinction coefficient in W. This change remains even after the temperature is lowered. We note that TiN has the added benefit of including nitrogen, making it less likely to react when placed in an N₂-rich environment at high temperatures. When exposed to an inert Ar environment, many of the refractory metals showed similarly substantial changes in refractive index beginning around 500°C. We complement the optical measurements with morphological and chemical analyses of the films’ surface by combining atomic force microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy of the substrates before and after being held at high temperatures, to analyze the possible physical and surface changes that have occurred. With these experimentally obtained temperature-dependent refractive indices, we provide an example application, quantifying the performance of these refractory metals in an example TPV absorber/emitter. Here, we find a 3-fold increase in efficiency by using refractory metals when compared to other current state-of-the-art absorber/emitters, proving their value for modern photonic applications.

In this talk I will discuss some thermomagnetic effects for radiative heat exchanges in non-Hermitian many-body systems [1]. Contrary to Hermitian systems, the non-Hermitian systems are optically non-reciprocal. This breaking of symmetry gives rise to some singular radiative behaviors that we detail in this talk.
The first effect is a thermal analog of Hall effect [2] which exists in magneto-optical networks in presence of an external magnetic field. This effect consists in the appearance of a temperature gradient and therefore a heat flux transversally to the direction of a primary temperature gradient.
The second effect is a giant thermal magneto-resistance [3] within these systems. Similarly to the famous magnetoresistance [4] reported in ferromagnetic/normal metal multilayers, variations of thermal resistance of about 50% has been highlighted in nanoparticle networks with magnetic fields of magnitude of 500mT opening so the way to an ultrafast tunability of heat transfers mediated by an external field.
Next we analyze the radiative heat transfers in Weyl semimetals networks. Contrary to magneto-optical systems these Weyl networks are intrinsically non-reciprocal even without applied magnetic field. Hence an anomalous Hall effect [5] can exist in these media.
To finish we discuss some potential applications for these effects to control heat flux at nanoscale and to cool down solids using an external magnetic field.

References :
[1] S.-A. Biehs, R. Messina, P. S. Venkataram, A. W. Rodriguez, J. C. Cuevas and P. Ben-Abdallah, Rev. Mod. Phys., 93, 025009 (2021).
[2] P. Ben-Abdallah, Phys. Rev. Lett. 116, 084301 (2016).
[3] I. Latella and P. Ben-Abdallah, Phys. Rev. Lett. 118, 173902 (2017).
[4] M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich, and J. Chazelas, Phys. Rev. Lett. 61, 2472 (1988).
[5] A. Ott, S.-A. Biehs and P. Ben-Abdallah, Anomalous photon thermal Hall effect, Phys. Rev. B 101, 241411 (2020).

Thermal emission observed at sub-wavelength distances from the emitting medium is referred to as the near-field thermal radiation. Thermal radiation in the near-field regime can exceed the far-field blackbody limit by orders of magnitude and be quasi-monochromatic. These unique properties of near-field thermal radiation have attracted significant attention for many potential applications such as waste heat recovery using thermophotovoltaic devices, near-field photonic cooling, and thermal rectification. Most of near-field applications require tuning the spectrum of near-field thermal radiation. So far, the near-field spectra are tuned by engineering materials at the sub-wavelength scale. These sub-wavelength-engineered materials are referred to as metamaterial. Thermal emission by metamaterials, which is significantly different from that of the corresponding bulk materials, can be tuned by modifying the geometry and size of the sub-wavelength features in the material. In this study, we propose tuning the spectrum of near-field thermal radiation by engineering the materials at the quantum scale. We show that modulating the electronic structure of materials by capitalizing on the quantum confinement effect provides a powerful mechanism for tuning the peak location of near-field thermal radiation.
For this purpose, we calculate the local density of states (LDOS) thermally emitted by lead chalcogenide quantum dots (QDs) of various diameters ranging from 3.3 nm to 6.8 nm at a near-field distance from the QDs. To find the thermal LDOS, the size-dependent dielectric function of the QDs is extracted from their measured absorbance spectra using the Maxwell-Garnett effective medium theory, the Kramers–Kronig relation and an optimization technique. The thermal discrete dipole approximation is used for finding the LDOS based on the framework of fluctuational electrodynamics and using the extracted dielectric function data. The thermal LDOS emitted by periodic and random arrays of QDs shows a peak around the bandgap energy of the QDs. The bandgap energy, which is affected by the quantum confinement in the QDs, can be tuned by changing the size and thus the level of quantum confinement of the QDs. The wavenumber of the peak LDOS emitted by PbS QDs can be shifted by up to 4490 cm^-1 by changing the diameter of the QDs from 3.7 nm to 6.8 nm. It is also demonstrated that the peak location of the near-field radiative heat transfer between two arrays of QDs shows the same level of tuning as the thermally emitted LDOS.
Acknowledgment:
This work is supported by the National Science Foundation under Grant No. CBET-1804360.

Thermal radiation from a hot body is well described by Planck’s law. However, Planck’s law is not applicable when the gap size or the characteristic dimensions of the involved objects are much smaller than the thermal wavelength. Specifically, when a hot and a cold object are placed at nanoscale gaps, orders of magnitude enhancement in the radiative energy transfer is expected due to contributions from the evanescent waves. This can dramatically enhance the performance of thermophotovoltaic (TPV) energy conversion, which involves a hot emitter radiating photons that excite electron hole pairs in a photovoltaic (PV) cell. The performance of a TPV system can be characterized by the electrical power density and the efficiency of the radiative energy conversion. Recent TPV studies in the far-field demonstrated impressively high efficiencies (~30%) but at low power densities due to operation in the far-field. Several theoretical proposals have shown that dramatic enhancement in power density can be achieved by reducing the gap to tens of nanometers. However, such demonstrations have been challenging due to difficulties in maintaining nano-gap separations between a hot emitter (>1000 K) and a cold cell (~300 K). In this talk, we will first describe the development of micro-fabricated doped silicon emitters that can be resistively heated to temperatures as high as 1270 K. Next, we will describe how high power (~5000 W/m²) and high efficiency (~7%) TPV energy conversion can be achieved when the hot emitter is placed at nanometric separations (<100 nm) from a custom fabricated thin-film InGaAs-based PV cell designed for high internal quantum efficiency and for minimized sub-bandgap absorption. Finally, we will present strategies for achieving even larger energy conversion efficiencies.

Recently, time reversal symmetry-breaking Weyl semimetals have attracted great attention in the field of thermal radiation because of their strong, intrinsic electromagnetic nonreciprocity in the mid-IR. Previous models have emphasized their potential to realize the breakdown of Kirchhoff’s law of radiation (i.e., the inequality of spectral directional emissivity and absorptivity) and highly directional emitters and absorbers. Their far-field nonreciprocity is linked to nonreciprocal surface plasmon polaritons, which suggests even stronger nonreciprocity is possible in the near-field, where surface modes dominate the radiative heat transfer. The effects of Fermi arc surface states (a defining characteristic of Weyl semimetals) on surface modes and near-field radiative heat transfer remain largely unexplored. Here, we model the near-field radiative heat transfer between two planar time reversal symmetry-breaking Weyl semimetals, explicitly including the effects of Fermi arc surface states in Maxwell’s equations. We show that the inclusion of Fermi arc surface states creates new surface modes that exhibit strong nonreciprocity and exotic behavior. These modes, which can exist at relatively large wavevectors, contribute non-negligibly to the heat transfer, contrasting our previous studies in which Fermi arc surface states did not significantly affect far-field radiative heat transfer. Our work further points to the applicability of Weyl semimetals in nonreciprocal, tunable radiative heat transfer systems and highlights the importance of Fermi arc surface states beyond as a hallmark of Weyl semimetals. This work is supported by ARO MURI (Grant No. W911NF-19-1-0279) via the University of Michigan.

Near-field radiative effects can be observed through two different manifestations. First, the distance dependence of the flux exchanged between two bodies at different temperatures is different from the prediction from the macroscopic theory. This is now well established for large bodies, such as parallel-surfaces and sphere-plane configurations [1]. Second, the temperature dependence of the flux exchanged is also expected to be different in the near field [2,3], as sizes and temperatures are tightly connected [4]. Only moderate temperature differences (up to 400 K) between emitter and receiver were probed so far, and only few materials were tested. We therefore focus on near-field radiative heat transfer measurements between microspheres and planar substrates [5] for record temperature differences larger than 900 K, with the emitter being heated up to 1200 K [6]. This allows observing the temperature dependence of near-field radiative heat transfer for various materials (SiO₂, graphite and InSb). It is shown that the temperature exponent n of the near-field radiative power, ranging between 2 and 3 in various cases, is lower than that in the far field, which itself can deviate from n = 4 well-known in the Stefan-Boltzmann law. These results are obtained thanks to a careful calibration of thermal cantilevers by combined means of Raman thermometry and FEM simulations. They indicate that power increase due to decreasing distance and rising emitter temperature do not act with equal footing, which has key implications for energy harvesting such as thermophotovoltaics.
References:
[1] C. Lucchesi et al., Nanoscale Horizons 6, 201 (2021) [2] T. Kralik et al., Phys. Rev. Lett. 109,224302 (2012) [3] Y. Tsurimaki et al., J. Quant. Spectrosc. Radiat. Transfer 187, 310–321 (2017) [4] K.L. Nguyen et al., Appl. Phys. Lett. 112, 111906 (2018) [5] C. Lucchesi et al., Nano Lett. 21, 4524 (2021) [6] C. Lucchesi et al., to be published.

Acknowledgments: Financial support by the French National Research Agency (ANR) grant ANR-6-CE05-0013 (Demo-NFR-TPV), by EU EIC Pathfinder FETPROACT-EIC-05-2019 grant 951976 (TPX-Power) and partial funding by the French "Investment for the Future" program (IDEXLYON ANR-16-IDEX-0005) are acknowledged.

The fast and large optical response changes in transparent conducting oxides (TCOs), namely, ITO and AZO, under external optical excitation have recently led to the demonstration of nonlinearity enhancements and all-optical addition of signal. In this talk, techniques to control the optical responses of TCOs will be discussed as well as nonlinear optical effects enabled by the so-called epsilon-near-zero (ENZ) region of the TCOs. Recent results on extending the ENZ regime from the visible to the mid-infrared wavelength region by utilizing metal- and optical doping will be reported. Specifically, we tailored the ENZ points of cadmium oxide from 11 µm to 5 µm by yttrium doping, to develop fast all-optical switches in the mid-IR regime. Secondly, utilizing undoped zinc oxide, we showed real-time control over the permittivity by interband pumping, achieving unity order refractive index change, and picosecond-speed reflectance modulation in the telecom regime. In addition, by employing titanium nitride with a nanosecond response, and aluminum-doped zinc oxide with a picosecond response, we tuned the switching speed of a metasurface from the nanosecond to the picosecond scale. This results could pave the way to designing novel nonlinear optical devices spanning the telecom to the mid-infrared regime.

Material systems with tunable optical properties are in high demand for mid- and far-infrared applications, including infrared sensing, thermal imaging, and radiative cooling. Such materials can also enable active metasurfaces, which can serve different optical functions simply by tuning the refractive index of a material layer. Here, we present the perovskite cobaltite La_1-xSr_xCoO_3-δ (LSCO) as a new material candidate for tunable nanophotonics that demonstrates increased light modulation depth and convenient electrical control for infrared applications.

We focus on electrolyte-gated transistors featuring perovskite LSCO at x = 0.5 (LSCO50), which is a metallic ferromagnet with a low formation enthalpy and high diffusivity of oxygen vacancies. Using electrolyte gating with ionic liquids/gels, it has been shown that positive gate voltage and resulting oxygen vacancy formation in ~10-nm-thick LSCO50 drives a topotactic transformation from the metallic perovskite phase to an insulating oxygen-vacancy-ordered brownmillerite phase, La_1-xSr_xCoO_2.5, and thus a metal-insulator transition (MIT).

Here, we show via ex situ spectroscopic ellipsometry and Kramers-Kronig consistent dispersion models that this electrochemically-induced MIT produces significant, nonvolatile refractive index changes between the perovskite and brownmillerite phases of LSCO50, both of which are air-stable. In particular, the insulating brownmillerite phase is a high-index, low-loss dielectric (n > 3, k < 0.4) in the mid-infrared, while the metallic perovskite phase remains highly lossy.

We then implement electrochemically-gated LSCO50 as the active layer in a reconfigurable, mid-infrared plasmonic absorber designed with finite-difference time-domain (FDTD) electromagnetic simulations. In the ungated state, strong optical losses in metallic-phase LSCO50 hinder surface plasmon resonance and yield a spectrum of higher reflectance, but the insulating phase accessed by electrochemical gating allows incident radiation to couple into highly-absorbing resonant modes, significantly lowering the reflectance of the metasurface. Such a device could enable many infrared applications including dynamic thermal imaging and infrared camouflage, and cements electrolyte-gated LSCO50 as a promising vehicle for tunable nanophotonics.

Metasurface design is a powerful approach to create exotic and desirable properties that do not appear in nature. The quasi-2D and deep-subwavelength structure suggests the device can be made ultrathin, lightweight, and flexible. By coupling the metasurface with mid-IR light, one can enhance performance or create new functionality in the frequency regime of phonons, which has wide applications in molecular vibrational spectroscopy, imaging, and thermal management. In particular, by employing index-tunable materials in metasurfaces, one can modulate the thermal radiation to stabilize the heat balance, which can be a game-changer for future energy-efficient wearable/portable thermoregulation. This dynamic mid-IR metasurface device is orders of magnitude more efficient than traditional thermoregulation such as thermoelectric devices or resistive heaters because it controls the heat transfer coefficient rather than directly pumping heat. Although there are various ways to achieve tunable mid-IR metasurface, special considerations are needed to optimize the thermoregulation performance in tuning range and switching time.
In this talk, I will introduce our recent research progress in using conducting polymers for mid-IR emissivity tuning and comparative study for ordinary versus metasurface design. Conducting polymers are widely known for their electrochemically tunable optoelectronic properties, and they have the suitable plasmon frequency to work as a mid-IR plasmonic material. The change of carrier density is driven by electrochemical reaction, which implies large contrast. The switching speed is not as fast as electrostatic gating but nevertheless more than enough for wearable technology and room temperature thermoregulation. Despite the potential impacts on applications for personal health and space heating/cooling energy efficiency, mid-IR electrochromic properties of conducting polymers have not been thoroughly studied, limiting their utility for tunable metamaterials. I will share our study on conductive polymer charge transport and optoelectronic properties, hoping to enrich the material palette for tunable mid-IR materials.

VO₂ is a phase change material that undergoes a first order crystalline phase transition at a critical temperature (T_c = 68^oC), resulting in significant changes in intrinsic electrical and optical properties, especially in the infrared. Optical changes with this phase transition are of particular interest as passive and active components of optoelectronic devices, specifically for thermal regulation and modulated signaling. Realizing this type of device often requires the integration of thin, conformal VO₂ films with complex, non-planar structures (like metamaterials). Thus, atomic layer deposition (ALD) is an ideal deposition method in these cases. In this work, we will discuss the ALD process optimization and characterization of thin VO₂ films itself and subsequent integration into different optoelectronic structures for active modulation. In particular, we will present a few examples based on nanopillar arrays.

Traditional metal-based plasmonic materials suffer from high optical losses, which has promoted research towards alternative low-loss materials that can support plasmonic-like effects. One such approach employs phonon-mediated collective-charge oscillations (surface phonon polaritons, SPhPs) that are supported by nanostructured polar dielectric materials (SiC, AlN, etc), which inherently are low-loss. Geometric design of the nanostructures enables spectral tuning of resonant features between the longitudinal and tranverse optical phonons of the polar material, typically in the infrared regions. However, the spectral position and amplitude of these resonances remain fixed after fabrication. Integrating phase change materials with these structures provides a way to achieve active modulation of resonances.

As an example, nanopillar arrays were etched into SiC and AlN to create narrowband resonances in the long-wave infrared region. These structures were subsequently coated with ALD VO₂ films with different thicknesses (8-75nm). As-deposited VO₂ films are highly conformal and amorphous, and cause the resonances to shift and broaden due to the different dielectric environment. However, after annealing the films at 525°C in 6x10^-5 Torr, the VO₂ films crystallize resulting in sharper resonances and spectral locations close to the initial uncoated structures. Temperature-dependence reflectance and emission measurements show that by heating through the VO₂ transition temperature, the amplitude of the resonances can be modulated. Full signal modulation (ie. on/off) requires at least a 16nm VO₂ film.

ALD VO₂ films can be applied to other optoelectronic pillar arrays derived from different, non-polar semiconductors, such as Si, to enable active focusing/defocusing or optical limiting [1] using metasurface concepts. Additionally, we have demonstrated the ability to use these in planar stacks for thermal homeostasis [2]. We will briefly go over several of these concepts the advantages and disadvantages of ALD VO₂ films in each.

[1] Howes et al, Nano Lett. 20(6), 4638-4644 (2020).
[2] Morsy et al, Scientific Reports 10, 13964 (2020).

The use of nano and microstructured metasurfaces provides new freedom in infrared optic design as it allows precise control over the phase, amplitude, and polarization at a surface. Realizing dynamic tuning of these properties is a critical step towards achieving more advanced functionality and improved operational bandwidth. In this talk, I outline several approaches towards achieving dynamic tuning by incorporating metasurfaces with phase change media. I will explore both thermal and electrical activation of these metasurfaces for achieving dynamic tunability in both the infrared regime. The metasurface can be used for a variety of purposes including spectral signature control, thermal management, and adaptive systems.

Objects around us constantly emit and absorb thermal radiation. The basic properties that characterize these two processes are emissivity and absorptivity, respectively. For reciprocal systems, the emissivity and absorptivity for a given direction, polarization, and frequency are tightly restricted to be equal by Kirchhoff's law of thermal radiation [1]. This restriction limits the control of thermal radiation and contributes to an intrinsic loss mechanism in photonic energy harvesting systems.

Existing approaches to violate Kirchhoff's law typically utilize conventional magneto-optical effects in the presence of an external magnetic field. However, these approaches require either a strong magnetic field (~3T) [2] or narrow-band resonances under a moderate magnetic field (~0.3T) [3], because the nonreciprocal effect in conventional magneto-optical materials is usually weak in the thermal wavelength range. The nonreciprocal effect is also only confined in a relatively narrow angular range. Meanwhile, in these designs, the angular distribution of emissivity and absorptivity is symmetric with respect to the normal direction, meaning that the emissivity in the θ direction is always equal to the absorptivity in the -θ direction. This equality significantly restricts the ability to separately control the emissivity and absorptivity in the angular degree of freedom.

In this talk, I will show that the axion electrodynamics in magnetic Weyl semimetals can be used to construct strongly nonreciprocal thermal emitters [4,5]. Such a thermal emitter can near completely violate Kirchhoff’s law over broad angular and frequency ranges, without requiring any external magnetic field. Meanwhile, I will introduce a general coupled-mode theory framework for nonreciprocal thermal emitters. I will discuss the necessary conditions for the nanophotonics design to decouple the link between emissivity in the θ direction and the absorptivity in the -θ direction so that one can achieve complete control of emissivity and absorptivity in the angular degree of freedom. Finally, I will present a case study using nanophotonic design and magnetic Weyl semimetal to achieve nonreciprocal thermal emission in semitransparent emitters [6].

Reference

[1] G. Kirchhoff, Annalen Der Physik 185, 275 (1860).
[2] L. Zhu and S. Fan, Phys. Rev. B 90, 220301 (2014).
[3] B. Zhao, Y. Shi, J. Wang, Z. Zhao, N. Zhao, and S. Fan, Opt. Lett. 44, 17(2019).
[4] B. Zhao*, C. Guo*, C. A. C. Garcia, P. Narang, and S. Fan, Nano Lett. 20, 1923 (2020).
[5] Y. Tsurimaki, X. Qian, S. Pajovic, F. Han, M. Li, and G. Chen, Phys. Rev. B 101, 165426 (2020).
[6] Y. Park, V. Asadchy, B, Zhao, C. Guo, J. Wang, and S. Fan, arXiv:2105.08954.

Vanadium dioxide displays thermochromic properties based on a reversible metal-insulator transition which is caused by the structural phase transition from monoclinic VO₂ (M) to rutile VO₂ (R) and vice versa. We developed a single layer coating comprising VO₂ (M) and SiO₂ starting from a mixture of pre-oligomerized tetraethoxysilane and vanadium(IV) oxalate dissolved in an alcohol-water mixture [1,2]. After dip coating and subsequent solvent evaporation under ambient conditions, we obtained a homogeneous mixture of both components as xerocoat on silica-coated float glass. During the thermal anneal at 450°C in inert gas atmosphere phase separation occurred. This yielded coatings comprising V-rich and Si-rich nanodomains, the size and shape of which depended on the V/Si ratio in the starting mixture. Size and shape of the nanodomains were determined by performing scanning transmission electron microscopy – energy dispersive X-ray studies on scraped off coating material. The thermochromic coatings resulting from these experiments combine high visible light transmission T_vis > 60% with a large solar modulation ΔT_sol ≥ 10%, as demonstrated via UV-vis-NIR spectrophotometric analyses performed below and above the switching temperature. In addition to T_vis and ΔT_sol, the switching hysteresis width studied. The relationship between the coating size, shape and concentration of VO₂ domains and the optical properties of the coating was studied in detail. Additionally scraped off coating material has been isolated and measured with differential scanning calorimetry to determine the switching enthalpy and relate that to the amount of VO₂ (M) in the obtained mixture of vanadium oxides. When applied in insulating glass units, the coating has a positive impact on energy savings for heating and cooling of buildings in intermediate climates, which is demonstrated through energy performance simulations. For a typical house in the Netherlands, energy savings up to 24% were forecasted [3]. In addition, we demonstrate a coating life time of > 10 years in insulating glass units through accelerated life time tests, including switching fatigue studies.

We present the results of our density functional theory simulations of the optical properties of some oxides at high temperature.
To validate, we simulate the band structure and dielectric constant of one of the archetypal oxides for which many experimental data is available i.e. MgO. To overcome the gap underestimation, we use the GGA+U approximation. At high temperatures, defects affect the optical properties, and therefore we simulate the dielectric constant of this system with O and Mg vacancies as the dominant defects. They induce respectively donor and accepptor levels in the gap and cause interband transitions of lower energies. The change of the dielectric constant as a function of defect concentration will be discussed in this work.

Birefringent and dichroic crystals are critical building blocks for devices such as polarizers, filters, and phase matching elements in optical systems. However, in the mid-wave (MWIR) and long-wave (LWIR) infrared spectral ranges, there is still a shortage of optically anisotropic materials. In this work, we show perovskite-derived chalcogenides—A_1+xTiS₃ (A=Sr, Ba)—are a platform for achieving giant optical anisotropy across the MWIR and LWIR and can play a key role in next-generation infrared optics. We grew high-quality A_1+xTiS₃ crystals of various shape and orientation by chemical vapor transport. Electron microscopy studies revealed 1D chains of face-sharing TiS₆ octahedra with a highly anisotropic electronic structure, which infers a large optical anisotropy. We used polarized infrared spectroscopy combined with spectroscopic ellipsometry to extract the optical properties of these materials and discovered refractive-index differences between the ordinary and extraordinary axes of up to two across the MWIR and LWIR, as well as large dichroism. We applied Density Function Theory (DFT) on A_1+xTiS₃ structure and discovered that such optical anisotropy originates from the structural modulation of TiS₆ octahedral. We anticipate A_1+xTiS₃ (A=Sr, Ba) and other quasi-1D materials will be useful for next-generation imaging and sensing applications, especially for miniaturized photonic devices. This study also lays ground for material designs towards highly electronic and optic anisotropic materials.

Programing materials with light and interrogating their properties within the resultant transient states is an important goal of modern condensed matter physics. Polaritons - where strong dipole active resonances hybridize with photons - are central to transient nano-optical exploration. Infrared nano-imaging can be used to detect these polaritonic waves, yeilding detailed information on the properties of host materials that support these polaritons. The naturally layered class of van der Waals (vdW) materials are particularly intriguing in this regard due, in-part, to their often strongly anisotropic behavior. In materials hosting both strong anisotropy and strong dipole active resonances sub-diffractional polaritonic wave packets, that travel as conical rays with hyperbolic dispersion throughout the materials’ bulk, can emerge. We utilized femtosecond photoexcitation to agitate crystals of the vdW semiconductor WSe2 and interrogated these crystals in the transient state with near-field optical microscopy [1]. Our time-resolved nano-imaging data reveals key signatures of optical hyperbolicity, appearing on the sub-picosecond timescale.
[1] AJS et al., Science 371, 617–620 (2021)

Narrow gap semiconductors and topological semimetals/metals are important in infrared applications and can be used for harvesting radiative thermal energy. The band structure of these materials can change with temperature when the material goes through a phase change and it affects the material's thermal, electrical, and magnetic properties. In this talk, I will discuss several materials, their band structure in different phases, and how the band structure and the electron-phonon scattering affect the electrical, and thermomagnetic properties. I will discuss our results on NbSe2 sample, a topological metal/semimetal, that goes through charge density wave (CDW) transition, and superconducting phase at lower temperatures. The band structure of both 1T and 2H phase appears semimetallic in the normal phase while a gap opens up in the CDW phase. I will also discuss the Mo1-xWxTe2 single crystal samples, another topological semimetal, which are going through 2H-1T phase change, and show how the Seebeck and the Nernst coefficients are affected. Finally, I will discuss the case of FeSb2, a highly correlated narrow-gap semiconductor with debates on the effect of phonon-drag as opposed to correlation on its transport properties and specifically on the Seebeck coefficient.

Photonic designs with active elements have the potential to exquisitely tailor the properties of thermal radiation, including modulation of the intensity, phase, linewidth, wavevector and scattered wavefront. We describe the coupling of tunable permittivity graphene elements to resonant nanophotonic structures for i) modulation of emissivity between 10% and 97% by Fermi level modulation, ii) active steering and focusing of thermal radiation, iii) dramatic emission line narrowing of thermal emitters from linewidths characteristic of blackbody emitters to those more closely resembling light-emitting diodes.

Pulsed laser excitation of graphene is shown to exhibit ultrabright non-thermal emission, arising from spontaneous emission of plasmons generated by cooling of the nonequilibrium carrier distribution. Our observation of the non-thermal contribution of Fermi-level-dependent radiation constitutes the first experimental demonstration of hot plasmon emission arising from a photo-inverted carrier distribution in graphene. This previously unobserved mechanism generates bright nonthermal radiation in the far field, as the generated plasmons are outcoupled from the excited graphene by gold nanoantennas, with implications for design of future ultrafast and ultrabright mid-infrared emission processes and light sources.

For the vast majority of materials, Kirchoff’s Law dictates that absorptivity and emissivity are equal for any particular wavelength or angle, a property that has fundamental implications for infrared photonics. While previous theoretical work has suggested that Kirchoff’s Law could be violated using magneto-optical materials, the required magnetic field strengths were impractically large. We demonstrate in the present work the first definitive experimental observation of Kirchoff Law violated in magnetoelectric heterostructures. We find that the required magnetic field for reciprocity breaking can be reduced by over an order of magnitude, bringing it within the range of practical experiments with simple permanent magnets. By combining the magneto-optical character of doped InAs crystals in the epsilon-near-zero wavelength regime with guided mode resonance waveguides, we have observed violations of Kirchoff’s Law as large as 50% in experiment. These results open up new opportunities for the manipulation of thermal radiation in the long wavelength infrared regime.

The reduced dimensions of electronic devices lends itself toward rapid bottlenecks in thermal dissipation; heat conduction becomes limited due to interface scattering of thermal carriers when material geometries approach the nanoscale. Thus, additional thermal transport pathways are necessary in further developing next-generation RF and power electronics. In this work, we consider two potential means of doing so: i) enhancing interfacial heat transport rates through non-equilibrium processes and ii) accessing polariton-mediated transport mechanisms.

In the first case, we elucidate a novel energy transduction pathway that can efficiently transfer heat across metal-semiconductor interfaces, where traditional thermal boundary resistances greatly mitigate heat flow. This energy transfer mechanism, termed Ballistic Thermal Injection, is demonstrated in gold/cadmium oxide (CdO) heterostructures. We find that under non-equilibrium conditions, photo-excited metals can undergo an electron-mediated ballistic energy transfer process that can remotely modulate the electronic environment of an underlying degenerate semiconductor; namely the plasmonic resonance of this semiconductor. We term this effect ‘ballistic thermal injection’ (BTI) to distinguish it from the well-studied charge injection effects caused by hot electrons traversing an energy barrier. Our results are explained through transient reflectivity (TDTR) measurements to understand the fundamental carrier dynamics at the Au/CdO interface. Additionally, we perform mid-infrared, ultra-fast pump-probe experiments to obtain spectral, temporal, and ultimately spatial resolution of the electronic environment within the CdO layer following excitation of the Au film. Ultimately, the carrier dynamics derived from the transient spectra fully agree with our TDTR results, and can only be accurately modeled by the BTI process rather than by charge injection.

With regards to polariton-mediated heat transfer pathways, from an experimental perspective, many of the recent advances have been investigated from the perspective of thermoreflectance measurements, such as TDTR or FDTR, where the role of these potential thermal carriers is inferred by trends in thermal conductivity measurements. While certainly of merit, these methods provide an experimental measure of the average temperature of a system rather than a direct probe of the vibrational carrier of interest. In this work, we further employ our tunable-wavelength, mid-infrared ultrafast pump-probe spectroscopy system to perform such a measurement. In particular, we investigate the phonon dynamics of both h-BN and SiC following pulsed excitation of a metal contact, providing a direct measure of the temporal dynamics of the optical phonon mode, the highly-reflective Reststrahlen band, and phonon-polaritons in these material systems during heating. These results demonstrate a unique regime of polariton-mediated heat transfer: the generation of a thermal radiation-induced phonon polariton. In other words, we demonstrate the ability to stimulate phonon polaritons in h-BN via remote heating of a metal contact, thus allowing for enhanced radiative heat transfer in device heterostructures.

The lattice symmetry of a crystal is one of the most important factors in determining its physical properties. Low-symmetry crystals offer powerful opportunities to control light propagation, polarization, and phase. When such materials featuring extreme optical anisotropy, they can support a hyperbolic response, enabling coupled light-matter interactions, also known as polaritons, with highly directional propagation and compression of light to deeply sub-wavelength scales. In this talk we show how low-symmetry crystals can support hyperbolic shear polaritons, a new polariton class arising in the mid- to far-infrared due to shear dissipation in the dielectric response. This feature can emerge in materials where the dielectric tensor cannot be diagonalized, that is, in low-symmetry monoclinic and triclinic crystals where multiple damped oscillators with non-orthogonal relative orientations contribute to the optical response. This results in a non-zero complex off-diagonal terms in the dielectric tensor which cannot be removed by coordinate rotation. Hyperbolic shear polaritons complement previous observations of hyperbolic phonon polaritons in orthorhombic and hexagonal crystal systems, unveiling new features, such as the continuous evolution of their propagation direction with frequency, tilted wavefronts, and asymmetric responses.
As an exemplar case, we will describe the hyperbolic shear polaritons supported by the polar optical phonons of the monoclinic semiconductor β-Ga₂O₃. We will describe an approximate framework for how to classify polaritons in low-symmetry materials with non-orthogonal oscillators, and how this predicts a continuous rotation of the polariton direction frequency. We will then go on to experimentally demonstrate how hyperbolic shear polaritons can be measured using orientation sensitive prism type coupling experiments. Finally, we will show the specific role of the off-diagonal terms play in creating new types of light propagation, and how this influences our treatment of the polaritons. This interplay between diagonal loss and off-diagonal shear dissipation in the dielectric response of these materials has implications for new forms of non-Hermitian and topological photonic states. We anticipate that our results will motivate new directions for polariton physics in low-symmetry materials, which include geological minerals, many common oxides, and organic crystals, significantly expanding the material base and extending design opportunities for compact photonic devices.

Engineering the coherent properties of thermal emission plays an important role in many applications, such as energy harvesting, light sources, imaging and remote temperature sensing. Whereas thermal emission from bulk materials is generally incoherent, it has been shown that structured material, which have at least one of its structure parameters comparable with the thermal wavelengths, can greatly modify the coherence of thermal emission due to excitation of optical modes in the photonic structures. Most of existing works so far can only realize simple function such as spectral and directional selectivity. More complex control such as focusing and holography, remains difficult to achieve. The limited control of thermal emission is a result of the simple structures used in most previous works, which are either periodic structures or small structures whose sizes are around the wavelength of thermal photons. They do not offer enough degrees of freedom to accomplish complex control. To overcome this limit, we use large-area emitters with inhomogeneous structures that offer a large design space. Fine tuning the structural features allows us to exploit collective wave effects of thermal photons and to realize sophisticated control of thermal emission such as focused thermal emission and thermal holography.

Considering the adoption of the sustainable circular bioeconomy, polyethylene (PE) fibers produced from green, renewable raw material are becoming more and more critical for everyday clothes, bedding textile and wearables due to their low environmental footprint and combination of material low-cost, durability with lightweight, high UV resistance and infrared (IR) transparency. [1-4] It is not surprising that the unique properties of green PE can be exploited to engineer high-performance sustainable fibers with mechanical properties tunable by design. Here, we explore the role of polyethylene molecular architecture in engineering mechanical properties and surface morphology of fibers based on the traditional fossil sourced raw PE and sugarcane-derived, renewable raw PE material. A range of fibers with different material parameters have been fabricated by a low-cost melt-extrusion method, mechanically and optically characterized, and classified according to their crystallographic structure as well as elongation, strength, recovery properties, and surface morphology. Differential scanning calorimetry and X-ray scattering techniques confirmed that by combining precise engineering of molecular structure we can adjust structural disorder in PE fibers, which results in crystallinity of fibers ranging from ~8% to ~60% and different crystallite orientation along the fiber axis. This tunability of the fiber molecular structure translates into a wide range of elongation at break, specific stiffness and specific strength values as well as tunable infrared transparency and thermal conductivity values. These results demonstrate feasibility of low-cost, large-scale fabrication of green PE fibers with a wide range of mechanical performance, smooth fiber surfaces, and variable cross-sectional shapes, paving the road to engineering mono-material high-performance sustainable textiles. These mono-material textiles can find applications in passive cooling technologies and are 100% recyclable at the end of their lifecycle.

This work was supported by the DEVCOM Soldier Center through the US Army Research Office (W911NF-13-D-0001), the Advanced Functional Fabrics of America (AFFOA) Institute (W15QKN-16-3-0001), the MIT Deshpande Center, and the US Department of Energy (DE-FG02-02ER45977). We thank B. A. Welsh (DEVCOM SC) for help with PE fiber fabrication and Braskem and Dow Chemical Company for providing raw olefin polymers.

[1] Alberghini, M.; Hong, S.; Lozano, L.M.; Korolovych, V.; Huang, Y.; Signorato, F.; Zandavi, S.H.; Fucetola, C.; Uluturk, I.; Tolstorukov, M.Y.; Chen, G.; Asinari, P.; M. Osgood III, R.; Fasano, M.; Boriskina, S.V. Sustainable polyethylene fabrics with engineered moisture transport for passive cooling. Nature Sustainability 2021, 1-10.
[2] Boriskina, S. V. An ode to polyethylene, MRS Energy Sustain. 2019, 6, E14.
[3] Lozano, L. M.; Hong, S.; Huang, Y.; Zandavi, H.; El Aoud, Y. A. ; Tsurimaki, Y.; Zhou, J.; Xu, Y.; Osgood, R. M.; Chen, G.; Boriskina, S. V. Optical engineering of polymer materials and composites for simultaneous color and thermal management. Opt. Mater. Express 2019, 9, 5.
[4] Tong, J.K.; Huang, X.; Boriskina, S.V.; Loomis, J.; Xu, Y.; Chen, G. Infrared-transparent visible-opaque fabrics for wearable personal thermal management. ACS Photonics 2015, 2, 769-778.

Photovoltaic systems show a strong reduction in conversion efficiency when their temperature rises due to hot carrier cooling and non-radiative recombination in the solar cells. One key factor that determines the steady-state cell temperature is the radiative cooling rate: every solar cell shows spontaneous emission of infrared light. The spontaneous emission is determined by the emissivity, which is linked to the absorptivity by Kirchhoff’s law (ε=A=1-R-T). To achieve the maximal radiative cooling rate for a silicon solar cell, and given the absorption spectrum of the earth’s atmosphere, the emissivity should be minimal in the spectral range from the Si bandgap at 1.1 to 4 micron and be maximal beyond that range.

Here, we propose two designs based on resonant microstructures; one for bare silicon solar cell and a second for a silica-based solar module glass. Both designs enhance the emissivity, and hence the radiative cooling rate, by reducing the reflection and transmission in a well-defined spectral range beyond 4 micron. They effectively enhance the absorption of infrared light. The design is based on Mie-like resonances in spherical or cylindrical structures that show strong forward scattering of light and thereby decrease the reflectivity. Finite-difference-time-domain (FDTD) simulations are used to optimize the geometry of the radiative cooling microstructures, by minimizing the reflectance in the infrared.

For the bare silicon solar cell, we find an optimized design composed of a close-packed hexagonal array of glass microspheres (diameter 20 micron) that is fabricated by drop casting from an aqueous suspension on silicon. The hemispherical reflectivity of the stack is measured using Fourier Transform Infrared Spectroscopy (FTIR) with an external integrating sphere. The measured reflectivity of bare silicon is reduced from 80 to 10 percent by adding the close-packed monolayer of glass spheres, for wavelengths larger than 7 micron.

To determine the radiative cooling power we measure, using a thermocouple, the temperature of different designs under direct sunlight on a sunny summer day in Amsterdam. The preliminary results of the silicon covered with glass spheres, show a clear decrease in temperature of approximately 5 degrees with respect to the bare silicon reference under solar illumination. Reference reflectivity measurements at wavelength below the Si bandgap show the effect is not due to reduced incoupling of below-gap sunlight into the cell, hence demonstrating a significant radiative cooling effect.

For the silica-based solar module glass, we use a periodic hexagonal array of quartz cylinders (diameter 3.5 micron, height 2.25 micron, pitch 6.13 micron). The cylinders are fabricated into a thin quartz plate with UV lithography and reactive ion etching using C₄F₈/Ar. The micropatterned plate is then placed on top of a silicon sample. The unpatterned stack shows a strong reflection peak at 9 micron, due to a stretching vibration in the quartz crystal. This peak is reduced from 75 to 30 percent, due to the forward scattering of the induced electric dipole in the microcylinders.

The new radiative cooling microstructure design concept can be applied to every kind of solar cell, with or without module glass. The resonant radiative cooling concept can be applied to all devices operating in outside ambient that benefit from a low operation temperature.

Thermophotonics (TPX) is a technology close to thermophotovoltaics (TPV), where a heated light-emitting diode (LED) is used as the active emitter of the system [1]. By doing so, the emission profile can be tuned: initially in the infrared range, it can be shifted by means of electroluminescence to a spectral range matching better the gap of a photovoltaic cell. With the development of LEDs and the increase of their achievable quantum efficiency, TPX has come out as an attractive concept for both energy harvesting and refrigeration [2]. The many studies on near-field (NF) thermal radiation and their application into efficient NF-TPV devices [3] highlight the possibility to extend the concept to near-field thermophotonics (NF-TPX) [4], where enhanced energy conversion is due to both electric control and wave tunneling.

This contribution analyzes numerically the capabilities of NF-TPX systems. Several methods modelling the charge carriers behaviour for NF-TPV systems [5-6] are compared for NF-TPX in terms of precision and computation time. For both the LED and the PV cell, the capabilities of PN and PIN AlGaAs junctions are analyzed. The results include detailed IV characteristics around the maximum power point, highlighting in particular the advantages in comparison to far-field TPX and NF-TPV. The impact of the doping levels and the thickness of each element are amongst the studied parameters.The capabilities of NF-TPX to harvest energy are assessed for thermal sources of few hundreds of degrees Celsius [7].

[1] N.P. Harder and M.A. Green, Semicond. Sci. Technol. 18,S270,2003 [2] T. Sadi et al., Nat. Phot.14, 205, 2020 [3] C. Lucchesi et al., Nano Lett.21, 4524, 2021 [4] B. Zhao et al., Nano Lett.18, 5224, 2018 [5] E. Blandre et al., Sci. Rep.7, 15860, 2017 [6] W.A. Callahan et al., Phys. Rev. Appl.15, 54035, 2021 [7] J. Legendre and P.O. Chapuis, submitted.

We acknowledge the funding of EU H2020 FET Proactive (EIC) programme through project TPX-Power (GA 951976).

Blocking near-infrared (NIR) is indispensable for saving energy consumed to maintain an interior temperature in buildings. However, it is hard to achieve both high visible transmittance and high NIR reflectance for energy-saving windows. Here, we demonstrate a TiO₂/Ag/TiO₂/SiO₂/TiO₂ multilayer film on a glass substrate to block the entire NIR (800 ≤ λ ≤ 2500 nm) while maintaining high visible transmittance. We first design and optimize the thickness of a TiO₂/Ag/TiO₂ structure; the metal layer reflects NIR and the dielectric layers increase visible transmittance with zero reflection condition. To further enhance NIR-blocking efficiency, we implement a TiO₂ back reflector with a SiO₂ spacer to TiO₂/Ag/TiO₂ structure_. TiO₂ layer with high refractive index sandwiched between glass substrate and SiO₂ with low refractive indices can induce additional Fresnel reflection without sacrificing transmittance in visible light. The optimal photonic multilayer film shows solar energy rejection 89.2% (reflection 86.5%, absorption 2.7%) in NIR, visible transmittance 69.9%. Furthermore, the blocking capability in NIR of the designed multilayer film is maintained over a wide range of incident angles of light.

As electronic devices continue to reduce towards nanoscale dimensions, understanding heat transport mechanisms at these scales becomes vital. In most non-metallic materials heat dissipates according to the movements of phonons or phonon-like waves. The efficiency of this phonon-mediated heat transport is dictated by various characteristics of these modes including their lifetime, scattering rate, and velocity. Thus, characterization of these properties is crucial to making more effective devices. From an experimental perspective, the majority of recent advances in such characteristics stems from ultrafast thermoreflectance measurements, namely TDTR and FDTR, where phonon transport is inferred from trends in thermal conductivity of materials. In other words, these methods measure the average temperature of a material following pulsed excitation rather than directly monitoring the individual mode of interest. In this work, we develop a novel ultrafast pump-probe technique that allows us to directly couple to vibrational modes of interest with sub-picosecond time resolution, thus characterizing the aforementioned phonon dynamics.

Our technique relies on a wavelength tunable mid-infrared probe pulse, allowing us to directly resonate with vibrational heat carriers, thus providing a direct measure of phonon dynamics in nanoscale material systems. In particular, we investigate the phonon dynamics in hexagonal boron nitride (h-BN) and silicon carbide; the high frequency optical modes in these materials lends itself to the potential for manipulation of thermal radiation in near room temperature conditions. Thus we investigate the ultrafast temporal dynamics of the transverse optical (TO) modes, the high reflectivity Reststrahlen band, and a unique regime of polaritonic heat transfer: the generation of a thermal radiation-induced phonon polariton.For the first time, we experimentally demonstrate the ability to stimulate phonon polaritons in h-BN through remote heating of a metal contact. Our results open the door for future experiments to manipulate and guide phonons via heterostructures, which could increase thermal efficiency of microelectronics.

Two bodies at different temperatures experience a photon-mediated heat exchange, even when separated by vacuum. This radiative heat flux is limited by Stefan-Boltzmann’s law in the far field, i.e. when the separation distance between the bodies is large compared to the thermal wavelength. The pioneering works of Rytov, Polder and van Hove and the development of the theoretical formalism of fluctuational electrodynamics allowed to show that in the near-field regime the heat flux can instead exceed this limit even by orders of magnitude for shorter distances. Surprisingly, the huge majority of works exploiting fluctuational electrodynamics neglect the possible existence of a temperature gradient due to the coupling between radiative heat transfer between the bodies and conduction within each body, and assume a perfectly uniform temperature inside each of them. We have recently developed a theory to describe the conduction-radiation coupling between solids of arbitrary size separated by a subwavelength gap [1]. The essence of this approach is based on the combination of Boltzmann’s equation to deal with the transport of heat carriers inside the solids (valid for any heat-transport regime) and fluctuational electrodynamics to calculate the radiative power locally dissipated in each body. By means of this approach we show that the interplay between conduction and radiation can indeed result in an observable temperature profile within each solid. Moreover, we show that, as a result of this temperature profile, the radiative heat flux exchanged between two parallel slabs at nanometric distances can be orders of magnitude smaller than the one predicted by the conventional theory. These results could have important implications in the fields of
nanoscale thermal management, near-field solid-state cooling, and nanoscale energy conversion. By exploiting our formalism, we have also highlighted a significant slowing-down of the thermal relaxation process [2] in the extreme near-field regime at distances where the heat
flux exchanged between the two solids is comparable or even dominates over the flux carried by conduction inside each solid. This mechanism, leading to a strong effective increase of the system thermal inertia, should play an important role in the temporal evolution of thermal state of interacting solids systems at nanometric and subnanometric scales.
References
[1] M. Reina, R. Messina, and P. Ben-Abdallah, Phys. Rev. Lett. 125, 224302 (2020)
[2] M. Reina, R. Messina, and P. Ben-Abdallah, arXiv:2106.15471 (2021, accepted on Phys. Rev. B)

Using resonant structures for enhancing light-matter interactions is a popular topic of research. The field localisation that they provide allows to amplify all kinds of processes, such as absorption and nonlinear effects. One limiting factor of resonant structures is their limited bandwidth, especially for the high-Q resonances the bandwidth is too small to effectively interact with short laserpulses. In this work we present a method, that allows to resolve this issue by dynamically switching the Q-factor of the resonance and allowing short pulses to effectively couple to the resonant mode. We demonstrate the effect using a simple theoretical model based on coupled mode theory and then show a potential implementation of the technique using a dielectric metasurface that we model using FDTD. In both models we consider the using our approach to increase the efficiency of third harmonic generation and show a significant increase in the generated harmonic.

Photonic structures with asymmetric responses to impinging light are highly desirable across the EM spectrum, even more so for infrared (IR) light, as they introduce new capabilities for integrated beam manipulation as well as thermal management applications. Here we discuss the key principles leading to extreme asymmetry in reflection or in both reflection and transmission in metagratings comprising building-blocks that lack inversion symmetry along the propagation direction. Although counterintuitive, we explain how such extremely asymmetric responses in these passive systems are entirely consistent with the reciprocity principle. We further showcase examples relevant to highly efficient uni-directional thermal IR sources and directional higher-harmonic generation.

The manipulation of light at small scales is one of the ultimate goals for nanophotonics. For this purpose, polaritons – hybrid light-matter waves that propagate in a confined length scale – are typically involved. Recent results of polaritons in van der Waals materials reveal a series of advances, including atomic scale localization, dynamic tunability, relative low-loss and topologically protected states. These advances are attributed to the unique physical properties in reduced dimensions and the configurability through van der Waals structuring and stacking. In this talk, I will show new merits of phonon polaritons that can be obtained through van der Waals configuration. I will talk about the tunability implemented into phonon polaritons by van der Waals heterostructuring hexagonal boron nitride with graphene and vanadium dioxide, where the polaritons can be tuned dynamically and reversibly via electrostatic gating and temperature control. I will also talk about the geometry and topology configuration of phonon polariton wavefront by twisting stacked slabs of molybdenum trioxide.

Individual electrons in conductors are transported via diffusion, yet current-voltage characteristics in a device are governed by the collective flow of current along streamlines. Despite being at the core of our understanding of electronic and optoelectronic charge transport, the intrinsic global pattern of current streamlines - described by the Shockley-Ramo theorem - has never been imaged. Here, we unveil a microscopy method to image the streamlines along which current naturally flows through ultrathin photodetector devices. Combining scanning photovoltage microscopy with a highly uniform, rotating magnetic field, we probe the photoresponse of atomically pristine thermospintronic heterostructures composed of platinum on yttrium iron garnet (YIG). Under focused laser excitation, a local out-of-plane temperature gradient produces a corresponding out-of-plane spin current in the YIG layer by the Spin Seebeck Effect, which, according to the Inverse Spin Hall Effect, scatters electrons in the platinum layer away from an external, saturating magnetic field. Much like tracers in a wind tunnel map the flow of air around an aerodynamic airfoil, we use a scanning laser beam as a source of directional charge current to map the flow around precisely engineered wing shaped devices, or electrofoils. Photocurrent streamlines can be contorted, compressed or expanded by changing the shape and angle of attack of the electrofoil devices. By imaging and manipulating photocurrent streamlines, we demonstrate an intensive imaging technique to visualize and characterize charge flow in quantum materials.

Air can play a key role within photovoltaic and photothermal materials, owing to its non-absorbing, low-index, low-thermal-conductivity properties. However, exploiting these unique properties is challenging because it requires an approach that accommodates the undesirable properties of air, namely the lack of structural support. Here, I discuss our recent progress in realizing thermophotovoltaic (TPV) cells¹ and refractory solar thermal absorbers^2,3 that incorporate air in a way that does not compromise their functional stability. In doing so, we overcome key limitations in TPV and solar thermal conversion, namely a lack of broadband spectral control, and realize record-high efficiencies. With such high-efficiency approaches, TPV-based thermal batteries can serve as a solution to the long-duration electricity storage problem, while concentrated solar thermal can access a high-temperature regime and, in turn, provide efficient, on-demand power generation and industrial process heat.

1. Fan, D., Burger, T., McSherry, S., Lee, B., Lenert, A. & Forrest, S. R., "Near-perfect photon utilization in an air-bridge thermophotovoltaic cell," Nature 586, 237–241 (2020). DOI: 10.1038/s41586-020-2717-7
2. Berquist, Z. J., Turaczy, K. K. & Lenert, A., "Plasmon-Enhanced Greenhouse Selectivity for High-Temperature Solar Thermal Energy Conversion," ACS Nano (2020). DOI: 10.1021/acsnano.0c04982
3. Gayle, A. J., Berquist, Z. J., Chen, Y., Hill, A. J., Hoffman, J. Y., Bielinski, A. R., Lenert, A. & Dasgupta, N. P., "Tunable Atomic Layer Deposition into Ultra-High-Aspect-Ratio (>60000:1) Aerogel Monoliths Enabled by Transport Modeling," Chem. Mater. 33, 5572–5583 (2021). DOI: 10.1021/ACS.CHEMMATER.1C00770

Birefringent and dichroic crystals are critical building blocks for devices such as polarizers, filters, and phase matching elements in optical systems. However, in the mid-wave (MWIR) and long-wave (LWIR) infrared spectral ranges, there is still a shortage of optically anisotropic materials. In this work, we show perovskite-derived chalcogenides—A_1+xTiS₃ (A=Sr, Ba)—are a platform for achieving giant optical anisotropy across the MWIR and LWIR and can play a key role in next-generation infrared optics. We grew high-quality A_1+xTiS₃ crystals of various shape and orientation by chemical vapor transport. Electron microscopy studies revealed 1D chains of face-sharing TiS₆ octahedra with a highly anisotropic electronic structure, which infers a large optical anisotropy. We used polarized infrared spectroscopy combined with spectroscopic ellipsometry to extract the optical properties of these materials and discovered refractive-index differences between the ordinary and extraordinary axes of up to two across the MWIR and LWIR, as well as large dichroism. We applied Density Function Theory (DFT) on A_1+xTiS₃ structure and discovered that such optical anisotropy originates from the structural modulation of TiS₆ octahedral. We anticipate A_1+xTiS₃ (A=Sr, Ba) and other quasi-1D materials will be useful for next-generation imaging and sensing applications, especially for miniaturized photonic devices. This study also lays ground for material designs towards highly electronic and optic anisotropic materials.

Individual electrons in conductors are transported via diffusion, yet current-voltage characteristics in a device are governed by the collective flow of current along streamlines. Despite being at the core of our understanding of electronic and optoelectronic charge transport, the intrinsic global pattern of current streamlines - described by the Shockley-Ramo theorem - has never been imaged. Here, we unveil a microscopy method to image the streamlines along which current naturally flows through ultrathin photodetector devices. Combining scanning photovoltage microscopy with a highly uniform, rotating magnetic field, we probe the photoresponse of atomically pristine thermospintronic heterostructures composed of platinum on yttrium iron garnet (YIG). Under focused laser excitation, a local out-of-plane temperature gradient produces a corresponding out-of-plane spin current in the YIG layer by the Spin Seebeck Effect, which, according to the Inverse Spin Hall Effect, scatters electrons in the platinum layer away from an external, saturating magnetic field. Much like tracers in a wind tunnel map the flow of air around an aerodynamic airfoil, we use a scanning laser beam as a source of directional charge current to map the flow around precisely engineered wing shaped devices, or electrofoils. Photocurrent streamlines can be contorted, compressed or expanded by changing the shape and angle of attack of the electrofoil devices. By imaging and manipulating photocurrent streamlines, we demonstrate an intensive imaging technique to visualize and characterize charge flow in quantum materials.

Infrared spectroscopy probes charge, spin and lattice dynamics and therefore is a useful tool to elucidate the physical origin of ordered phases in quantum oxides and interfaces. However, conventional infrared measurements are diffraction limited and do not provide nanoscale resolution. The technique of scattering-type scanning near-field optical microscopy (s-SNOM) based on the near-field interaction between an atomic-force microscope (AFM) tip and the sample, overcomes the diffraction limit, while keeping all the advantages of conventional spectroscopy. We present variable temperature s-SNOM mapping and infrared near-field spectra of surface phonon polaritons in SrTiO3 and coupled phonon-plasmon polariton modes in (nano-)structures realized at LaAlO3/SrTiO3 interfaces [1,2]. We report a long phonon-polariton propagation length exceeding 100 microns, which shows a significant temperature dependence. Our results suggest that strontium titanate and related oxide interfaces are potentially promising for (far-)infrared plasmonic applications.

[1] W. Luo, M. Boselli, J.-M. Poumirol, I. Ardizzone, J. Teyssier, D. van der Marel, S. Gariglio, J.-M. Triscone and A.B. Kuzmenko, Nature Communications 10, 2774 (2019).
[2] M. Boselli, G. Scheerer, M. Filippone, W. Luo, A. Waelchli, A.B. Kuzmenko, S. Gariglio, T. Giamarchi, and J.-M. Triscone, Phys. Rev. B, 103, 075431 (2021).

Transition metal dichalcogenides belong to the family of van der Waals materials and exhibit unique optical and electronic properties, including high tunability, strong nonlinearity, and anisotropy stemming from the van der Waals layers of the material [1]. Most importantly, these materials have a high refractive index and small non-radiative losses in the infrared spectral range. One can operate in the regime when the period of the nanoantenna array is comparable to the wavelength of multipole resonance of a single nanoantenna, which results in optical coupling between nanoantenna and the excitation of lattice resonances [2].

We consider various multipole excitations in such van der Waals and high-index nanostructures, and we design metasurfaces with the properties of directional scattering based on the excited multipoles. We take advantage of both the same-multipole couple and cross-multipole coupling of nanoscatterers in the lattice. We show the nanostructure Mie resonances and coupling between nanoantennas and substrate in the van der Waals nanoantenna array. Such nanoantenna arrays with collective excitation and Mie resonances have a high potential to be utilized in infrared metasurfaces and photonic elements.

The computational efforts are supported by UNM Research Allocations Committee, award RAC2022.

References
[1] V. E. Babicheva and J. Moloney, "Lattice Zenneck modes on subwavelength antennas," Laser & Photonics Reviews 13, 1800267 (2019).
[2] V. E. Babicheva and A. Evlyukhin, “Multipole lattice effects in high refractive index metasurfaces,” Journal of Applied Physics 129, 040902 (2021).

Optoelectronics devices operating at different wavelength ranges require materials with different bandgaps. Although varying chemical compositions or integrating materials with different bandgaps could tailor the operating wavelength range depending on their target applications, increased complexity of their fabrication and necessity for precise control of thickness and/or compositions of materials have made it impractical for realization of actively tunable optoelectronics. Strain is an effective method to modulate the electronic and optical properties of semiconductors by mechanically altering their structure. Reversible and controllable tuning of bandgap in semiconductors can be achieved even within a single device via application of elastic strain. While bulk crystalline materials possess a limited maximum amount of strain, two-dimensional (2D) layered materials, such as graphene and transition metal dichalcogenides (TMDCs), can sustain much larger elastic strain, thus allowing for wide tuning of their optical bandgap using strain.
Black phosphorus (bP) has shown unique response upon strain, including anomalous strain dependence of bandgap, tunable van der Waals interactions, and piezoelectricity. These unprecedented strain-dependent characteristics are attributed to a puckered lattice structure of bP, of which bandgap has extraordinary sensitivity to strain. To fabricate devices where reversible strain application is viable, semiconductor materials are required to be in a form of thin membrane. As opposed to bulk crystalline semiconductors, this is readily achievable in bP due to its thin-membrane nature and by mechanical exfoliation to obtain the desired thickness of the bP membrane. Especially, bP with thickness greater than 6 layers has its bandgap spanning over near-infrared (NIR), short-wavelength infrared (SWIR), and mid-wavelength infrared (MWIR) ranges where there is a rapidly increasing demand for optical communications, thermal imaging, health monitoring, spectroscopy, and gas sensing. Strain-induced bandgap modification in bP has recently been studied by several studies and they have shown potentials of bP as tunable optical materials. However, despite the considerable interest in IR regime for optoelectronic applications, such as light-emitting diodes and photodetectors, a device-compatible approach towards tunable optoelectronics with high performance at room temperature is yet to be demonstrated.
Here, we present bP with strain-tunable bandgap and its applications for room-temperature infrared optoelectronic devices with actively variable operating wavelength ranges. Covering from 2.3 μm to 5.5 μm, we demonstrate wide-range tuning of bP bandgap with a combination of compressive and tensile strain. Repeatable and continuous tuning of electrically-pumped MWIR light emission is achieved by utilizing a strained bP-MoS₂ heterostructure on a flexible substrate. Moreover, the low Auger recombination velocity of bP offers much higher theoretical limit of the quantum efficiency in our tunable bP-LED, compared to other materials with similar bandgaps (e.g. InAs and PbSe). We further exploit strain-tunable bandgap of bP to develop a spectrally variable photodetector with high responsivity that can extend its cutoff wavelength by applied strain. Finally, by leveraging the tunable light emission properties of strained bP, we demonstrate multiplexed non-dispersive infrared (NDIR) gas sensing, where multiple gases (e.g. CO₂, CH₄, and H₂O) are detected using a single light source. Our approach bridges the existing technological gap in optoelectronics at infrared wavelength regimes, where III-V and II-VI materials have limited their widespread utilization due to their high cost and stringent requirement for fabrication process by epitaxial growth. Also, our solution decouples the complexity in the device structure and tuning capability in the operating wavelength range without the necessity for sacrificing its performance.

Mid-infrared (IR) spectral range has significant potential application in thermal imaging, free-space communication, molecule sensing and others. Thus mid-IR sources, detectors and other opto-electronic components have a vast demand in this era. Furthermore, polarized IR light is a valuable addition in IR imaging techniques as it can distinguish various features of an object. However, conventional wire grid mid-IR polarizers are bulky, designed using complex lithography techniques and exhibit form birefringence. Hence, the integration of conventional mid-IR polarizers with chip-scale devices is challenging. Recently, van der Waals (vdW) materials exhibiting natural in-plane hyperbolicity in the mid-IR spectral region, like α-MoO₃, have garnered research attention. These vdW single crystals demonstrate several benefits over conventional devices, e.g., non-lithographic fabrication approach, facile integration with nano-scale platforms and room temperature operation. In this work, we investigate the infrared optical response of α-MoO₃ thin film for the application of a reflecting type polarizer.

We fabricate the α-MoO₃ using the thermal physical evaporation technique and transfer the as grown α-MoO₃ flakes to a silicon substrate using the mechanical exfoliation method. XRD, HRTEM and SEED confirm the orthorhombic nature of MoO₃ flake. Further, polarization-dependent Raman studies reveal the in-plane anisotropic nature of α-MoO₃. Moreover, the optical response of α-MoO₃ thin film in the mid-IR spectral region displays anisotropic reflectance for p- and s- polarized incident light, which can be exploited as a reflecting type mid-IR polarizer. We explain our results as a manifestation of strong in-plane hyperbolic anisotropy of α-MoO₃ single crystals in the mid-IR spectral region. We verify these experimental results using full-wave numerical simulations and using semi-analytical model based on the transfer matrix method. We perform extensive device optimization and the optimized thickness of α-MoO₃ thin film lies between 1500 nm to 2500 nm where it shows an extinction ratio of more than 5 dB over a bandwidth more than (200 cm^-1) with reflectance above 60%. We further experimentally evaluate the temperature resilience of our device and discuss our findings in the context of polarizer performance metrics. This work opens up new avenues for naturally hyperbolic vdW thin-films to be utilized to realize sub-wavelength optical devices with ease of fabrication and chip-scale integration.

To address the critical energy issues, the utilization of solar energy has been one of the main approaches to produce clean energy for many technical, environmental, and ecological advantages. Conventional solar harvesting has been limited to 2D surfaces such as photovoltaic (PV) solar cells. We report solar harvesting via multiple layers of thin films which are spectral-selectively rendered transparent. In this fashion, solar harvesting is transformed from 2D to 3D with much greater energy density. The key requirements are that these films must not only exhibit strong UV and NIR absorptions for harvesting sufficient solar photon energies, but also high average visible transmittance (AVT) for permitting light passing through many layers. We have designed and synthesized photothermal materials from two families of materials: iron oxide (Fe₃O₄@Cu_2-xS) and porphyrin compounds (chlorophyllin), both are optically characterized with significant UV and NIR absorptions and high AVTs. Analogous to multilayer capacitors, we have constructed a photothermal generator (PTG) with parallel multilayers of transparent thin films of Fe₃O₄@Cu_2-xS and chlorophyllin sealed in a cuboid. As simulated solar light passing through these transparent films, they are photonically activated to generate sufficient heat on each layer, raising the maximum temperature to 76.1 °C. The thermal energy generated has increased from 5074.3 J (one layer) to 55, 465.8 J (ten layers) for the Fe₃O₄@Cu_2-xS system, and increased from 3826.3 J (one layer) to 56, 143.9 J (ten layers) for the chlorophyllin system, both have more than tenfold of increase. The solar photothermal conversion efficiencies of the PTG have reached greater than 60% for both systems. Also discussed is the mechanism underlying the structural effects on optical absorption and photothermal heating.

Many naturally occuring materials obey Kirchoff’s Law, the statement that the absorptivity and emissivity of a material are equal for any given wavelength and solid angle. This fundamental property both underlies and limits a variety of applications, from energy harvesting to remote sensing. In the last several years, there has been growing interest in strategies for breaking Kirchoff’s Law, with effort focused on strategies using magneto-optical materials. However, these strategies require use of an external magnet for implementation. Here, we consider an alternate method, which instead uses time-dependent modulation of the material refractive index. This approach would eliminate the need for an external magnet, potentially allowing the violation of Kirchoff’s Law in electronically modulated, semiconductor systems. In this work, we use numerical simulations to design a nonreciprocal infrared thermal emitter with strong Kirchoff violation.

We start with a dielectric grating that supports guided-mode resonances and impose a spatially- and time-varying index modulation. We calculate the conversion of incoming light to sideband frequencies using a coupled finite-element method (FEM) simulation formalism. For the lossless system, as in previous work, we find that modulation gives rise to nonreciprocal reflection effects. In the forward direction, mode conversion due to index modulation significantly reduces the reflection. In the reverse direction, the absence of mode conversion results in much higher reflection. We thus observe a strong direction-dependence to the reflected signal.

We then introduce loss into the system to create a material capable of thermal emission. We observe continued direction-dependence in the reflectivity, along with the emergence of a resonant reflection dip in the reverse direction. We then use a generalized coupled-mode formalism valid for time-modulated systems to relate the absorptivity and emissivity at a given angle and frequency to the scattering matrix of the modulated material. In the weak-modulation limit, we choose the modulation to correspond to the maximum conversion rate of the lossless system. In this case, we predict a large contrast between absorptivity and emissivity at wavelengths in the mid-infrared, using realistic modulation frequencies of 10’s of GHz. In the strong-modulation limit, we further increase the modulation strength to observe directional Rabi splitting. In this case, we observe even higher contrast between emissivity and absorptivity.

Our results thus suggest the feasibility of achieving violations of Kirchoff’s Law without the use of magneto-optics, providing a route to the use of nonreciprocal emissive effects in a variety of novel energy conversion and sensing applications.

Abstract: During the last decade, the opto-electronic properties of direct bandgap and metastable GeSn semiconductors have been intensively investigated from short-wave IR (SWIR: 1.5-3 μm) to mid-wave IR (MWIR: 3-8 μm) wavelengths.[1] By engineering materials properties a plethora of infrared photonic devices have been developed, such as lasers, LEDs, and photodetectors[2,3,4,5] However, GeSn thin films are typically under compressive strain due to the large lattice mismatch with Ge interlayer used in the growth on Si substrate. And plastic relaxation during lattice-mismatched growth results in the formation of defects and dislocations, thereby reducing device performance. By exploiting nanowire (NW) morphology, elastic strain can be relaxed due to the compliant nature of the nanoscale core, thus avoiding the formation of structural defects and resulting in a higher crystalline quality compared to thin film materials, without compromising optoelectronic properties.[6]
With this perspective, we discuss in this work discuss the opto-electronic devices made of Ge/GeSn core/shell NWs and operating at room-temperature across SWIR-MWIR wavelengths. NWs with a tunable Sn content ranging from 8 at.% to >20 at.% were grown in a CVD reactor using either a Si or Ge (111) substrate and their structural properties will be discussed down to the atomic level by combining Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT). Single Ge/Ge_0.92Sn_0.08 NW SWIR photodetectors that were fabricated on a SiO₂/Si substrate exhibit excellent light detection performance at room temperature, with a responsivity of 2.7 A/W under 1550 nm illumination at 1 V and an extended cutoff wavelength of 2.1 μm compared to the 1.6 μm typical to Ge. Moreover, the single NW photodetector shows a degree of polarization with the polarization ratio up to 32%. The back-gated single NW FET geometry allows to estimate the mobility in the Ge/Ge_0.92Sn_0.08 core/shell NW at 50 , which is two times higher than that in Ge NWs. The tunability of the cutoff wavelength of the NW photodetector from 2.1 μm to higher wavelengths up to 6 μm will be demonstrated using NWs at Sn content of 20 at.% and higher .and their performance will be discussed by combining temperature-dependent responsivity measurements The broadband tunability of GeSn NWs across the infrared range has a tremendous potential for the development of on chip data communication and sensing devices using scalable, Si-compatible fabrication processes.

[1] O. Moutanabbir, et al. Appl. Phys. Lett. 118, 110502 (2021)
[2] S. Wirths, et al. Nat. Photonics 9.2, 88-92 (2015)
[3] S. Assali, et al. Appl. Phys. Lett. 112.25, 251903 (2018)
[4] M.R.M. Atalla, et al. Adv. Funct. Mater. 31.3, 2006329 (2021)
[5] H. Tran, et al. J. Appl. Phys. 124, 013101 (2018)
[6] S. Assali, et al., Nano Lett 17, 1538−1544 (2017)

Germanium-tin alloys may provide a path toward efficient mid-infrared (IR) wavelength optoelectronics in a materials system that is all group IV, and therefore chemically compatible with silicon-based complementary metal oxide semiconductor (CMOS) technologies. They, therefore, hold promise as a potential IR source for on-chip optical interconnects, and for monolithic integration of IR detectors with silicon circuitry. A direct bandgap can be achieved in Ge by incorporating a metastably high Sn composition of about 8–10 at%. However, the optoelectronic properties of these materials have been limited to date, which is thought to be caused largely by a high concentration of grown-in vacancies and vacancy-containing defect clusters. Isolated vacancies act as acceptor-type defects and are present due to the low-temperature at which these alloys are grown¹. Moreover, complexing of Sn atoms and vacancies is theoretically predicted and can create trap levels near mid gap², thus promoting non-radiative carrier recombination. In order to measure the presence and behavior of these complexes in largely strain-free³ single crystal Ge-Sn shells grown by reduced pressure chemical vapor deposition around Ge nanowires, we have used extended x-ray absorption fine structure spectroscopy (EXAFS). EXAFS is one of the few techniques able to probe the detailed local structure of point defect complexes. We report measurements sensitive to the presence and concentration of Sn-vacancy complexes in Ge/Ge-Sn core/shell nanowires, to provide clearer understanding of how these defects affect the material’s optoelectronic properties.


1. Nakatsuka et al. Mobility behavior of Ge_1-xSn_x layers grown on silicon-on-insulator substrates. Jpn. J. Appl. Phys. 49, (2010).
2. Tahini et al. Diffusion of tin in germanium: A GGA+U approach. Appl. Phys. Lett. 99, 1–4 (2011).
3. Meng et al. Coupling of coherent misfit strain and composition distributions in core–shell Ge/Ge_1-xSn_x nanowire light emitters. Mater. Today Nano 5, 100026 (2019).

In the past 30 years, material scientists have largely capitalized on the grand appeal of utilizing quantum confinement to obtain size-tunable inter-band optical transitions and implement colloidal quantum dots (CQDs) in optoelectronic applications throughout the electromagnetic spectrum. The infrared region is particularly exciting with applications in telecommunications, night-time surveillance, and satellite imaging for agricultural water conservation. While most progress with CQDs in the infrared (IR) has been achieved using inter-band transitions in Pb- and Hg-based heavy metal compounds, intra-band optical transitions originating from external- or self- dopants can potentially expand the library of materials to generate IR-optoelectronic devices with non-toxic materials. In this talk, I will focus on my group’s work on recently discovered silver chalcogenide (Ag₂Se) quantum dots that exhibit distinct optical absorption in the mid wave-IR wavelength spectrum. These CQDs demonstrate a narrow bandgap metastable tetragonal phase, not available in bulk, and contain excess electrons in the lowest level of the conduction band. This allows for intra-band optical transitions between the first and the second conduction energy level which can significantly decrease Auger recombination rates and avoid the need for cryogenic cooling. I will present a detailed study of the size-dependent inter-band to intra-band optical transition and compare the competing effects of quantum confinement, environmental Fermi level and particle stoichiometry to provide guidelines for stable electron occupation of the 1S_e state and obtaining tunable mid wave IR absorption.

Semiconductor metasurfaces have become attractive for applications in nonlinear optics, control of light emission and switching. In particular, metasurfaces made from direct bandgap semiconductors with low carrier lifetime combined with design concepts that include topology and symmetry can lead to new ways to emit and detect far-infrared radiation.

Ultrafast photodetection with semiconductor devices typically requires structures with fast photoresponse with minimal volume of the photodetection active area. In particular, photoconductive (PC) detectors for far-infrared (THz) pulses rely on devices made from semiconductors exhibiting sub-picosecond lifetime, high absorption of a short gating pulse and high on-off photoconductive contrast. These simultaneous requirements have been extremely difficult to meet using standard and uniform photoconducting semiconductors. However, perfect absorption in semiconductor metasurfaces has created a new path to a new family of photoconductive detectors with unmatched performance. Perfect absorption in dielectric metasurfaces require overlapping two resonances of opposite symmetry in the meta-atoms, critically coupled to the intrinsic absorption of the semiconductor. Moreover, in order to collect the photocurrent, the metasurfaces should provide a continuous current path to the collecting electrodes. Using these concepts, we show a new family of THz photoconductive detectors that can operate with ultralow gating powers (tens of uW) and exceed most other performance parameters of conventional PC detectors. These highly efficient PC detectors are applicable to a wide range of THz spectroscopy and imaging systems, particularly those where low power operation is required such as detector arrays and cryogenic environments.

Finally, I will also present our latest results on THz emission from these ultrafast semiconductor metasurfaces.

Monoclinic vanadium dioxide VO₂ (M) is a thermochromic material based on its reversible metal-insulator transition (MIT). The MIT results from the underlying structural phase transition (SPT) from monoclinic VO₂ (M) below to rutile VO₂ (R) above the transition temperature (T₀) and vice versa. The SPT and associated MIT are of great interest for the implementation in energy efficient glazing as VO₂ (M) exhibits a high transmission for solar infrared (IR) radiation below T₀ while above T₀ IR light is blocked. This can lead to significant reduction in energy consumption for heating and cooling of buildings in intermediate climates.
In order to apply VO₂ (M) in thermochromic energy efficient glazing, dopants are required to lower T₀, from 68°C to 20-30°C. Another challenge is the preparation of high quality doped VO₂ nanoparticles for use in coatings and polymer films. To obtain such nanoparticles, we prepared W-doped VO₂ powders and subsequently subjected them to bead milling. We investigated the switching kinetics of undoped and W-doped VO₂ (M) micron-sized powders [1] and nanoparticles. These studies were carried out using data obtained from differential scanning calorimetry, and isoconversional methods were applied for the kinetic analysis. Undoped and W-doped VO₂ (M) powders were prepared via solution-phase reduction of V₂O₅ with oxalic acid followed by solvent removal and thermal annealing. Nanoparticles were obtained through wet bead milling of the powders dispersed in 2-propanol.
The results show that the activation energy of the SPT is temperature dependent and decreases when the difference between the material temperature and T₀ increases. Additionally, the activation barrier for both VO₂ (M) to VO₂ (R) and vice versa is similar for undoped and W-doped VO₂ (M) powders, and ranges between 138-563 kJ mol^-1. This indicates that similar defects are involved in the SPT of VO₂ (M) to VO₂ (R) and vice versa [1]. Wet bead milling effectively decreased the VO₂ particle size from 10-50 µm in powders to 50-500 nm for nanoparticles dispersed in 2-propanol. This leads to a decrease in switching enthalpy from 47 to 28 J g^-1, amongst others caused by partial loss of crystallinity during bead milling. The activation energy decreases with decreasing particle size. Simultaneously an asymmetry in the switching kinetics occurs and increases with progressive milling. This suggests that the defects introduced into the crystal structure by bead milling affect the transitions VO₂ (M) to VO₂ (R) and vice versa differently.
In conclusion, W-doping of the VO₂ was successful in tailoring the transition temperature and from the kinetic studies we find that the switching temperature selected should be 2-3°C below the desired in order to facilitate a rapid SPT and MIT. Additionally, wet bead milling was successfully applied to achieve VO₂ nanoparticles. Milling induces asymmetry in the switching kinetics. The produced particles can be further used for the implementation in coatings and films for studying the relationship between the particle size and switching enthalpy and the optical properties of the resulting coatings and films for smart window applications.

References
1. Calvi L, Leufkens L, Yeung CPK, et al. A comparative study on the switching kinetics of W/VO₂ powders and VO₂ coatings and their implications for thermochromic glazing. Sol Energy Mater Sol Cells. 2021;224(2021):110977. doi:10.1016/j.solmat.2021.110977

In nature, the infrared emissivity of most materials and surfaces are quite large (>0.8), such as woods, glasses, stones, human body surfaces, many plastics, and so on. Apart from polished metals, low-emissivity materials and surfaces (<0.2) are relatively scarce, especially among those dark color materials with high visible absorption. However, those materials and surfaces with low infrared emissivity but high visible absorption are highly demanded for many specific applications, such as visible and IR camouflage, solar thermal harvesting, and thermal management. For instance, even with higher surface temperatures, the low emissivity endows those materials/surfaces with the counter-intuitive ability to conceal themselves from IR detection due to the limited thermal radiation, showing great potential in IR camouflage. Moreover, owing to their dark color, they can easily blend themselves with the environment at night or under weak light. In other words, these visible black but infrared white materials are attractive in multispectral (visible-IR) camouflage. In additional, in solar thermal areas, such low-emissivity black materials can achieve high solar-thermal conversion efficiency due to the suppression of radiative heat losses. However, the lack of such ideal intrinsic materials has been a long-standing challenge for decades. For this reason, great efforts have been made in developing subwavelength photonic crystals and metamaterials/metasurfaces using complicated and costly nanofabrication methods. It is preferable if there was a flexible intrinsic material with strong, broadband solar absorption but low mid-IR emissivity.
In this study, for the first time, we discovered that the black 2D Ti₃C₂T_x MXene film shows strong reflection up to 90% for the mid-IR wavelengths, resulting in a quite low emissivity of ~0.1. Associated with its high solar absorption (90%), it achieves a record-high spectral selectivity (8.2) among reported black materials to the best of our knowledge. Excellent intrinsic spectral selectivity is offered by the Ti₃C₂T_x MXene in a free-standing form as well as coatings on versatile substrates, even on porous, rough surfaces, showing much wider applications than metamaterials. The great potential of 2D Ti₃C₂T_x in visible-IR camouflage was demonstrated. Further studies also reveal that some other commercially available MXenes are also potential IR white materials.

The utilization of man-made, electricity-powered light sources as external stimuli for anti-counterfeiting and other applications is limited by the cost, energy consumption, and lack of human-compatibility. In this work we explore the use of human hand as a natural and powerless infrared (IR) light source and demonstrated the integration of human hand into the anti-counterfeiting systems to enhance the human-compatibility of the system. This hand-based IR anti-counterfeiting approach enables both multilevel information encryption and unclonable coding. We also demonstrate the simultaneous decoding of both the visible code and the IR code overlaid on a flexible label without interference, which makes it promising for high level dual- or multiple-mode anti-counterfeiting. A smartphone-based IR camera is also integrated into this anti-counterfeiting system, which offers the possibility of instantly uploading the coding data to the database to further enhance the effectiveness of the anti-counterfeiting process. Compared to traditional passive light sources and other external stimuli, using human hand as active stimuli enhances the security level of the system, as well as the portability and accessibility of the anti-counterfeiting system, which will help further open up the possibility of integrating not only human hand, but also potentially other human components into the functional systems and processes.

During the decades, the infrared survivability of aircraft has been an important issue for ally’s air force enhancement in the modern battlefield. In the development of IR sensors, the signature from airframes in the long-wave infrared (LWIR, 8-12 μm) band could be used for target aircraft tracking. Infrared seekers detect aircraft using the contrast radiant intensity between target aircraft and background. For this reason, we thought that we could reduce contrast signature intensity for infrared camouflage. When the background is the sky, because the infrared signature of the sky is low, many studies have been conducted to lower the surface emissivity of the target in the detection wave band. However, when the background is ground, soil, or forest such as look-down situation, we confirmed that there is an optimal emissivity to minimize the contrast signature. Because the polyurethane (PU) based paint is applied to aircraft, in general, through spray coating, we developed PU/Al composite coating that can be applied to aircraft with the same coating method. Polyurethane (PU)/Al composite coating with low infrared emissivity was prepared via spray coating method using PU paint, Al-particles, and ethanol as adhesive, functional pigment, and agent for spraying, respectively. The effect of floating position and amount of Al-particles on infrared emissivity has been discussed. In order to uniformly spray Al-particles, we mixed particles with ethanol at a weight percent of 0.5% and sprayed them using an air-spray gun. The vertical position of Al-particles in a thin PU paint layer was controlled by varying the drying time of PU paint, which takes 48 hours to completely dry. Also, the amount of Al-particles on the coating surface was controlled by varying the number of spraying. To analyze optical properties, we measured the optical emittance and reflectance using FT-IR, and also the apparent temperature using IR camera in infrared band. The results show that the reflectivity increases, therefore, the surface emissivity decreases in the LWIR band, as the vertical floating position is closer to the top surface. Also, the surface emissivity decreases when there are more particles in the coating, and especially, this effect is greater when the floating position of the particle is on the top surface.

Optical confusion refers to a creature’s camouflage technique by which it assimilates itself with the surroundings through manipulation of its colors and patterns. Inspired by these coloration and patterning strategies, artificial tunable materials to control their visible colors or infrared signatures have researched in display, energy, and military application fields. With the development of multiband imagery sensors, optical confusing camouflage must not only be able to control the signature intensity but also be able to mimic patterns of surroundings. Moreover, it must have multiband characteristics. Conventional visible camouflage mimics the colors and patterns of the surroundings and, therefore, creates pixelated digital camouflage patterns. However, conventional infrared camouflage has been focused on reducing signature of target by assuming averaged background signature. Therefore, optical confusion by patterning the colors and signatures in the visible and infrared ranges simultaneously is the most appropriate approach to developing advanced multispectral camouflage.
Herein, we propose a tunable multiband coloration metamaterial (TMCM) devised by arranging pixelated silicon nanowire fractal structures (SiNW-FSs). In this study, we fabricated SiNW-FSs by using one of the wet etching methods, which is metal-assisted chemical etching (MaCE). We calculated the fractal properties of the SiNW-FSs to verify their fractality. As a result, we found that the SiNW-FSs exhibited constant fractal dimension values and a 39% increase in lacunarity, which quantified their morphological heterogeneity with etching time increase. Moreover, we measured their multispectral optical properties, namely, reflectivity and emissivity in the visible and infrared ranges, respectively. we concluded that we could tune the reflectivity and emissivity in each waveband. Finally, through imagery measurements, we confirmed the superior multiband optical confusing effect of the TMCM against a modeled background, which imitates the real environment, compared with that of conventional single-spectral camouflage surfaces. The TMCM is expected to help improve the understanding of the multispectral characteristics of randomly distributed nanostructures and aid the development of state-of-the-art stealth technology.

Mid-infrared light sources are key components for future chemical sensing, on-chip optical interconnect and LIDAR technologies. GeSn alloys can be synthesized under silicon CMOS compatible conditions. Therefore, GeSn can open pathways for miniaturization of mid-infrared devices such as on-chip molecular sensors, optical interconnects and thermal cloaking devices. Most importantly, direct band gap GeSn is a promising material for on chip integration of light emitters. Previous research on GeSn films has shown mid-infrared lasing at increasing temperatures, initially (in 2015) at less than 90K and, in more recent reports, at or near room temperature. It has also been shown that GeSn nanowires with high-Sn content exhibit strong direct-gap photoluminescence at room temperature. Compared to GeSn thin films, GeSn nanowires are less constrained to lattice match with misfitting (e.g. Ge, Si) substrates and therefore may achieve higher Sn contents without strain and defects that promote non-radiative recombination. Additionally, unlike in a planar GeSn film where the emission angle is limited to < 13⁰ by its high refractive index (n>4), GeSn nanowires provide a platform to design the optical density of states for highly efficient light extraction directly from the light emitting material. GeSn nanowires may also constitute a superior medium for absorption and generation of photocarriers.
We demonstrate room temperature mid-infrared photodetection using resonantly absorbing GeSn/Ge core-shell nanowire photonic crystals. We have synthesized GeSn/Ge core-shell single crystal nanowires with up to 6% Sn that exhibit strong direct-bandgap photoluminescence at room temperature, with a fixed periodic, varying radius and Sn compositions. To synthesize the nanowire device, gold catalysts were patterned on Ge [111] substrate and then two step VLS growth was performed to synthesize Ge core and GeSn shell. The vertical nanowire photodetection device consists of arrays of nanowires in-filled with PMMA insulating layer, an ITO top contact (electron selective) and an aluminum bottom contact (hold selective). We performed selective area FTIR reflectance characterization, which indicates the reflectance maximum changes predictably as the radius and composition of the photonic crystal arrays varies. Reflectance data are in agreement with FDTD full wave simulation of resonant wavelengths and band structures of photonic crystal arrays. We performed photocurrent characterization with an FTIR spectrometer at room temperature. The photocurrent spectrum of Ge/GeSn nanowire photonic crystals have absorption edges up to 2.4μm, a many-fold enhancement due to resonant absorption, and a photocurrent spectrum that is tunable by varying the photonic crystal geometry.

Vanadium dioxide (VO₂) shows a metal-insulator phase transition around 70 °C (T_c), and is drawing attention due to the anomalous physical properties. The optical property in infrared (IR) light region is drastically changed with the transition due to increased free carrier by 4 digits. The VO₂ at metal state (over T_c) has high reflectivity in IR region, and low reflectivity at insulating state (below T_c). Also, it is well known that the transition temperature is modified by the additional elemental doping such as W and Mg.
VO₂ is a promising material to realize future devices such as “smart glass” and “thermal memory” due to temperature-switchable IR properties. Typically, VO₂ shows a temperature hysteresis during heating and cooling processes. The “smart glass” requires narrow temperature-hysteresis-width. In contrast, the “thermal memory”, which can record and read information with temperature states, requires broad temperature-hysteresis-width to store thermal information. Thus, it is essential for realizing such future devices to control the hysteresis width in optical properties of VO₂. In this study, we have fabricated nanostructured VO₂ with different morphologies and measured the temperature width of the hysteresis by IR reflectivity measurements. We found that the hysteresis width is highly dependent on the VO₂ nanostructures [1].
We fabricated VO₂ nanostructures by sputtering deposition and subsequent lamp annealing. For the sputtering deposition, we used a V metal disc as a sputtering target, and deposited V thin films on Si substrates at 350 °C. For the lamp annealing, the samples were annealed at 600 °C by changing the annealing times (1-90 min) and O₂ pressures (2-15 Pa). We observed the morphology of VO₂ by SEM, and recorded IR reflectivity changing the sample temperature in both heating and cooling processes. We found that the fabricated samples were formed by VO₂ nanoparticles on the Si substrates, and that the aggregation of VO₂ nanoparticles was changed by controlling the annealing times and the O₂ pressure during annealing. The isolation of VO₂ nanoparticles progressed with annealing time, and the long-time annealing (90 min) induced highly isolated VO₂ nanoparticles. From the temperature-dependent IR reflectivity measurements, we estimated the hysteresis width (△T) at the wavelength of 2.5 μm. For the highly aggregated VO₂ nanoparticles, which were fabricated by short-time annealing (1 min) and low O₂ pressure (2 Pa), the values of △T were small. The value of △T increased with the decrease of aggregation of VO₂ nanoparticles. We have fabricated VO₂ nanostructures with the different △T of 17 °C and 61 °C by changing the annealing conditions. Also, in cooling process, two-step decreases in reflectivity were observed for the slightly aggregated VO₂ nanoparticles. Such two-step decreases in IR reflectivity were not observed for highly aggregated or highly isolated VO₂ nanoparticles.

[1] K. Nishikawa, M. Yoshimura, and Y. Watanabe “Growth of nanostructured VO₂ via controlling oxidation of V thin films: Morphology and phase transition properties” Journal of Applied Physics 129, 185303 (2021).

Photothermal heating method has been emerged as a contactless shape manipulating system in materials community. Although numerous methods are documented to achieve 2D to 3D construction, photothermal method has an advantage of using inexpensive near infrared (NIR) light and uniform stimulus with contactless system. The common photothermal method uses black ink patterned hinges on shape memory polymers (SMP) with local light absorption. However, remaining hinges could induce further undesirable actuation after 3D construction. In this study, we employed adhesive tape patterns instead of black ink patterns to achieve transparent 3D structures. The black adhesive tape patterns were irradiated with NIR light to induce local light absorption. When the temperature of the black adhesive patterned area exceeds the polymer’s glass transition temperature, biaxially-stretched polymeric chains recover its conformation to random coils, resulting in contraction. We present the 3D complex structures from radial and chiral 2D tape patterns to control the curvature. We also demonstrate sequential folding by distinct light absorption time. Furthermore, we will discuss shape-morphing with multi-material including papers and conductive paste to reconfigure the 3D structures and conductivity.

Transmission infrared (IR) spectroscopy is one of the best methods for quantitative detection of chemical species with high accuracy. However, the evaluation of chemical reactions in aqueous solutions is usually difficult due to the strong IR absorption of water itself. We avoided this difficulty by fabricating an optical cell with an optical path length of about a few micrometers: the thickness of water through which IR rays can penetrate^1,2. To evaluate the absorbance: A using a single optical cell, we measure the transmitted light I₀ and I_s for the empty cell and the cell filled with a specimen, respectively, and calculate A = -log(I_s/I₀). However, since the measurement times of I₀ and I_s are different, stable quantitative measurement is difficult due to instrument drift (e.g., temperature change of the interferometer or detector) or accidental fluctuations. In this study, we fabricated a microfluidic cell with two adjacent channels and developed a transmission IR spectroscopy system that can simultaneously measure I₀ and I_s using a linear array detector.
A two-channel microcell was fabricated by photolithography technique on a CaF₂ substrate and sealed with a CaF₂ window plate. This was mounted on a transmission IR spectroscope (IRT7000, Jasco Inc.) equipped with a linear array (16 parallel detectors). The linear array detectors were cooled by a 5L liquid nitrogen tank, allowing continuous measurements for about a week. One channel was filled with dry nitrogen gas, while the other channel was filled with ethanol solution (20 mass%) for simultaneous measurement of I₀ and I_s. Simultaneous measurements were made 512 times during 100 seconds, and data was transferred in 20 seconds, which was repeated as one cycle.
When the measurements were made at the same time: t, the time variation, or drift, of I₀(t) and I_s(t) was fluctuated synchronously. In order to evaluate the stability of this IR measurement system, we calculated the time variation of the ratio of Int(I_s(t))/Int(I₀(t)). Here, Int(I₀(t)) and Int(I_s(t)) were calculated by integrating the respective spectra of I₀(t) and I_s(t) in the range of 2000 to 2250 cm^-1 after sensitivity correction. For example, in the case of simultaneous measurements, Int(I_s(t))/Int(I₀(t))=0.98733±0.00012. In a normal measurement, i.e. using one cell, it takes about one hour to change the cell contents. Therefore, the time difference of Int(I_s(t+τ))/Int(I₀(t)) was evaluated as τ=1 hour. The error in this case was ±0.00150. In other words, the stability of the measurement was improved by a factor of 12.5.
The system was found to be capable of stable quantitative absorbance measurements over a long period of time, even in the presence of external disturbances. As a result, this system can be applied to the measurement of slow chemical reactions in aqueous solution, such as photosynthesis, photocatalysis, and cellular metabolism.

References
1. T.Ube, T.Harumoto, and T.Ishiguro, In situ transmission infrared spectroscopy of hydroxylation of aluminum thin film and subsequent dehydration processes, J. Appl. Phys., 53(2014), 066701-1 - 066701-4,
2. T.Ube, Y. Yoneyama, and T.Ishiguro, In situ Measurement of the pH-dependent Transmission Infrared Spectra of Aqueous Lactic Acid Solutions, Anal. Sci., 33(2017)1395-1400

Incandescent sources such as hot membranes and globars are widely used for mid infrared spectroscopic applications. The emission properties of these sources can be tailored by means of resonant metasurfaces: control of the spectrum, polarization and directivity has been reported. For detection or communication applications, fast temperature modulation is desirable but is still a challenge due to thermal inertia. Reducing thermal inertia can be achieved using nanoscale structures at the expense of a low absorption and emission cross section. Here, we introduce a metasurface that combines nanoscale heaters to ensure fast thermal response and nanophotonic resonances to provide large monochromatic and polarized emissivity. The metasurface is based on platinum and silicon nitride and can sustain high temperatures. We report a peak emissivity of 0.8 and an operation up to 20 MHz, six orders of magnitude faster than commercially available hot membranes.

Understanding radiative heat transfer in nanoscale gaps and devices is of considerable interest for creating novel energy conversion devices. In this talk, I will first describe ongoing efforts in our group to experimentally elucidate nanoscale radiative heat transfer. Specifically, I will present our recent experimental work where we have addressed the following questions: Can existing theories accurately describe radiative heat transfer in single nanometer sized gaps? Can radiative thermal conductances that are orders of magnitude larger than those between blackbodies be achieved? In order to address these questions we have developed a variety of instrumentation including novel nanopositioning platforms and microdevices, which will also be described. Further, I will discuss possible applications of near-field thermal radiation for energy conversion and photonic cooling. Finally, I will briefly outline how these technical advances can be leveraged for future investigations of nanoscale heat transport and near-field thermophotovoltaic energy conversion.

Understanding near-field radiative heat transfer (NFRHT) underpins the management of thermal energy for the development of novel heat-control technologies. Evanescent near-fields couple across nanoscale gaps, making these platforms a classic case study for NFRHT phenomena. Most studies on NFRHT in gaps provide limited spectral information, with the exception of scanning probe microscopies [1], in which the spatial resolution is limited to tens of nm. Spectral information can also be obtained using far-field IR probes, in which case much of the spatial sensitivity is lost and optically dark modes are unreachable. Challenges remain in understanding the spectral and spatial properties of NFRHT channels provided by bright and dark surface phonon polaritons (SPhP) in dielectric-based nanogaps. Detecting all of the available SPhP modes present in a gap and their spatial distribution; understanding the role of gap size and shape in tuning the spectral response; and nanoscale imaging of the local temperature profile across the gap is needed to further our understanding of NFRHT limits and nanoscale thermal emission.
We study the near-field spectral response of SiC nanogaps as small as 5 nm, with different temperature gradients across the gap. We use an atom-sized electron beam in a monochromated transmission electron microscope to conduct electron energy loss spectroscopy (EELS) measurements of the IR response [2]. We image the spatial distribution of both bright and dark SPhP modes, providing rich spectral information with nm spatial resolution [3]. For temperature measurements, we use a recently-developed method to locally measure thermal profiles across the gap using phonon EELS signal [4].
We fabricate SiC nanogaps between two rods using a focused ion beam, producing gaps ranging from 40-280 nm. Under heating, the gaps may shrink, down to 5 nm. Our EELS data reveals a broad spectral peak: the amalgamation of a rich spectrum of SPhP modes. We notice that the shape, intensity, and maximum of the EELS peak depends on the gap size and shape, and on the probe location in the gap. For example, the high energy modes within the peak are stronger in the centre of the gap than near the open sides, indicating strong high energy near-fields at the gap centre due to the formation of highly localized modes in these narrow gaps (<50 nm). Our results suggest that spectral tunability can be obtained even with small gaps, potentially affecting spectral heat flux and thermal emission. To gain further insight into the variety of SPhP in the gap, we conduct boundary element simulations of coupled nanorods with varying cross-sectional geometries. We find that a large variety of coupled modes exist, including new face-type modes with extremely localized near-fields, dependent on the cross-sectional shape. Our results provide relevant information on channels for NFRHT beyond the traditional coupled dipolar modes.
Our fabricated gaps are designed to exhibit different temperature profiles; some gaps have the same temperature on either side while other gaps have hot and cold sides, according to our local temperature measurements. In thermally symmetric gaps, the near-field intensity drops towards the centre, but is symmetrically distributed on either side and increases under heating. In thermally asymmetrical gaps, the near-fields near the hotter rod are stronger, due to an increase in thermal phonon population [4]. Our work studying the spatial distribution and temperature gradients present in coupled SPhP represents progress towards understanding the available channels for NFRHT and thermal emission in nanogaps.

Acknowledgements
MJL and NDB acknowledge NSERC of Canada for financial support. We thank P. Batson for access to the Rutgers University Nion microscope.

References
[1] Cuevas & García-Vidal, ACS Photonics, 5, 10, 2018.
[2] Krivanek et al., Nature, 514, 7521, 2014.
[3] Lagos et al., Nature, 543, 7646, 2017.
[4] Lagos & Batson, Nano Lett., 18, 7, 2018.

The Solid State Incandescent Light Emitting Device (SSI-LED) was reported recently (1,2,3). It emits the warm white light from visible to near IR wavelength range, which is similar to that emitted from a conventional incandescent lamp or that of the sunlight in the same wavelength range. The principle of the light emission of this kind device is the thermal excitation of nano-resistors formed from the breakdown of a MOS capacitor with a metal oxide high-k gate dielectric. It can be operated continuously for more than 20,000 hours in air without failure. The electrical, optical, and material properties of the SSI-LED have been investigated. In general, the intensity and spectrum of the emitted light is related to the material and structure of the nano-resistor, i.e., the original gate dielectric properties, and the applied voltage. In this paper, authors investigated the relationship between the (light emission area/whole gate area) ratio and the operation condition as well as th gate dielectric material using the J Mapping method. The cause of the high light emission density on the edge of the device will also be discussed.
1. Y. Kuo and C.-C. Lin, Appl. Phys. Letts., 102(3), 031117 (2013).
2. Y. Kuo, IEEE Trans. Elec. Dev., 62(11), 3536-3540 (2015).
3. A. Shukla and Y. Kuo, ECS J. Solid State Sci. Technol., 9, 065017 (2020).

The near-field effects of radiative heat transfer arise when at least one of the characteristic lengths of the system, i.e., the size of the objects and their separation distance, is smaller than the dominant thermal wavelength (approximately 10 μm at room temperature). Radiative heat transfer in the near field can be quasi-monochromatic, polarized, coherent, and it can exceed the far-field blackbody limit by several orders of magnitude. These unique properties are highly promising for many advanced and state-of-the-art technologies in waste heat recovery, nanoscale-resolution imaging, photonic cooling, thermal management of electronic devices, and thermal rectification through a vacuum gap. The quasi-monochromatic nature of near-field radiative heat transfer plays a central role in these technologies. For example, quasi-monochromatic thermal emission has been reported to improve the performance of nano-gap thermophotovoltaic devices which are proposed for waste heat recovery. Another example is a thermal diode, which allow heat to flow only in a preferred direction. In this case, thermal rectification is achieved by capitalizing on temperature dependence of the resonance frequency of thermal emission. Development of the near-field applications requires materials with designer near-field thermal spectra. Materials with designer near-field spectra are rarely found in nature. As such, metamaterials that are engineered at the sub-wavelength scale have been proposed for controlling the spectral distribution of near-field thermal radiation.
Achieving customized near-field spectra using metamaterials requires reliable and computationally efficient theoretical models for designing the material as well as robust experimental techniques for characterization of the designed materials. This talk starts with an overview of near-field thermal radiation, its significance and applications, and it continues with a discussion of various metamaterials, including nanopillars, gratings, and Mie-resonance based metamaterials, proposed for tuning the spectrum of near-field thermal radiation. Next, the numerical and approximate models used for designing metamaterials is reviewed, and an experimental technique for characterizing the spectrum of near-field thermal radiation is presented. Finally, the measured near-field and far-field thermal radiation spectra for metamaterials made of silicon carbide nanopillars are presented and discussed.

Metasurfaces have recently emerged as a breakthrough platform for manipulating light at the nanoscale with the use of engineered dielectric and plasmonic nanostructures. They can be tailored to operate over a broad spectrum ranging from terahertz to middle-infrared (Mid-IR) and ultraviolet. Mid-IR metasurfaces are particularly important for the wavelengths between 3 to 20 μm because this range includes two atmospheric windows and fundamental absorption bands of chemicals and biomolecules. Infrared absorption spectroscopy taps into these absorption bands, so called “fingerprints”, for label-free and chemical-specific biosensing. Nevertheless, conventional infrared absorption suffers from several limitations such as low signal levels, difficulty to operate in aqueous environment and bulky and expensive instrumentation. In this talk I will present our recent work with plasmonic and dielectric Mid-IR metasurfaces for realizing novel Mid-IR biosensors through surface enhanced infrared absorption spectroscopy (SEIRA) and incorporation of dielectric metasurfaces with phase change materials for programmable phase control [1-4]. I will also present a versatile nanofabrication process enabling wafer-scale and CMOS-compatible manufacturing of efficient dielectric and plasmonic mid-infrared metasurfaces [5].

With metallic nanoantennas our lab has been developing ultrasensitive SEIRA based biosensors. Most recently, we introduced a deep learning-augmented Mid-IR nanoplasmonic metasurface, which provides a universal platform to study dynamic biological processes involving analytes from all four major biomolecular classes [1]. The optofluidic device incorporating multiresonant plasmonic metasurface and microfluidics enabled to collect a vast amount of spectrotemporal data, which is then used to construct a deep neural network for accurate analyte discrimination. The capabilities of the new method are demonstrated by dynamically monitoring a multistep bioassay containing sucrose- and nucleotides-loaded liposomes interacting with a small, lipid membrane-perforating peptide.

In low-loss dielectric metasurfaces, we harness their high-Q resonances to implement compact Mid-IR biosensors. One of our devices utilizes large-area imaging with a 2D pixelated metasurface to detect Mid-IR molecular fingerprints without the need for spectrometry and frequency scanning [2]. The metasurface is designed to provide strong near-field enhancement, where the resonance positions of individual metapixels are varied over a target Mid-IR range. With this one-to-one mapping between spectral and spatial information, enhanced absorption signatures of analytes are sampled at multiple spectral points and translated into a 2D barcode-like spatial absorption map for imaging-based detection. In another device, we expanded the operational spectral range of dielectric metasurfaces for spectroscopy by using angle scanning and polarization aspect together. The system provided an impressively wide spectral coverage from 1100 cm-1 to 1800 cm-1 at 1.4 cm-1 resolution [3].

Current metasurfaces are often locked to a fixed functionality due to their static constituent materials. Extending into tunable metasurfaces can enable active optical elements such as tunable lenses, dynamic holograms and spatial light modulators for key application areas including adaptive spectroscopy. Recently, we introduced a tunable all-dielectric Mid-IR Huygens’ metasurface consisting of multi-layer Ge meta-units with strategically incorporated phase change material [4]. Through hyperspectral imaging measurements we showed optical programming of a light phase profile into the metasurface with single meta-unit precision and a maximum light phase tunability of 81% of full 2π phase shift.
References:
[1] John-Herpin et al. Advanced Materials (2021)
[2] Tittl et al. Science (2018)
[3] Leitis et al. Science Advances (2019)
[4] Leitis et al. Advanced Functional Materials (2020)
[5] Leitis et al. Under Review (2021)

By engineering light absorption in dielectric nanoantennas, we explore new opportunites for the manipulation of temperatures at the nanoscale as well as the design of reconfigurable metasurfaces.
In the last decade, optical nanoantennas have revolutionized light manipulation and control at the nanoscale. Light absorption was initially considered a purely detrimental process, reducing the efficiency of optoelectronic devices. Recently, however, it has attracted growing interest, enabling novel light-energy conversion pathways and offering intriguing opportunities for reconfigurable systems. In particular, going beyond metallic nanostructures, I will introduce the opportunities offered by dielectric nanoantennas for thermonanophotonics and, more specifically, for thermo-optical reconfigurable devices.
In the first part of the talk, I will discuss self-induced optical heating effects in highly absorbing Silicon (Si) and Germanium (Ge) nanoresonators. In particular, I will show recent calculations demonstrating that, due to thermo-optical effects, self-heating can give rise to a complex, non-linear relationship between illumination intensity and temperature, even for moderate illumination intensities relevant for applications such as Raman scattering [1]. Subsequently, I will discuss how self-induced optical heating could be employed in optical devices and metasurfaces. In particular, I will present our recent efforts in the design of thermally reconfigurable metalenses [2].
[1] T.V. Tsoulos, G. Tagliabue – Self-induced thermo-optical effects in silicon and germanium dielectric nanoresonators - Nanophotonics 9 (12), 3849-3861
[2] A. Archetti, R.J. Lin, F. Kiani, N. Restori, TV Tsoulos, G. Tagliabue – Thermally Reconfigurable Metalens - in preparation.

Two-dimensional (2D) materials offer extraordinary potential for control of light and light-matter interactions at the atomic scale. In particular, twisted 2D materials has recently attracted a lot of interest due to the capability to induce moiré superlattices and discovery of electronic correlated phases [1,2]. In this talk, we present nanoscale optical techniques such as near-field optical microscopy and photocurrent nanoscopy, and reveal with nanometer spatial resolution unique observations of the infrared and photothermal properties of twisted 2D materials.
We report on the photo-thermoelectric nano-imaging within the Moire unit cell of small-angle twisted graphene [4] and interband collective modes in charge neutral twisted-bilayer graphene near the magic angle [3]. The freedom to engineer these so-called optical and electronic quantum metamaterials [1] is expected to expose a myriad of unexpected phenomena. As an example for indirectly patterned polaritons, we introduce and demonstrate a novel multimodal reflection mechanism of the ray-like optical excitations in hyperbolic materials, such as hBN.


References
[1] Song, Gabor et. al., Nature Nanotechnology (2019)
[2] Cao et al., Nature (2018)
[3] Hesp et al., Nature Physics (2021). Arxiv 1910.07893.
[4] Hesp et al., Nature Communications (2020)
[5] Epstein et al., Science (2020)
[6] Herzig Sheinfux et al., (2021)

The large mismatch between the free-space light wavelength and the spatial extension of localized optical modes, particularly at infrared frequencies, results in a poor light-mode coupling, which poses a serious problem in the design of practical structures for light and thermal radiation management. As a possible way to address this issue, we propose the use of perfect scatterers, which enable complete coupling between free space radiation and arbitrarily small localized modes when they are placed at suitable positions and the external light is also optimally structured in the far field. We discuss quantum perfect scatterers consisting of two-level atomic/molecular systems, as well as classical perfect scatterers provided by Mie-mode-supporting dielectric cavities. Our results open a practical route to circumvent the long-standing photon-polariton wavelength mismatch problem in nanophotonics.

Phonon polaritons - light coupled to optical lattice vibrations - in 2D materials exhibit ultra-short wavelengths, long lifetimes and strong field confinement, which allow for manipulating infrared light at the nanometer scale. Here, we discuss real-space nanoimaging studies of ultra-confined infrared phonon polaritons, essentially in thin hexagonal boron nitride layers and nanostructures, using scattering-type scanning near-field optical microscopy (s-SNOM) and nanoscale infrared Fourier transform (nano-FTIR) spectroscopy. We visualize and analyze phonon polaritons in nanoscale waveguides and resonators. Particularly, we will demonstrate that phonon polaritons can be utilized to achieve vibrational strong coupling with nanoscale amounts of organic molecules.

Air can play a key role within photovoltaic and photothermal materials, owing to its non-absorbing, low-index, low-thermal-conductivity properties. However, exploiting these unique properties is challenging because it requires an approach that accommodates the undesirable properties of air, namely the lack of structural support. Here, I discuss our recent progress in realizing thermophotovoltaic (TPV) cells¹ and refractory solar thermal absorbers^2,3 that incorporate air in a way that does not compromise their functional stability. In doing so, we overcome key limitations in TPV and solar thermal conversion, namely a lack of broadband spectral control, and realize record-high efficiencies. With such high-efficiency approaches, TPV-based thermal batteries can serve as a solution to the long-duration electricity storage problem, while concentrated solar thermal can access a high-temperature regime and, in turn, provide efficient, on-demand power generation and industrial process heat.

1. Fan, D., Burger, T., McSherry, S., Lee, B., Lenert, A. & Forrest, S. R., "Near-perfect photon utilization in an air-bridge thermophotovoltaic cell," Nature 586, 237–241 (2020). DOI: 10.1038/s41586-020-2717-7
2. Berquist, Z. J., Turaczy, K. K. & Lenert, A., "Plasmon-Enhanced Greenhouse Selectivity for High-Temperature Solar Thermal Energy Conversion," ACS Nano (2020). DOI: 10.1021/acsnano.0c04982
3. Gayle, A. J., Berquist, Z. J., Chen, Y., Hill, A. J., Hoffman, J. Y., Bielinski, A. R., Lenert, A. & Dasgupta, N. P., "Tunable Atomic Layer Deposition into Ultra-High-Aspect-Ratio (>60000:1) Aerogel Monoliths Enabled by Transport Modeling," Chem. Mater. 33, 5572–5583 (2021). DOI: 10.1021/ACS.CHEMMATER.1C00770

This talk presents new synthesis methods and characterization of optically transparent mesoporous silica thin films and monoliths with tunable infrared properties. The material featured porosity that can be adjusted from 50 to 90% with narrow pore size distribution varying between 5 and 20 nm and silica building blocks between 2 and 10 nm. As a result, the materials are transparent in the visible while their infrared properties can be tuned to achieve very high and/or spectrally selective thermal emission at room temperature. The material optical behavior observed across the spectrum is interpreted thanks to numerical simulations of electromagnetic wave transport through realistic computer-generated mesoporous materials. Particular attention is paid to the effects of dependent scattering, absorption, and fractal architecture of the mesoporous structure. The novel materials investigated could be used for passive thermoregulation of buildings.