Symposium EP11—Hybrid Materials and Devices for Enhanced Light-Matter Interactions

The strong light-matter coupling is the core of cavity quantum electrodynamics (CQED), which leads to discoveries of fascinating phenomena in solid state system such as Bose-Einstein condensation and photon blockade. Indirect evidence indicates non-fluorescence processes such as phonon scattering are critical to these phenomena. However, the understanding of such processes remains elusive due to their non-radiative nature smeared by the overwhelming fluorescence in cavities. Here we directly probe phonon scattering based on Raman spectroscopy in the strong coupling regime in a plasmonic cavity embedded with a monolayer MoS₂. The studied non-fluorescence process is significantly modified by the hybrid properties of the newly formed half-light half-matter quasiparticles, i.e., polaritons. For the first time, we observe nonlinearly enhanced valley-dependent phonon modes, involved with stimulated lattice vibrations and inter-valley scatterings. This work provides a new perspective to investigate fundamental quantum processes in the strong coupling regime.

Vibrational polaritons, composed of strongly coupled cavity optical modes and molecular vibrations, have recently been utilized to modify excited-state dynamics (Nature Communications 2016, 7, 13504) and chemical reaction rates (Acc. Chem. Res. 2016, 49, 2403). Ultrafast vibrational spectroscopy has the potential to reveal how strong coupling modifies vibrational relaxation and energy transfer that can mediate molecular reactivity. Toward that end, we have carried out transient infrared studies on cavity-coupled molecules in solution. Our results have revealed several important phenomena, including (1) the presence of polariton excited states that, in the systems studies, relax more quickly than do the molecules in free space; (2) a reservoir of dark states which can dominate the transient spectral response; and (3) ultrafast modulation of the coupled system between the weak and strong coupling regimes, leading to dramatic changes in the transmission and reflection of the system. Ultrafast modulation of transmission spectra is uniquely useful in single pulse studies where saturable absorption is substantially modified due strong coupling effects that result in much larger saturation intensities (~15 x) compared to those measured for the uncoupled molecular system. This behavior has potential applications for nonlinear optical and photonic devices.

Monolayer molybdenum disulfide (MoS₂) is at the focus of current research for different applications ranging from the photovoltaics, photodetection to optoelectronics. The absorption from monolayer MoS₂ is four times to that of graphene, but it is still to be optimized to enhance the performance of photonic devices. Multiple excitonic absorption states in the energy range from ultraviolet to visible, and the exceptionally high exciton binding energy excitonic states presents many opportunities to enhance the interactions of incident light to these excitonic states. Hybrid metal–MoS₂ structures can be formed with different metals, and tuned with the shape and size of these metal nanostructures to enhance the light-matter interaction by coupling with the specific excitonic energy state as expected. The exciton dynamics followed by the optical excitation and the interaction dynamics such as hybrid exciton formation, exciton-exciton annihilation etc. are strongly influenced due to coupling with the lattice vibrational modes. The plasmon modes, when coupled with phonon modes, control the coherent interaction among the excitonic states as well as the interaction between the excitonic states with the plasmonic states. Here we report the activation of a new vibrational mode in the hybrid silver (Ag)-MoS₂ monolayer structure at 35 meV, and the dressing of the Raman mode with the plasmon mode significantly contributes in the light-matter interaction process as measured using the pump-probe spectroscopy. By selecting an optical excitation, which is in resonance to the plasmon modes, the absorption of the incident light is increased due to the enhanced electric field of the localized plasmons. In addition, the new Raman mode in the hybrid structure along with the usual active Raman modes in MoS₂ are also resonantly driven by the pulses of the femtoseconds (fs) laser source. The driving field due to resonantly excited plasmons with the pump pulses creates the power broadening of the excitonic optical transitions. Also, the driving field generates quantum coherence effect that significantly modifies the absorption of probe light forming the so-called dark state as illustrated using a density matrix model. There is a strong coherent coupling of the plasmon modes with the excitonic states in MoS₂. A hybrid exciton-plasmon band of half-width about 250 meV is formed around A and B excitonic states in the transient absorption spectrum at a delay time of about 500 femtoseconds after the optical excitation. The formation of the hybrid states offers an opportunity to strongly enhance the light-matter interaction and hence improve the performance of optical and electrical devices based on MoS₂.

The term “hot electron” is frequently used in the literature to describe electrons in a solid with energies exceeding their thermally-created counterparts at ambient temperature. In plasmonic structures, the nonradiative dephasing of plasmons generates excited (i.e., athermal) electrons in the conduction band of metals and a subsequent electron thermalization process leads to the equilibration of the athermal electrons and the formation of a hot-electron distribution. The high energy nature of athermal and hot electrons enables varieties of optically-driven processes with fascinating applications in photochemical and photovoltaic devices. More recently, exploring the transient dynamics of such energetic charge carriers has gained a growing attention with the hope for the implementation of active plasmonic platforms. Indeed, the optical modulation of the electron temperature in metals enables the tuning of their refractive index and therefore, allows for the all-optical modulation of the plasmonic response at low light intensity. However, the intrinsically slow decay of the hot-electron temperature via the electron-phonon interaction, impedes ultrafast all-optical modulation in plasmonic structures. In this work, we show that the ultrafast transfer of hot electrons from plasmonic metals to electron accepting materials allows for the sup-picosecond (< 190 fs) recovery of the refractive index change in plasmonic metals. Our experimental findings suggest that the activation of hot-electron transfer pathways screen the contribution of electron-photon interactions and instead provide an electron-dominated relaxation mechanism. Through the design of a subradiant, high-Q, and polarization-sensitive plasmonic crystal we demonstrate ultrafast modulation of phase, polarization, and intensity of light in an all-optical fashion.

1. Taghinejad, M.; Taghinejad, H.; Xu, Z.; Liu, Y.; Rodrigues, S. P.; Lee, K. T.; Lian, T.; Adibi, A.; Cai, W. Hot-Electron-Assisted Femtosecond All-Optical Modulation in Plasmonics. Advanced Materials 30, 17049015-17049021 (2018).
2. Taghinejad, M.; Taghinejad, H.; Xu, Z.; Lee, K. T.; Rodrigues, S. P.; Yan, J.; Adibi, A.; Lian, T.; Cai, W. Ultrafast Control of Phase and Polarization of Light Expedited by Hot-Electron Transfer. Nano Letters 18, 5544-5551 (2018).
3. Taghinejad, M.; Cai, W. All-Optical Control of Light in Micro- and Nano-photonics. Invited Perspective ACS Photonics, In Press (2019).

Colloidal plasmonic nanocrystals (NCs) are known for their size- and shape-dependent localized surface plasmon resonances. Here we show these plasmonic NCs can be used as building blocks of mesoscale materials. Chemical exchange of the long ligands used in NC synthesis with more compact ligand chemistries brings neighboring NCs into proximity and increases interparticle coupling. This ligand-controlled coupling allows us to tailor a dielectric-to-metal phase transition seen by a 10¹⁰ range in DC conductivity and a dielectric permittivity ranging from everywhere positive to everywhere negative across the whole range of optical frequencies. We realize a "diluted metal" with optical properties not found in the bulk metal analog, presenting a new axis in plasmonic materials design and the realization of optical properties akin to next-generation metamaterials. We harness the solution-processability and physical properties of colloidal plasmonic NCs to print NC superstructures for large-area, active metamaterials. We demonstrate quarter-wave plates with extreme bandwidths and high polarization conversion efficiencies in the near- to-mid infrared. By combining superparamagnetic Zn_0.2Fe_2.8O₄ NCs and plasmonic Au NCs, we fabricate multifunctional, smart superparticles, which in suspensions switch their polarization-dependent transmission in the infrared in response to an external magnetic field. Finally, by juxtaposing plasmonic NCs and bulk materials, we exploit their different chemical and mechanical properties to transform lithographically defined two-dimensional structures, upon ligand exchange, into three-dimensional structures and use this approach to achieve chiral metamaterials.

Mobile device has developed display, memory, and various sensors for helping human life. CMOS image sensor is important technology to include human eye function into mobile devices. RGB imaging in visible wavelength region (430 ~ 565 nm) can be disturbed by near infrared light (NIR, 700 nm ~ 1200 nm). In order to protect NIR light, we have developed NIR absorbing dye materials and its formulations composed of copper complexes, organic dyes, and binding polymers. Depending on ionic ligand types, absorption wavelength and solubility can be controlled. By adding coordinative ligands, absorption intensity and hydrothermal stability can be improved by John-Teller Distortion effect and binding energy, respectively. In this presentation, we will discuss about what is the failure origin to be stable under hydrothermal condition by analysis with XPS and FT-IR. Finally, designing photo-crosslinkable ligand without hydroxyl group, NIR absorbing Cu (II) phosphate/sulfonate materials will be presented.

The performance and functionality of integrated optical devices are governed by their fundamental physical properties. In passive systems, such as waveguide arrays and interconnects, this limitation has pushed the field towards ultra-low loss materials that are compatible with conventional CMOS processing. However, this singular focus has resulted in a decrease in the functionality of these components and placed limitations on the ultimate system footprint. For example, one of the current drivers in the field is Si photonics. However, the refractive index of silicon places a fundamental minimum size in order to confine the optical mode. Similarly, due to its low second and third order optical nonlinear coefficients and high dispersion, it is challenging to achieve active components without high input power. By expanding the optical material toolbox, new strategies for designing optical devices and circuits will be possible.
One emerging strategy for changing device performance is based on surface engineering. Originally focused on adding biologically active groups to a device for bio/chem detection, more recent work has explored developing passivating surface chemistries to stabilize or protect device surfaces. However, the majority of these efforts involved changing the device properties by adding material layers instead of directly changing the device material. In the present work, we develop and demonstrate an approach to directly change the Raman gain of the optical device by modifying the Raman of the dangling surface functional groups. The initial device studied as a proof of concept is an optical resonator; however, the basic strategy is suitable for any on-chip device.
Optical resonators act as optical amplifiers, allowing low input powers to be significantly increased. One type of resonator is a traveling wave resonator or whispering gallery mode resonator. This cavity confines light in circular orbits, and due to photon storage times (or quality factors, Q) in the nanosecond regime, the input power can be amplified by over 100,000x. The large circulating optical intensities can reveal nonlinear phenomena that otherwise would require large pulsed laser systems. For example, previous work using silica cavities has demonstrated Raman lasing with only a few hundred micro-Watts of input power. However, the efficiency of the Raman generation was poor due to the intrinsically low Raman gain of silica.
In this present work, we fabricate silica resonators on chip. However, instead of relying on the intrinsic bulk Raman gain of the silica, we leverage surface chemistry to modify the disordered surface or boundary layer. As a result, a monolayer of oriented organic molecules is aligned with the circulating optical field in a microcavity. This optimization improves the light-matter interaction at the boundary layer, and the total Raman gain of the device.
The two ordered monolayers studied are methylsilane (MS) and dimethylsilane (DMS). After attachment on the device surface, the devices still maintain ultra-high Q factors (over 10⁷). Due to the alignment of the Raman modes of the siloxane molecule with the optical field, the efficiency and lasing thresholds are significant improved. With an excitation source of 765 nm, low threshold SRS located at ~465 cm^-1 is observed in both the MS and DMS functionalized devices with SRS efficiencies of approximately 40%. These efficiencies represent over 5 times enhancement as compared to bare silica devices. This work represents a new strategy for dramatically increasing the nonlinear optical performance of integrated photonic devices using organic molecules with a hybrid structure.

Trace-amount toxins in drinking water are not only serious environmental pollution but also threaten human health through long term accumulation. In need for sensors capable of detecting low-concentration, trace-amount molecules, highly sensitive and selective detection technologies have been long developed. Research has recently expanded to multiplexing, which can significantly reduce analysis time and sample volume while also providing a more thorough information of a given system such as that of the environment or a living cell. So far, multiplexing has been largely realized through assays based on highly sensitive fluorescent or electrochemical sensors, yet they lack in ability to differentiate from similar molecules and selectively recognize target molecules within mixed samples. Surface-enhanced Raman spectroscopy (SERS) has recently drawn much attention as a multiplexing technique owing to its fingerprint-spectra specificity and single-molecule level sensitivity. However, conventional approaches using lithography or metallic nanoparticles are either costly and time-consuming or show insufficient reproducibility. Recently, our group has demonstrated highly reproducible 3D cross-point Au nanostructures for SERS based on high-resolution nanotransfer printing with uniform average enhancement factor of 4.1 × 10⁷ over a macro-scale area. Herein, we present thermally soldered Au nanogrids with enhanced plasmon quality for highly sensitive detection of trace-amount molecules and accurate quantitative multiplexing via SERS. The Au nanogrids were fabricated by a thermal annealing method to solder multi-stacked 3D Au nanostructure into a single entity while preserving the sub-20 nm nanostructures. The enhanced plasmon qualities of Au nanogrids can be attributed to the reduced grain-boundary areas, enabling highly sensitive detection of low-concentration molecules through SERS. Au nanogrids were also functionalized with multiple single single-stranded DNA (ssDNA) aptamer probes each specifically designed for different toxin molecules to achieve selective multiplexing and quantitative analysis. We used scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to characterize the nanostructure and grain morphology of Au nanogrids, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) to confirm aptamer-functionalization. As a result, we achieved three-component multiplexing and quantification of drinking water toxins each down to 10^-11 M via SERS even without the use of Raman reporter dye-molecules.

Nanostructured materials with tailored geometries on the atomic scale having tuneable physico-chemical properties are of immense interest for applications in photodetection. Among different reported nanostructure, metal nanoparticle on semiconducting layers have been proposed to improve performance parameters such as minimum dark current and enhanced UV response of a photo-conducting device.^1-3 However, the mentioned device geometries suffer from poor UV-to-Visible rejection ratio, as the top metal nanoparticle coating responds to visible illumination.² Therefore, we envisioned to develop the ZnO@Au core-shell nanostructures with metal (Au) core and semiconducting (ZnO) shell for photoconductive device, to enhance UV emission/ detection as a result of SPR mediated carrier generation and transport with suppressed dark current. In present work, we have synthesized the ZnO nanostructures, without and with Au core, using wet chemical route. The ZnO and ZnO@Au nanostructures, were then deposited on ITO coated glass substrates followed by thermal annealing at 300 °C, under ambient conditions, forming ZnO/ITO and ZnO(Au)/ITO nanostructures thin-film, respectively. Optical properties revealed that the incorporation of Au core in ZnO nanostructures reduces the overall optical bandgap of the material and also suppresses the visible emission originating from ZnO defect. Further, the electrical measurements shows suppressed dark current with enhanced light current under 380 nm UV illumination for ZnO@Au/ITO device, leading to 7 folds increase in photocurrent to dark current ratio (at 10 V applied bias) compared to the ZnO/ITO device. Additionally, the ZnO@Au/ITO device also demonstrates an improved UV-to-Visible rejection ratio (~1×10³) compared to the ZnO/ITO device (~1.8×10²) at 10 V applied bias and thus suggests its application in designing high performance Visible-blind UV photodetector.

References:
1. Liu K, Sakurai M, Liao M, Aono M, J. Phys. Chem. C, 114 (2010) 19835.
2. Gogurla N, Sinha A K, Santra S, Manna S, Ray S K, Sci. Rep., 4 (2014) 6483.
3. Wang X, Liu K, Chen X, Li B, Jiang M, Zhang Z, Zhao H, Shen D, ACS Appl. Mater. Interfaces, 9 (2017) 5574.

Solid-state quantum emitters make up the most crucial components of any quantum information processing device. The quest for finding the ideal quantum system has led to a large effort on different materials, including organic molecules, semiconductor quantum dots, diamond color centers, rare earth ions in inorganic crystals and a variety of two-dimensional materials. While each system offers interesting features, the ideal system is still not within reach. One possible strategy to address this issue is, thus, to modify and improve the properties of the existing quantum emitters, e.g. by modifying their radiative decay rates and pathways using concepts from Cavity Quantum Electrodynamics. In our laboratory, we pursue this strategy and investigate the coupling of solid-state quantum emitters to microcavities and plasmonic nanoantennas.

In the first part of this presentation, I will discuss the room-temperature coupling of collodial quantum dots to plasmonic nanoantennas. Semiconductor quantum dots are capable of emitting one, two or more photons after each excitation because of the possibility of generating multiple excitons within the same quantum dot. In practice, however, such multiphoton emission is often inefficient due to fast nonradiative decay channels such as Auger recombination. We have recently demonstrated that the biexciton emission efficiency can be significantly improved by a large radiative enhancement in the near field of a gold nanocone antenna fabricated by focused ion beam milling. We show that the quantum efficiency of the biexciton emission is increased by more than one order of magnitude to 70% in the coupled system. Moreover, by performing many quantitative in-situ measurements on the very same quantum dot, we demonstrate more than 100-fold radiative enhancement by the gold nanocone antenna for both excitonic and biexcitonic emission channels.

In the seond part of my talk, I will show results on the cryogenic coupling of single organic molecules to open Fabry-Perot microcavities at the onset of strong coupling. Organic dye molecules are very strong emitters with near-unity quantum efficiency. However, their internal vibrational degrees of freedom and their coupling to the phononic landscape of their host matrix give rise to various sources of decoherence such that their use in quantum engineering becomes limited. To get around this effect, we couple an organic dye molecule to an ultrasmall Fabry-Perot cavity made of a curved (fabricated by focused ion beam milling) and a flat mirror. The ability to tune the cavity and scan it laterally across a thin moleculear crystal allows us to perform several experiments with an exquisite control. Our results show that a dye molecule can be turned into a two-level quantum coherent system.

In closing, I discuss the prospects of hybrid quantum material based on plasmonic nanoantennas for use at room temperature.

Photons are a potential resource for a growing host of applications in quantum technologies and quantum information sciences. A particular interest is in effectively harvesting pure single photons from simple quantum emitters operating at room temperature, and in finding effective ways for strong interactions between only a few photons.
In this talk I will review our progress towards a realization of bright, high-purity single photon sources from impure photon emitters, by designing hybrid nano-emitter – nano-antenna devices that can efficiently extract and direct single photons using indeterministic two- photon states. I will also introduce our recent results showing on-chip low-loss guiding and large electrically-controlled enhancement of effective photon interactions in an optical waveguide by utilizing strong interactions of photons with dipolar excitations in semiconductors.

Hybridisation of quantum emitters and plasmonic nano-structures has attracted much attention over the last years, due to their interest in the design of plasmon-based nano-lasers [1,2] or to achieve long-range qubit entanglement [3,4]. Recent theoretical studies [5,6] suggest a plasmonic super-radiant mechanism to increase the rate of emitters, similar to Dicke super-radiance [7].
In this talk, we will report a review of our work in these domains and explain the salient features of the involved effects.
As such, i) we provide experimental evidence of plasmonic super-radiance of organic emitters close to a metal nanosphere at room temperature. This observation of plasmonic super-radiance at room temperature opens questions about the robustness of these collective states against decoherence mechanisms which are of major interest for potential applications. A complete quantum theoretical/numerical approach will be presented, which will give full account of the obtained experimental results [8].
ii) We proposed a new type of nanodevice, capable of both path-selectivity and anisotropic lasing that is based on loss-compensation and amplification by a localized plasmon polariton [9]. The nano-device is a Y-shaped plasmonic nanostructure embedded in an anisotropic host medium with gain. The anisotropy leads to the path selectivity, an effect which is more pronounced once gain is included. The path-selectivity may be coupled with activation of a rotation of the anisotropic host medium for inducing a light-guiding switching functionality. On the experimental side, we used a DNA origami structure to precisely localize three different fluorescent dyes close to the tips of hollow gold nanotriangles. A spectral dependence of plasmon-enhanced fluorescence is evidenced through co-localized AFM and fluorescence measurements. The experimental results match well with explanatory FDTD simulations. Our findings open the way to the bottom-up fabrication of plasmonic routers operating through plasmon energy transfer. They will allow one to actively control the direction of light propagation [10].

References
[1] J.G. Bohnet et al. Nature 484, 78–81 (2012).
[2] M.A. Noginov et al., Nature460, 1110–2 (2009).
[3] R. Kolesov et al., Nature Physics 5, 470–474 (2009).
[4] A. Gonzalez-Tudela et al., Phys. Rev. Lett.106, 020501 (2011).
[5] V.N. Pustovit et al.,Phys. Rev. Lett.102, 077401 (2009).
[6] D. Martín-Cano et al., Nano Letters 10, 3129–3134(2010).
[7] R.H. Dicke. Phys. Rev.93, 99-110 (1954).
[8] P. Fauché et al., manuscript in preparation.
[9] A. Yamada, D. Neuhauser and R. A. L. Vallée. Nanoscale 8, 18476–18482 (2016).
[10] P. Ivaskovic, A. Yamada, J. Elezgaray, D. Talaga, S. Bonhommeau, M. Blanchard-Desce, R.A.L. Vallée and S. Ravaine, Nanoscale, 10, 16568-16573, (2018).

Spectrally-selective materials are of great interest for optoelectronic devices in which wavelength-selectivity of the photoactive material is necessary for applications such as multijunction solar cells, narrow-band photodetectors, transparent photovoltaics, and tailored emission sources. Achieving controlled transparency or opacity within multiple wavelength bands in the absorption, reflection, and transmission spectra is difficult to achieve in traditional semiconductors that typically absorb at all energies above their electronic band gap and is generally realized by the use of external bandpass filters. Here, we propose an alternate method for achieving spectral selectivity in optoelectronic thin films: the use of band engineering in 2D photonic crystal slabs within the absorbing region of a semiconductor in which resonant and confined photonic states are strongly coupled to the external reflecting and transmitting fields that share lateral wave-vectors.
As a first step, we use optical simulations to systematically study the effect of material absorption on the properties of the photonic bands in a slab-type photonic crystal structure. We find that adding weak loss, which is realized by introducing a small (<10%) imaginary part to the permittivity, does not appreciably change the frequencies of the photonic bands but does weaken the definition of the band structures. This is the result of reduced quality factors of the associated resonance modes due to the presence of dissipation. The results are also confirmed by qualitative analysis using perturbation theory. Critically, in the case where external propagating waves are incident upon or out-coupled from the photonic crystal slab structure, the radiating photonic bands induce multiple strong Fano resonance features in the slab transmission and reflection spectra, due to coupling between the bands and external fields. The Fano resonances display large transparency contrasts across the center of the resonances that should enable transmission and reflection wavelength selectivity and introducing material absorption into the model widens the bandwidth of this selectivity. Moreover, the absorption spectra also peak at these resonance frequencies, due to the associated concentrated field intensity inside the slab, with peak widths increasing under increasing dissipation, giving rise to the possibility of achieving spectral selectivity in the absorbing region of semiconductors.
We also demonstrate this tuning method experimentally by fabricating a proof-of-principle photonic structure consisting of a self-assembled polystyrene bead monolayer infiltrated with PbS CQDs that displays both near-infrared absorption enhancement and visible transparency enhancement over a homogeneous control film, qualitatively matching predictions and showing promise for wavelength-selective optoelectronic applications.

Optical quantum technologies will likely operate at the level of single or few photons. To guarantee high fidelity optical quantum computation and fast quantum communication rates, efficient, scalable, and versatile photonics is required. However, many promising material platforms, which host interesting quantum emitters, have challenging fabrication protocols. Moreover, to comply with the stringent fabrication constraints, traditional optimization methods are slow and device functionalities are limited.

Diamond hosts versatile color centers, some of which exhibit very long coherence times and/or minimal inhomogeneous broadening. Moreover, site-controlled implantation of color centers enables precise integration into on-chip optical circuits. To develop a diamond photonic platform that can match the scalability potential of the diamond color centers, major improvements in fabrication and device design are necessary. In our work, we have optimized fabrication methods based on quasi-isotropic etching and developed a computational approach to inverse design photonics based on desired performance. Our design approach can include fabrication constraints as part of the optimization and thus results in robust designs. As our optimization methods can probe the entire parameter space our device designs are non-intuitive, but are fabricable using standard techniques, are robust against typical fabrication errors, and outperform state-of-the-art counterparts in footprint, efficiency and stability.

High light transmission microcavities are a greatly studied and applied topic [1, 2] . Lately, joined
structures have drawn even more attention [3] , like Fibonacci conjugated mirrors [4] , because of
its optical properties, as perfect transmission or field localization. Here, we study a simpler and
novel joined structure, which presents high transmission (quasi-perfect for short arrays) in a
resonant condition. In particular, we explore the transmission and field localization via simulations
to manufacture the best arrangement in porous silicon: a Microcavity-3 Bragg Mirrors array. This
is not a common microcavity, since it is composed by 3 Bragg mirrors, where the first two are in
a symmetric configuration (microcavity), while the third one is in a conjugated configuration.
This sequence improves all the characteristics of common microcavities, particularly those for
microcavities formed by absorbent materials.

[1] E. C. N. D. Philippe Delaye, "Enhanced nonlinear interaction in a microcavity under coherent
excitation.," Optics Express, vol. 23, pp. 29964-29977, 2015.
[2] B. Z. N. G. J. J. B. Mathias Kolle, "Stretch-tuneable dielectric mirrors and optical microcavities,"
Optics Express, vol. 18, pp. 4356-4364, 2010.
[3] V. G. L. Y. Yun-Feng Xiao, "Coupled optical microcavities: an enhanced refractometric sensing
configuration," Optics Express, vol. 16, pp. 12538-12543, 2008.
[4] J. T.-M. J. A. d. R. G. G. N. R Nava, "Perfect light transmission in Fibonacci arrays of dielectric
multilayers," J. Phys.: Condens Matter, vol. 21, 2009.

The interaction between molecular (atomic) electron(s) and the vacuum field of a reflective cavity generates a significant interest thanks to the rapid developments in nanophotonics. Such interaction which lies within the realm of cavity quantum electrodynamic can substantially affect transport properties of molecular systems^1-3. For the last several years cavity-induced modifications of charge transport properties of molecular systems have drawn a considerable interest ^1,4,5. In our work we consider non-adiabatic electron transfer process in the presence of a cavity mode. We worked out a generalized framework of interaction between a charged molecular system and a quantized electromagnetic field of a cavity and applied it to the problem of electron transfer between a donor and an acceptor that are put in a confined vacuum electromagnetic field. The effective system Hamiltonian presents a unified Rabi and spin-boson model which includes a self-dipole energy term. This term which is usually neglected in the standard light-matter interaction models plays a crucial role in our formalism. Two limiting cases were considered: in the first case the electron is much faster than the cavity mode (slow mode ) whereas in the second case the tunneling time of the electron is significantly larger than the period of the mode (fast mode). In the latter case the presence of the cavity does not alter the electron dynamics. In the case of slow mode, a Marcus-like electron transfer (ET) rate is obtained by summing over all final states and averaging over initial states of the electromagnetic field. We computed ET rates varying different system parameters. It was found that at high temperatures, high reorganization energies and/or low mode frequency the total rate can be described by a single Marcus rate with an increased reorganization energy whereas at low temperatures, low reorganization energy and/or high mode frequency the energy gap dependence of the rate exhibits local maxima which positions are defined by a resonance condition. Our study showed that a significant rate enhancement can be produced by the coupling to the field mode if the system is in the inverted region. The cavity-induced enhancement factor is larger at lower temperatures and/or at higher energy gaps. In the normal region the cavity-modified rate is smaller than the cavity-free one.
The results of this work may offer a new way to control electron transfer. Using tunable highly-confined IR nanocavities one can easily manipulate the magnitude of ET rate by adjusting the mode volume or/and changing the relative orientation of the system with respect to the polarizability vector of the field. This opens new opportunities for manufacturing of nanodevices and stimulates new developments in infrared plasmonics

References

1. E. Orgiu, J. George, J.A. Hutchison, E. Devaux, J.F. Dayen, B. Doudin, F. Stellacci, C. Genet, J. Schachenmayer, C. Genes, G. Pupillo, P. Samorì, and T.W. Ebbesen, Nat. Mater. 14, 1123 (2015).
2. D.M. Coles, N. Somaschi, P. Michetti, C. Clark, P.G. Lagoudakis, P.G. Savvidis, and D.G. Lidzey, Nat. Mater. 13, 712 (2014).
3. J. Feist and F.J. Garcia-Vidal, Phys. Rev. Lett. 114, 196402 (2015).
4. F. Herrera and F.C. Spano, Phys. Rev. Lett. 116, 238301 (2016).
5. M. Kowalewski and S. Mukamel, Proc. Natl. Acad. Sci. 114, 3278 (2017).

Smart nanostructured materials with optical properties responsive to external stimuli are gaining increasing interests due to their intrigue potential applications in printing, sensing, signage, security documents, displays, and other color-related devices. In this presentation, I will update our recent progress on the development of novel self-assembly approaches for the fabrication of various nanostructured materials whose optical properties can be dynamically tuned by controlling the spatial arrangement of the nanoscale building blocks. In particular, I will discuss the self-assembly of anisotropically shaped magnetic colloidal particles such as ellipsoids and cubes into three-dimensional photonic crystals with strong field-direction-dependent diffraction. Then I will report the assembly and disassembly of plasmonic metal nanostructures and the associated opportunities in the development of novel optical devices such as humidity-responsive colorimetric sensors.

Recent experiments have demonstrated that light and matter can mix together to an extreme degree, and previously uncharted regimes of light-matter interactions are currently being explored in a variety of settings, where new phenomena emerge through the breakdown of the rotating wave approximation [1]. This talk will summarize a series of experiments we have performed in such regimes. We will first describe our observation of ultrastrong light-matter coupling in a two-dimensional electron gas in a high-Q terahertz cavity in a quantizing magnetic field, demonstrating a record-high cooperativity [2]. The electron cyclotron resonance peak exhibited splitting into the lower and upper polariton branches with a magnitude that is proportional to the square-root of the electron density, a hallmark of cooperative vacuum Rabi splitting, known as Dicke cooperativity. Additionally, we have obtained clear and definitive evidence for the vacuum Bloch-Siegert shift [3]. The second part of this talk will present microcavity exciton polaritons in a thin film of aligned carbon nanotubes [4] embedded in a Fabry-Perot cavity, also exhibiting cooperative ultrastrong light-matter coupling with unusual continuous controllability over the coupling strength through polarization rotation [5]. Finally, we have generalized the concept of Dicke cooperativity to a condensed matter system, demonstrating that it also occurs in a magnetic solid in the form of matter-matter interaction [6]. Specifically, the exchange interaction of N paramagnetic erbium(III) (Er³⁺) spins with an iron(III) (Fe³⁺) magnon field in erbium orthoferrite (ErFeO₃) exhibits a vacuum Rabi splitting whose magnitude is proportional to N^1/2. Our results provide a route for understanding, controlling, and predicting novel phases of condensed matter using concepts and tools available in quantum optics, opening up a variety of exciting possibilities to combine the traditional disciplines of many-body condensed matter physics and cavity-based quantum optics.

References
1. For a review, see, e.g., P. Forn-Díaz, L. Lamata, E. Rico, J. Kono, and E. Solano, arXiv:1804.09275.
2. Q. Zhang et al., Nature Physics 12, 1005 (2016).
3. X. Li et al., Nature Photonics 12, 324 (2018).
4. X. He et al., Nature Nanotechnology 11, 633 (2016).
5. W. Gao et al., Nature Photonics 12, 362 (2018).
6. X. Li et al., Science 361, 794 (2018).

When a collection of electronic excitations are strongly coupled to a single mode cavity, mixed light-matter excitations called polaritons are created. The situation is especially interesting when the strength of the light-matter coupling W_R is such that the coupling energy becomes close to the one of the bare matter resonance w₀. For this value of parameters, the system enters the so-called ultra-strong coupling regime, in which a number of very interesting physical effects were predicted. Using metamaterial coupled to two-dimensional electron gases, we have demonstrated that a ratio W_R/w₀ close to or above unity can be reached.

We also demonstrated that such ultra-strong light-matter coupling can be achieved using special geometries where the only less than 100 electrons are effectively coupled to the resonator. Other metamaterial engineering include the inter-meta-atom coupling using a surface plasmon polariton resonance. This feature enables to restore the dispersion to the metamaterial ensemble and to control the linewidth of the latter. One very intriguing feature of the ultra-strong light-matter coupled system is the prediction that photon pairs will be emitted through non-adiabatic modulation of the coupling. To this end, we have realized metamaterials based on high Tc superconductors that retain a high quality factor resonance for magnetic field up to 9T and coupled them to two-dimensional electron gases. Because the resonator is designed to be switchable using the superconducting transition, experiment can now be conducted using very intense terahertz fields. We have also used transport to probe the ultra-strong light-matter coupling. As shown in Fig. 1, the longitudinal magneto-resistance of a two-dimensional electron gas is modulated by the irradiation by a weak, tunable THz source revealing the dispersion of the polariton branches. The effect of the vacuum field could be evidenced by a special metamaterial in which the vacuum field could be tuned by a mechanical plate brought in the vicinity of the resonator.
[1] G. Scalari et al., "Ultrastrong Coupling of the Cyclotron Transition of a 2D Electron Gas to a THz Metamaterial," (in English), Science, vol. 335, no. 6074, pp. 1323-1326, Apr 15 2012.
[2] C. Maissen et al., "Ultrastrong coupling in the near field of complementary split-ring resonators," (in English), Physical Review B, vol. 90, no. 20, p. 205309, Nov 24 2014.
[3] J. Keller et al., "Few-Electron Ultrastrong Light-Matter Coupling at 300 GHz with Nanogap Hybrid LC Microcavities," Nano Letters, vol. 17, no. 12, pp. 7410-7415, Dec 2017.
[4] J. Keller et al., "Coupling Surface Plasmon Polariton Modes to Complementary THz Metasurfaces Tuned by Inter Meta-Atom Distance," (in English), Advanced Optical Materials, vol. 5, no. 6, p. 1600884, Mar 01 2017.
[5] J. Keller et al., "High Tc Superconducting THz Metamaterial for Ultrastrong Coupling in a Magnetic Field," ACS Photonics, rapid-communication pp. 1-7, Oct 04 2018.
[6] G. L. Paravicini-Bagliani et al., "Tomography of an ultrastrongly coupled polariton state using magneto-transport in the quantum regime," arXiv.org, vol. quant-ph, May 02 2018.

Similar to electron waves, the phonon states in semiconductors can undergo changes induced by external boundaries. The possibility of controlling the acoustic phonon spectrum in periodic structures has led to an explosive growth in the field of phononic crystals. A possibility of the acoustic phonon confinement effects in individual nanostructures has also been demonstrated experimentally [1]. The same periodic structures with properly tuned dimensions can act as photonic crystals affecting the light–matter interactions further. In this work, we conducted experimental and computational study to develop pillar structures, which act simultaneously as phononic and photonic crystals. The “pillar-with-hat” structures were fabricated using the electron beam lithography on a silicon (100) substrates, followed by the inductively-coupled plasma (ICP) cryogenic dry etching. The hats of the pillars were created with a special design to have exactly the same orientation plane as the substrate. The pillars, with the diameter and height of 280 nm and 362 nm, respectively, were positioned in the square arrays of 1×1 µm² dimensions. We used Brillouin-Mandelstam light scattering spectroscopy as a tool to measure the dispersion of acoustic phonons with energies in the range from 0.5 GHz up to 900 GHz near the Brillouin zone center [2]. Changing the angle of light incidence with respect to the substrate allowed us to vary the probing phonon wave-vector, and determine the dispersion near the Brillouin zone center. We analyzed the contributions from all three mechanisms, which contribute to light scattering in our samples. They include scattering from the bulk, i.e. the volume of the substrate via the elasto-optic mechanism, from the surface of the substrate via the surface ripple mechanism, and from the side-facets of the pillars via the surface ripple mechanism. The finite-element modeling guided the nanostructure fabrication. We have found clear signatures of the phonon spectrum modification in the appearance of confined phonon sub-bands at the energies in the range from 2-20 GHz. The light reflectance has been strongly enhanced in certain directions. The experimental results confirmed the dual function of the structure as the phononic-photonic crystal. We argue that the dual function of the structure allows better control of the light–matter interactions, with important implications for engineering the radiative and non-radiative processes in the materials.
The work of the Balandin group was supported, in part, by the DARPA project W911NF18-1-0041 Phonon Engineered Materials for Fine-Tuning the G-R Center and Auger Recombination. Numerical simulations were supported in part by the NSF EFRI-1433395. L.B. also acknowledges support from AFOSR under grant number FA9550-17-1-0377.
[1] F. Kargar, B. Debnath, J.-P. Kakko, A. Säynätjoki, H. Lipsanen, D. L. Nika, R. K. Lake, and A. A. Balandin, Direct observation of confined acoustic phonon polarization branches in free-standing semiconductor nanowires, Nature Communications, 7, 13400 (2016).
[2] F. Kargar, E. H. Penilla, E. Aytan, J. S. Lewis, J. E. Garay, A. A. Balandin, Acoustic phonon spectrum engineering in bulk crystals via incorporation of dopant atoms, Applied Physics Letters, 112, 191902 (2018).

Plasmon-exciton coupling (plexciton) is a subject undergoing intense research to understand the electromagnetic interaction between plasmonic (metallic) and excitonic (semiconducting) materials and their promising applications in photonics. We investigate the extent on the plexciton (plasmon-exciton) coupling in a superlattice built with periodically-distributed plasmonic cores with exitonic shells. We calculate the optical properties (extinction, transmittance, and reflectance) and the photonic band structure of miconscale cubic superlattice composed of 40-nm gold core (Au NPs) with 5 nm quantum shell (with exciton at 630 nm) arranged in a cubic array as a function of the inter-nanoparticles spacing (or lattice constant) from 5 to 35 nm. The photonic band structure of the smallest lattice constant present a band gap from 300 to 545 nm and a lower energy band density than those of large lattice constant in which the band gap disappears. The calculated extinction cross-section, transmittance, and reflectance show the plasmon-exciton coupling of the core-shell NPs and its decoupling as the distance between NPs increases. For these gaps, oscillations in their transmittance and reflectance appear as Bragg resonances above 600 nm wavelength. The resonance modes can be used to excite and enhance the interaction between the plasmons and the excitons.

Interest continues to grow in photonic and phononic analogues of topological electronic phases. These systems are typically non-interacting, and have the same band structure and edge state structure as their fermionic counterparts. In this talk, I’ll discuss recent theory work in my group on a class of photonic systems where this correspondence fails. They involve using parametric “two-photon” driving, and have Hamiltonians that superficially resemble those of topological superconductors. Among the surprising effects that emerge are the presence of topologically-protected instabilities that can be harnessed for non-reciprocal quantum amplification, and effective non-Hermitian dynamics in a bosonic analogue of the Kitaev-Majorana chain. From an application point of view, parametric driving is an attractive route to topological photonics, as the generated gain inherently helps offset the impact of losses. I’ll discuss how our ideas could be realized in a variety of different experimental platforms.

Metasurfaces, which are two-dimensional equivalent of metamaterials, offer a unique and efficient platform to study and control light-matter interactions in the sub-wavelength limit. When combined with semiconductor heterostructures, the metasurfaces can be coupled to fundamental excitations such as intersubband transitions in quantum wells. Such hybrid devices can provide opportunities for both fundamental studies of light-matter interactions as well as for new ultrathin optical devices such as voltage tunable optical modulators and nonlinear frequency generators. In the first part of this talk, I will present a low dissipation optical modulator using a hybrid plasmonic metasurface where the tuning mechanism relies on field induced tunneling of electrons in semiconductor heterostructures. In the second part of the talk, I will concentrate on hybrid dielectric metasurfaces that use using leaky mode resonances coupled to intersubband transitions for high efficiency and broadband second harmonic generation. I will finally conclude by presenting fundamental studies of strong-light matter interaction between Mie modes in dielectric resonators and intersubband transitions in semiconductor quantum wells.

Large-area and actively tunable broadband optical absorption is important for a wide range of applications, including enhanced light-matter interactions, sensing, imaging, and energy harvesting. Conventional plasmonic absorbers based on lithographic fabrication are limited by low throughput and small surface area coverage, and often have only fixed, as-fabricated optical absorption characteristics. Here we report large-area, lithography-free, metal-insulator-metal (MIM) plasmonic metasurfaces fabricated using wafer-scale physical vapor deposition, with tunable dielectric spacers. Random plasmonic nanostructures were formed by adjusting the deposition rate and thickness such that the top metal films remained below the percolation threshold. Gold was used for the metallic structures and different dielectrics were explored for the insulating spacer, including SiO₂, and polymers such as poly(methyl methacrylate) (PMMA), and Nafion. In particular, polymers open the possibility of active tuning through both changes in chemical and mechanical response. Samples were characterized using scanning electron microscopy, microspectroscopy, and spectroscopic ellipsometry, and show nearly 100% absorption over bandwidths as large as 200 nm in the visible spectrum across cm² surface areas. It is shown that discontinuous top metal films near the metal-dielectric transition, having nanostructures capable of supporting both localized surface plasmon resonances (LSPRs) and surface plasmon resonances (SPRs), play a crucial role in this broadband absorption. We also explored the role of the dielectric spacer, and experimentally showed that the absorption bands may be shifted dramatically by adjusting the spacer thickness, e.g. increasing the thickness from 100 nm to 160 nm resulted in a change of reflected peak wavelength from 450 nm to 650 nm, respectively. The unique combination of wafer-scale broadband absorption and a dynamically tunable spacer suggests a path forward to create new active, plasmonic devices.

Recently, many researches have been investigated on oxide semiconductor materials as alternative substance to silicon for various applications such as thin film transistor (TFT), photosensor, gas sensor, etc. It’s because they have superior electrical characteristics, compatibility with flexible substrates, high transparency, high field effect mobility, and low off-state current compared to amorphous silicon. Among the applications, photosensor is attracting attention because it can be applied in various fields such as health care industry, intelligent display, and smartphone. However, the photosensor with oxide semiconductor has some issues that the wavelength range of the detectable light is limited due to its wide bandgap. (>3 eV). Therefore, there are various studies about oxide semiconductor materials based photo transistors to detect visible light by stacking additional absorption layer composed of quantum dot, organic materials, and metal nanoparticle. However, they have disadvantages of high fabrication process, poor uniformity, and vulnerability to external environment.
In addition, some papers researched about the phototransistors with the only oxide semiconductor by controlling the sub-gap states in the oxide semiconductor to improve the light absorption characteristics in the visible light region (400-700 nm). They applied zinc tin oxide (ZTO) or indium zinc oxide (IZO) which have relatively higher oxygen related defects compared to indium gallium zinc oxide (IGZO). These oxide semiconductors act both channel layer and light absorption layer so that oxygen related defects could aggravate electrical characteristics and stability. By these problems, the phototransistor with oxide semiconductor cannot operate as photosensor device for a long time. Additionally, oxygen related defects mainly affect deep trap site, so the wavelength range of absorbable visible light by the oxide semiconductor is limited. Because of these factors, further studies are needed to absorb the entire range of visible light including red light. In this paper, we proposed indium-gallium-zinc-oxide (IGZO) phototransistors with simple fabrication process of stacking solution processed oxide light absorption layer (SAL). The phototransistor with the SAL was designed to have superior electrical and optical characteristics by separating the role of channel layer and light absorption layer. We generated sub-gap states in bandgap of SAL by low annealing temperature resulting in improvement of light absorption in visible light region (400 ~ 700 nm). The SAL was composed of IGZO which was similar with IGZO channel layer. The low annealing temperature process affect two type of defects, oxygen related defects and organic residue defects. The mechanism of visible light absorption by the SAL was verified by comparing the two type of defects, oxygen related defects and organic residue, and results of optical characteristics of the phototransistor with the SAL. These sub-gap states caused SAL to absorb visible light despite the wide bandgap of IGZO (~3.0 eV). As a result, IGZO phototransistor with SAL has superior light absorption characteristics such as high photoresponsivity of 127 A/W and photosensitivity of ~106 in red (635 nm) and green (532 nm) light region.

Spectral upconversion is a promising approach to circumvent the sub-bandgap transparency of single-junction solar cells by converting longer wavelength sunlight into high energy photons that can be readily absorbed. However, low intensity of one-sun illumination at relevant wavelengths of spectral upconversion inherently limits their practical application in photovoltaics. Here we present a composite module of GaAs solar cells that can provide meaningful enhancement of their one-sun photovoltaic performance by additionally capturing sub-bandgap photons via plasmonically enhanced spectral upconversion¹. Ultrathin GaAs solar cells with a specialized epitaxial design are integrated on an upconversion medium containing NaYF₄: Er³⁺, Yb³⁺ upconversion nanocrystals (UCNC), coated on a plasmonic reflector composed of hole-post hybrid silver nanostructure. The solar-to-electric conversion efficiency of GaAs solar cells on a UCNC-incorporated plasmonic substrate is improved by ~6.4% (relative) and ~11.8% (relative), respectively, compared to those on a nanostructured silver reflector without UCNC and on a plain silver reflector with UCNC, which is attributed to the combined effects of local electric-field amplification to enhance the absorption of UCNC, augmented upconverted emission via coupling into radiative modes, waveguided light concentration, as well as photon recycling.
¹ Chen et al, ACS Photonics, DOI: 10.1021/acsphotonics.8b01245

We demonstrate a new paradigm for large-scale fabrication of metal nanoparticles on crystalline oxide nanostructures for surface enhanced Raman spectroscopy (SERS). A single-cell vapor-solid deposition was introduced to yield high-throughput SnO₂ nanostructures with flexible control of size and geometry. Subsequent high-density Au nanoparticles with three-dimensional coverage on the oxide nanostructures were achieved through nucleation control in Volmer-Weber growth mode using physical vapor deposition at elevated temperatures. Au nanoparticles on SnO₂ nanostructures operating in two different “hot spot” modes exhibit the remarkable sensitivity of detecting trace concentrations molecules. This strategy also enables surface modification to alter hydrophilic to super-hydrophobic surfaces with contact angle over 150°. The manifestation of high-density metal nanoparticle formation presents a route to develop multifunctional nanostructures.

Recently, oxide semiconductors have attracted attention as new materials that can be widely used due to the outstanding characteristics of oxide semiconductor based devices, such as high mobility, low off current, and transparency compared to amorphous silicon based devices. Owing to these characteristics, oxide semiconductors are applied for a lot of electronic devices such as sensors and thin film transistors. In spite of this versatility, however, there is a limit to use oxide semiconductors as a phototransistor for detecting visible light region due to the high band gap energy (> 3 eV) of oxide semiconductors. Since the light of visible region has an energy of about 2 eV, the oxide semiconductor having high energy band gap cannot absorb the visible light. In order to overcome such disadvantages, researches have been conducted to fabricate visible light phototransistors by depositing a visible light absorption layer using quantum dots, nanowire, and 2D materials but there are problems such as complicated material synthesis process and difficulty in large area application.
In this study, indium–gallium–zinc oxide (IGZO) based visible light phototransistors were fabricated using a selenium passivation layer (Se PVL) as a visible light absorption layer. Selenium (Se) has photoconductive properties and exhibits conductive property only when light is irradiated. The Se PVL could be easily deposited by thermal evaporation process without any post treatment. It was confirmed that the IGZO phototransistors with Se PVL had the highest photoresponse characteristic at the Se PVL thickness of 150 nm. The IGZO phototransistors with Se PVL exhibited high optoelectronic characteristics such as photoresponsivity of 303.12 A/W, photosensitivity of 6.86 x 10⁸, and detectivity of 5.18 x 10¹² Jones under 635 nm light illumination. Se showed the characteristics of p-type semiconductor and formed a large difference of valance band maximum in energy band alignment with IGZO layer. Therefore, electrons generated through absorption of visible light in Se PVL could be easily transferred to IGZO and affect electrical characteristics, while holes are not easily migrated due to valance band energy difference, so that recombination is prevented. The IGZO phototransistors with Se PVL exhibited excellent durability due to constant I_illuminated and I_dark in time dependent photoresponse characteristics of over 8000 s. It was also confirmed that Se PVL plays a role of passivation through slight threshold voltage shift improvement in positive bias stress of 10000 s.

In this work, we present a new approach to demonstrate rewritable, light-driven recordings in polydimethylsiloxane (PDMS) by manipulating the light-induced photobleaching phenomenon [1]. The selectively photobleached red dyes turned transparent, thereby creating patterned letters that emit different colors such as blue or green, from their backgrounds. Furthermore, the diffusion of dye molecules in the bleached region promoted a self-healing characteristic that could erase the patterned letters, which could enable the rewriting of different letters using the same PDMS. Using the proposed approach, we could pattern multi-color-emitting PDMS by taking advantage of the light-induced photobleaching phenomenon that is usually regarded as an undesirable effect in microscopy research. The introduction of this novel way to realize rewritable, light-driven technology is highly meaningful because current approaches have been based on photochromic materials, which require a complex synthesis process. Using the conventional-dye-incorporated PDMS and photobleaching phenomenon for the reversible color changes would be an alternative approach to resolve the remaining challenges. We believe that this novel concept may find potential applications in many research fields, such as secure printing and optical storage devices.
[1] S. Song, H. Takezoe and S. M. Jeong, J. Mater. Chem. C, 6, 10704 (2018).

Light trapping and the broadband absorption of the solar radiation are important for photonic and nanophotonic applications ranging from sensing to harvesting of the solar energy. For example, appropriate light trapping supports the realization of ultra-thin photovoltaic cells with enhanced efficiencies1-3. Efficient light trapping was demonstrated with surface arrays of subwavelength structures such as nanopillar (NP) arrays, nanocone arrays, etc4. In the current study we numerically explore light trapping based on surface arrays of subwavelength trumpet non-imaging light (NLC) concentrators (henceforth, trumpet arrays)5. Non-imaging optics was formulated in the regime of geometrical optics in the early 1970s. There are various members to the NLC family such as light cone (LC) NLC, paraboloid NLC, compound parabolic concentrators (CPC) and its derivatives, etc. The trumpet NLC (or hyperboloid NLC) is an important NLC with an ideal concentration ratio as it accounts for both meridional rays as well as skew rays.
We use finite-difference time-domain (FDTD) electromagnetic calculations to examine light trapping and broadband absorption of the solar radiation for laterally infinite cubic-tiled substrate-less silicon trumpet array under normal illumination. The absorptivity spectra of trumpet arrays are characterized by strong absorption peaks, some of which are just below the Yablonovitch limit, which is solely attributed to efficient
occupation of the array Mie modes. We show that the absorption enhancement at the near infrared is an order of magnitude higher than that of optimized NP arrays. We show superior broadband absorption of the solar radiation in trumpet arrays (with unoptimized geometry) compared with that of optimized nanopillar arrays (~26% enhancement). We show that low reflectivity is governed by modal excitations at the upper part of the trumpets (which is also supported by the weak dependency of the reflectivity on the array height), whereas the transmissivity is governed by modal excitation at the lower part of the trumpets. We show that the strong absorption peaks of trumpet arrays are governed by the interplay between reflectivity and transmissivity, and the corresponding excitations, which is tuned by adequate selection of the trumpet bottom diameter. The higher optical absorption in trumpet array is governed by low transmissivity, in contrast with nanopillar array in which the absorption is governed by low reflectivity6.
References
1. James R. Maiolo III, Brendan M. Kayes, Michael A. Filler, M. C. P. & Michael D. Kelzenberg, Harry A. Atwater, and N. S. L. High aspect ratio silicon wire array photoelectrochemical cells. J. Am. Chem. Soc. 129, 12346–12347 (2007).
2. Brongersma, M. L., Cui, Y. & Fan, S. Light management for photovoltaics using high-index nanostructures. Nat. Mater. 13, 451–60 (2014).
3. Ingram, D. B. & Linic, S. Water Splitting on Composite Plasmonic-Metal/Semiconductor Photoelectrodes: Evidence for Selective Plasmon-Induced Formation of Charge Carriers near the Semiconductor Surface. J. Am. Chem. Soc. 133, 5202–5205 (2011).
4. Fountaine, K. T., Cheng, W.-H., Bukowsky, C. R. & Atwater, H. a. Near-Unity Unselective Absorption in Sparse InP Nanowire Arrays. ACS Photonics 3, 1826–1832 (2016).
5. Prajapati, A., Chauhan, A., Keizman, D. & Shalev, G. Approaching the Yablonovitch limit with free-floating arrays of subwavelength trumpet non-imaging light concentrator driven by extraordinary low transmission. Nano Energy Under review, (2018).
6. Shalev, G., Schmitt, S., Brönstrup, G. & Christiansen, S. Maximizing the ultimate absorption efficiency of vertically-aligned semiconductor nanowire arrays with wires of a low absorption cross-section. Nano Energy 1–9 (2015). doi:10.1016/j.nanoen.2015.01.048

Neutral transparent solar cells (TSCs) are highly desirable for building integrated photovoltaics and automobile applications. Transparency and photovoltaic conversion efficiency (PCE) have a trade-off relationship in the photovoltaics. Achieving high performance of TSCs while maintaining transparency is therefore extremely challenging. Here, we report the use of light absorption engineered Si microwire array – transparent polymer composite films (SiMPF) as free-standing form to fabricate transparent hybrid solar cells with neutral color and high efficiencies. The controllable spacing between microwires enables to tune transparency of the devices. Moreover, the slanted SiMPF by a solvent-assisted wet etching exhibits excellent anti-reflection property while maintaining the transmittance of the devices compared to the flat ones. Finite-difference time-domain simulation reveals that electromagnetic losses are minimized and slanted SiMPF exhibited enhanced electric field distribution, encouraging the light-matter interaction. Finally, we apply p-type conductive polymer on the top and transparent conductive oxide on the bottom of the SiMPF, enabling us to fabricate neutral-colored semitransparent solar cells. Such devices demonstrate ~ 8 % power conversion efficiency for average visible transparencies of almost 10% retaining color-neutrality. These slanted SiMPF platforms are shown to have a significant advantage beyond the trade-off relationship between transparency and PCE, making them ideal TSCs.

Large-scale perfect light absorption in visible and near-infrared (VIS-NIR) spectrum are vital for efficient solar energy harvesting technologies. A wide range of metamaterial absorbers have been demonstrated, but they are limited by issues such as their high manufacturing cost, narrow band absorption, and incident angle sensitivity. We show a facile method to fabricate wafer-scale Ag-Sb2S3 nanoporous plasmon enhanced absorbers on plastic, crystalline and glassy substrates. This method is non-lithographic, and wet chemistry-free that exploits the immiscibility of Ag and Sb2S3 to control the surface morphology of the nanoporous structure[1]. Experimental results and numerical modelling show that the nanoporous structure has a high absorptance >80% over a wide spectral range from 300 to 2400 nm[1]. The high absorptance of the proposed Ag-Sb2S3 nanoporous structure is attributed to the excitation of Ag plasmon resonances and semiconductor bandgap absorption[1]. The high absorptance is independent of the polarisation state of the light and insensitive to the angle of incidence[1], which is a substantial advantage over many other metamaterial absorbers. We have also found that the Ag-Sb2S3 nanoporous structure is an effective absorber on both flexible and rigid substrates[1]. The simple room temperature co-sputter deposition method ensures that the Ag-Sb2S3 nanoporous structure can be grown on substrates such as flexible plastics, glass and ceramics, and other substrates at an industrial scale. In addition, these Ag-Sb2S3 plasmonic absorbers have other interdisciplinary applications including biosensing, which will also be introduced in this presentation[2].

[1] W. L. Dong, T. Cao, K. Liu, R. E. Simpson, Nano Energy 2018, 54, 272.
[2] K. V. Sreekanth, W. L. Dong, Q. L. Ouyang, S. Sreejith, M. ElKabbash, C. T. Lim, G. Strangi, K. T. Yong, R. E. Simpson, R. Singh, Acs Applied Materials & Interfaces 2018, 10, 34991.

Acknowledgements
This work was supported by Singapore China Joint Research Program (JRP) with grant number 2015DFG12630 from the International Science & Technology Cooperation Program of China and grant number 1420200046 from the Singapore Science & Engineering Research Council (SERC). T.C. acknowledges support from Program for Liaoning Excellent Talents in University (Grant no. LJQ2015021). W.D. is grateful for her SUTD President's Graduate Fellowship.

Two-dimensional (2D) van der Waals materials have emerged as a very attractive class of optoelectronic material due to the unprecedented strength in its interaction with light. In this talk I will discuss approaches to control this interaction by integrating the 2D materials with microcavities, and metamaterials. I will first discuss the formation of strongly coupled half-light half-matter quasiparticles (microcavity exciton-polaritons) in in the 2D transition metal dichacogenide (TMD) systems and approaches to optically/electrically control them [1-3]. The possibility to enhance the nonlinear optical response of the polariton states by exploiting the higher order Rydberg states in TMDs will also be discussed. We will also present our recent work on chiral metasurfaces to address and route the valley excitons in 2D TMDs. Finally, we will discuss the realization of room temperature quantum emitter array using strain engineered hexagonal boron nitride [4] and coupling them to high quality factor resonators.
[1] X. Liu, et al., Nature Photonics 9, 30 (2015)
[2] Z. Sun et al., Nature Photonics 11, 491 (2017)
[3] B. Chakraborty et al., Nano Lett. 18, 6455 (2018)
[4] N. Proscia, et al. Optica 5, 1128 (2018).

The isolation of stable atomically thin two-dimensional (2D) materials on arbitrary substrates has led to a revolution in solid state physics and semiconductor device research over the past decade.1 A variety of other 2D materials (including semiconductors) with varying properties have been isolated raising the prospects for devices assembled by van der Waals forces.2 A fundamental challenge in using 2D materials for opto-electronic devices is enhancing their interaction with light, ultimately responsible for higher performance and efficiency in the devices. In particular, for photovoltaics; inorganic materials (e.g., Si, GaAs and GaInP) can concurrently maximize absorption and carrier collection. But thin film absorbers have lacked the above ability often due to due to surface and interface recombination effects. In contrast, Van der Waals semiconductors have naturally passivated surfaces with electronically active edges that allows retention of high electronic quality down-to the atomically thin limit. In this seminar, I will show our recent work on photovoltaic devices from transition metal dichalcogenides of molybdenum and tungsten such as MoS2, WSe2 etc. We have recently demonstrated near-unity absorption in the visible part of the electromagnetic spectrum in < 15 nm films of these semiconductors by placing them on reflecting metal substrates such as gold and silver. We have further shown that these highly absorbing, ultrathin films can be further used for fabrication of simple Schottky junction photovoltaic devices with microfabricated metallic top contacts. 3 While, this work helps solve the light-absorption problem, the external quantum efficiency EQE was < 10% for our Schottky junction devices. Very recently, we have extended this early work to fabricate p-n heterojunctions from p- WSe2/n-MoS2 and use graphene as a transparent top contact to amplify our current collection efficiency and push the EQE up to 50%,4 approaching that of many emerging photovoltaic technologies with active layers in the 100s of nm range. This represents a significant development as both light-absorption and charge collection have been addressed in these devices.
Finally, I will then present ongoing work on addressing the key remaining challenge for application of 2D materials and their heterostructures in high efficiency photovoltaics which entails engineering of interfaces and open-circuit voltage. in addition I will present our recent developments on using nanophotonic scanning probes to perform, high-resolution near-field optoelectronic spectroscopy on 2D semiconductors.6 I will conclude by giving a broad perspective5 on 2D materials for mainstream as well as specialty applications as well as highlight some other novel applications of combining nanophotonic concepts with two-dimensional semiconductors. 1. Jariwala, D. et al. ACS Nano 2014, 8, 1102–1120. 2. Jariwala, D. et al. Nat. Mater. 2017, 16, 170-181. 3. Jariwala, D. et al. Nano Lett. 2016, 16, 5482-5487. 4. Wong, J.; Jariwala, D. et al. ACS Nano 2017, 11, 7230–7240. 5. Jariwala, D.et al. ACS Photonics 2017, 4, 2962-2970. 6. Jariwala, D. et al. 2D Materials, 2018, 5, 035003

Graphene has gained enormous research interest over the past two decades due to its exceptional electronic and optical properties. A single atomic layer of carbon atoms with the honeycomb structure has a universal optical absorption around (πα) ~ 2.3% that is defined by the fine structure constant α. This is scientifically unprecedented for materials, but is too small for useful optoelectronic applications. Here we report the production of nearly perfectly light absorbing graphene nanostripes (GNSPs) from a single-step, high-yield, plasma enhanced chemical vapor deposition (PECVD) method using an aromatic precursor (Dichlorobenzene) that enables vertical growth of graphene nanostripes on substrates. The quality of the GNSPs was evaluated using Raman spectroscopy, EDS and XPS techniques to confirm the absence of any dopant, and UPS measurements of the work function at ~ 4.45 eV further confirmed the purity of GNSPs was comparable to pristine graphene. The optical absorption spectra of vertically grown GNSPs revealed nearly perfect absorption (>~ 99.9%) over the visible spectral range from 400 nm to 900 nm. The mechanism for nearly complete light absorption may be understood from the gapless nature of graphene that can absorb all wavelengths through carrier excitations and the presence of multiple internal light scattering between the vertical sheets of graphene nanostripes that are stacked in a turbostratic fashion (i.e., rotationally disordered). To investigate whether GNSPs, being a good light absorber, may also be a good light emitter, we investigated the photoluminescence (PL) of GNSPs using a confocal laser scanning Light Sheet Microscope (LSM 880). GNSPs were transferred from the growth copper substrates by either mechanical exfoliation onto a glass substrate or liquid phased exfoliation using N-methyl-pyrolidone (NMP). We observed a broadband nonlinear emission in the visible spectral range for continuous wave (CW) laser excitation wavelengths at 405nm, 458nm, 488nm, 514nm, 561nm, 594nm and 633nm. The nonlinear nature of emission was manifested by the experimental observation of a blue shifted emission tail above the energy of the excitation laser. Under the same laser power, the emission spectrum was found to redshift with decreasing laser excitation energy, and the emission intensity also decreased substantially with decreasing laser excitation energy, showing ~ 5 times weaker emission intensity for excitation wavelength at 633nm relative to that for excitation wavelength at 405nm. While emission for a given laser excitation energy only appeared above a threshold laser power, strong light emission was already observed for laser powers as low as ~ 0.03mW, which was much stronger than most conventional fluorophores. These preliminary findings suggest that the mechanism of light emission may be qualitatively attributed to thermal emission of hot electrons that interacted with strongly coupled optical phonons (SCOP) and topological defects (e.g., the Stone–Wales defects as confirmed by TEM and AFM) in GNSPs, although more systematic and quantitative studies are still necessary to elucidate the underlying physics. All in all, the extraordinary light absorption and photoluminescence found in PECVD-grown GNSPs suggest that GNSPs are promising for use as an active layer in high efficiency photovoltaic devices and tunable light emitting devices.

In this talk we discuss our recent effort in the context of hybrid metasurfaces formed through nanophotonic engineering metasurfaces and 2D materials, reporting our recent theoretical and experimental results in the context of hyperbolic plasmon propagation and embedded eigenstates. During the talk we will discuss their highly unusual light-matter interactions and potential opportunities for nanophotonic devices.

Solid state quantum emitters are a mainstay of quantum nanophotonics as integrated single photon sources (SPS) and optical nanoprobes[1,2]. Integrating such emitters with active nanophotonic elements is desirable in order to attain efficient control of their optical properties but typically degrades the photostability of the emitter itself[2]. In our group, we have developped optomechanical[3] and optoelectrical[4] approaches to either tune energy and decay rate of single photon sources. In this talk, I will present recent experiments[4] that demonstrate a tuneable hybrid device which integrates lifetime-limited single emitters (linewidth 40 MHz) and 2D materials at sub-wavelength separation without degradation of the emission properties. Our device’s nanoscale dimensions enable ultra-broadband tuning (tuning range > 400 GHz) and fast modulation (frequency 100 MHz) of the emission energy, which renders it an integrated, ultra-compact tuneable SPS. Conversely, this offers a novel approach to optical sensing of 2D material properties using a single emitter as a nanoprobe. present a new type of hybrid system, consisting of an on-chip graphene NEMS suspended a few tens of nanometres above nitrogen-vacancy centres (NVCs), which are stable single-photon emitters embedded in nanodiamonds
References
[1] Benson. Assembly of hybrid photonic architectures from nanophotonic constituents. Nature 480, 193 (2011)
[2] Moerner et al. Single-Molecule Optical Detection, Imaging and Spectroscopy. (Wiley, 2008).
[3] Reserbat-Plantey, A. et al. Nature Communications. 7, 10218 (2016).
[4] Schädler et al. submitted. (2018)

While graphene is a promising material for novel photonic devices due to its broadband optical absorption, ultrafast carrier dynamics and electrical tunability [1], the quantum efficiency of graphene devices is intrinsically limited by low absorption of graphene (2.3% of normal incident light). To enhance light-matter interaction, optical focusing elements such as plasmonic metal nanoparticles (NP) can be utilized [2]. Here, we report our recent results on the graphene plasmonic NP hybrid structures. Femtosecond pump−probe measurements of graphene nearby plasmonic gold nano-disc structures confirm the presence of a strong near-field interaction leading to hot carrier generation in the graphene; however, the results suggest that the hot carriers arise dominantly from direct photoexcitation in the graphene with a minimal contribution from charge transfer from the gold [3]. Next, asymmetric plasmon-nanobar electrical contacts are employed with a view to create an electronic temperature gradient across a homogeneous graphene channel, which results in photothermoelectric (PTE) current generation [4]. At certain conditions, the plasmon-induced PTE photocurrent can be directly isolated. In this regime, the device effectively operates as a sensitive electronic thermometer providing access to plasmon induced local carrier temperature which is estimated to be of the order of 2000 K. These results are of particular importance for development of hot carrier based plasmonic devices.

[1] T. Mueller et al. Nat Photonic 4, 297 (2010);
[2] Z. Fang et al. Nanoletts 12, 3808 (2012);
[3] Gilbertson, A. M. et al. Nano Lett. 2015, 15, 3458–3464.
[4] Shautsova et al. Nat Com. 2018 (accepted)

Over the past decade, significant progress in controlling light-matter interactions at the nanoscale has been achieved. Most of the advances, however, have relied on static systems limited to the as-fabricated or as-synthesized metal nanostructures. This talk will discuss how the ability to tune plasmonics responses reversibly may address key challenges in future nanoscale optics. We will highlight how the fabrication and scalability of responsive nano-optical substrates can be used in diverse applications from stretchable nano-lasing to reconfigurable lensing and imaging.

Optical absorption in nanowire arrays is intensively explored to enhance performance of photovoltaic devices. At normal incident light, vertically standing nanowires act as optical nano-antennas and show up resonant optical modes in their absorption spectra at special wavelengths due to their comparable dimensions to optical wavelengths. To achieve high performance photovoltaic devices, near-unity absorption spectra of nanowires are necessary and are gained through optimizing geometrical parameters of the nanowires [1]. Conventionally, longer nanowires or denser arrays of nanowires (smaller pitch) are used to approach near-unity absorption of the excited optical modes at the expense of larger material volume of nanowires. However, employing light trapping mechanisms in nanowires are the alternatives to enhance absorption of the excited optical modes without increasing the material volume [2], reducing the total cost. The light trapping through increasing optical path length of the optical modes improves absorption of an excited optical mode in constant material volume, enhancing absorption efficiency due to both reducing light reflection at the interface of air/nanowires, and reducing light transmission at the interface of nanowires/substrate. In this study, to achieve improved unselective near-unity absorption spectra in nanowires, we take advantage of light trapping mechanisms in nanowires through embedding distributed Bragg reflectors (DBRs) in the bottom as well as in the top part of InP nanowires, respectively. Our calculations show that by employing the DBRs of only two periods of InGaAs/InP, an unselective absorption spectrum is obtained at normal incidence. At the presence of oblique light incidence, the integration absorption efficiency enhances up to about 85% at 50^○ compared to 79% at normal incidence. Increasing the period number of the DBRs from two to five, this value increases even upper to about 95% [3]. The improvement is due to decreasing light transmission of the excited optical modes from the nanowires into the substrate. The results are expected to be valid for other direct bandgap III-V semiconductor materials. Taking advantage of DBRs to enhance optical absorption in nanowires offers a great potential for novel high-performance photovoltaic applications.


1. M. Aghaeipour et al., opt. Express 22, 29204-29212 (2014)
2. E Garnett and P. Yang, Nano let. 10, 1082-1087 (2010)
3. M. Aghaeipour and H. Pettersson, Nanophotonics 5, 819-825 (2018).

Quantum dot antennas have potential to increase the light trapping efficiency and concentration factor for planar luminescent solar concentrators. We report here on quantum dot antennas designed for a fabricated tandem luminescent solar concentrator (LSC) that consists of a Si bottom cell and a top cell LSC with highly efficient CdSe/CdS quantum dot (QD) isotropic luminophores evenly dispersed in a poly(laurylmetacrylate) (PLMA) waveguide, optically coupled to a micro-array of InGaP top cells. The QDs absorb light in in 300-500 nm wavelength range and emit luminescence radiation at 635 nm, a wavelength matching the band edge of the InGaP cells.
In previous LSC top cell designs, wavelength-selective mirrors have been used to spectrally reflect and trap the radiated QD emission within the waveguide. We compare the light trapping and concentration factor for planar waveguide designs employing Bragg filter top mirrors and metasurface bottom mirrors with quantum dot antennas. Metasurface mirrors are formed from a high index (n>3) material patterned as a hexagonal array of cylindrical pillars of subwavelength thickness onto a glass (n=~1.5) substrate. This LSC is integrated with a Si bottom cell to form a 4 terminal tandem with a maximum theoretical efficiency of 29.4% with a projected tandem module cost of $81.30/m².
A limiting factor in current LSC designs is related to the isotropic emission of the QDs. The radiated emission can be lost through the escape cone or be parasitically lost within the waveguide. By forming quantum dot antennas, consisting of QDs coupled directly to the pillars of the metasurface bottom mirror, we can induce highly directional, oblique anisotropic, QD emission, reducing escape cone losses, and thus potentially eliminating the need for a spectrally-selective top mirror. We simulate quantum dot antennas composed of aluminum antimonide, silicon, and silicon carbide and find a maximum achievable efficiency of 32.2% with a lower projected tandem module cost of $63.87/m².

The efficiencies of state of the art single-junction solar cells are approaching the Shockley-Queisser limit (c-Si, GaAs); in contrast, their thicknesses are far from the theoretical limits and could be reduced by more than one order of magnitude with efficient light-trapping. In this work, we first present a benchmark of recent advances of ultra-thin solar cells (c-Si, CIGS, GaAs) using the short-circuit current as a function of the absorber thickness. We show that current state of the art solar cells operate close to single-pass absorption and we highlight the different light-trapping strategies proposed in the literature to approach the Lambertian limit. We then introduce our strategy for efficient light-trapping based on multi-resonant absorption and we apply these concepts to CIGS and GaAs ultra-thin solar cells. The goal is to reduce the thickness of the semiconductor absorber by one order of magnitude while preserving the short-circuit current. This study is not only pertinent from an academic viewpoint, but is of practical relevance for example for CIGS manufacturers for reducing material consumption and time deposition or for space power applications, where ultra-thin solar cells based on III-V semiconductors outperform long-term efficiency and power production of thicker cells due to their intrinsically higher radiation tolerance.
The light-trapping architecture we propose is based on a periodic nanostructured TiO₂/silver back-mirror optimized to couple diffraction modes inside the solar cell absorber. Using RCWA electromagnetic simulations, we design a 150 nm-thick CIGS solar cell with a nanostructured back/contact-mirror. We predict a short-circuit current up to 36.3 mA/cm², a potential efficiency of 20% using realistic FF and V_OC, and we present our roadmap for implementing these concepts in an industrial CIGS solar cell fabrication process [1].
We then apply a similar strategy to the fabrication of a ultra-thin 205 nm-thick GaAs solar cell featuring localized ohmic contacts and a periodic nanostructured back-mirror. The GaAs solar cell is epitaxially grown by MOCVD at Fraunhofer ISE and transferred on a nanostructured TiO₂/silver mirror at C2N-CNRS. The nanostructured mirror is fabricated using a low-cost and scalable technique based on direct Nanoimprint of TiO₂ sol-gel derived film [2]. For the first time we experimentally demonstrate an effective absorption enhancement exceeding the theoretical double-pass absorption (J_SC= 24.6 mA/cm² under 1 sun AM1.5G illumination) while preserving FF and V_OC, resulting in a certified state of the art efficiency of 19.9%. A detailed loss analysis is conducted and provides a pathway for reaching 25% conversion efficiency in ultra-thin GaAs solar cells using the same light trapping scheme [3].

[1] J. Goffard et al. “Light trapping in ultrathin CIGS solar cells with a nanostructured back mirror”, IEEE Journal of Photovoltaics 7, 1433 (2017)
[2] A. Cattoni et al, “Degassing-Assisted Paterning of hybrid and inorganic films”, submitted
[3] C. Hung-Ling et al “A 19.9%-efficient ultrathin 205nm-thick GaAs Solar Cell with a Silver Nanostructured Back Mirror”, in preparation

Nanosized structures introduce new ways for manipulating light injection, coupling and trapping at the subwavelength scale. A number of nanoscale light trapping structures that can efficiently control and enhance light absorption have been reported such as, inverted nanopyramids¹, nanowires², quasi-random patterns³, and nanocones⁴, mainly focusing in the optical domain, with very few modifications. The famous nano-structure in this study area is known as “black silicon” exhibiting nearly zero reflection over a broad solar spectrum range. The complexity of these structures comes when are integrated into real devices. The challenging problem is not to compromise the optical enhanced character of silicon by the electrical losses such as the recombination at the solar cell surfaces. Since black silicon devices exhibit large surface areas, minimising surface recombination is mainly difficult. There are reports on high efficient black silicon solar cells thus far have yet not been able to compete with conventional silicon devices. Jihun and the co-workers have reported an 18.2% efficient all back contact (ABC) black silicon device⁶. They find that the dominant recombination process can be reduced by careful monitoring of the doping profile of the structured surface. They report that efficiency limitation is mainly due to Auger recombination whereas surface recombination only dominates at low doping density profiles. In contrast, Hele Sevin et al. report a 22.1% black silicon IBC device by passivating the surface recombination through the deposition of a conformal layer of alumina⁷.
Here, we demonstrate the fabrication of quasi-random nanophotonic structures that have similar optical properties as black silicon and integrate them by both wet and dry processing. We also fabricate state-of-the-art light trapping structure, micron-sized pyramids and realize into silicon solar cell. We present a quantitative comparison of these nanophotonic structures both in the electrical and optical domain, evaluating the impact of plasma dry etching by comparing the same nanostructures made of wet chemical etching. Furthermore, it is an interesting study to understand whether the taper walls of the wet etched structure improves the antireflective (AR) property compared to the vertical walls realized by dry etching. Ultimately, we also show a detailed comparative study of micro vs nano, structures to address the question of whether nanostructuring to improve the optical performance of solar cell devices without compromising their electrical efficiency.
1. Zhou, S. et al. Wafer-Scale Integration of Inverted Nanopyramid Arrays for Advanced Light Trapping in Crystalline Silicon Thin Film Solar Cells. doi:10.1186/s11671-016-1397-6
2. Garnett, E. & Yang, P. Light trapping in silicon nanowire solar cells. Nano Lett. 10, 1082–7 (2010).
3. Martins, E. R. et al. Deterministic quasi-random nanostructures for photon control. Nat. Commun. 4, 2665 (2013).
4. Wang, K. X., Yu, Z., Liu, V., Cui, Y. & Fan, S. Absorption enhancement in ultrathin crystalline silicon solar cells with antireflection and light-trapping nanocone gratings. Nano Lett. 12, 1616–1619 (2012).
6. Oh, J., Yuan, H.-C. & Branz, H. M. An 18.2%-efficient black-silicon solar cell achieved through control of carrier recombination in nanostructures. Nat. Nanotechnol. 7, 743–748 (2012).
7. Savin, H. et al. Black silicon solar cells with interdigitated back-contacts achieve 22.1% efficiency. Nat. Nanotechnol. 10, 1–6 (2015).

Since metal nanomaterials have strong electromagnetic fields at the surface, photodynamic therapy in the near-infrared light (NIR) region has been investigated widely for biomedical applications. Among them, most light-triggered nanomaterials have been designed for anticancer treatment, but there were few studies to improve antibacterial properties. To achieve multiple functions into one nanosystem, it is critical to synergize several properties together. Therefore, in the present study, different gold, silver, and selenium nanoparticles have been synthesized with tunable sizes and morphologies under varying temperature and pH values. Compare to nanospheres, rod and branched structure have indicated enhanced antibacterial effects. Moreover, gold and selenium nanoparticles have demonstrated the good biocompatibility to normal cells. Furthermore, NIR light has been used to mediate reactive oxygen species (ROS) production and control the photothermal therapy. After studying the growth curve, colony counting, and bacteria live/dead assay, both Gram-positive bacteria [Staphylococcus epidermids] and Gram-negative bacteria [Pseudomonas aeruginosa] have been reduced significantly using the synergized nanomaterials. Therefore, a novel NIR photoactive nanosystem has been developed for antibacterial applications.

Increasing access to clean drinking water is a global sustainability goal¹. Solar energy can disinfect water through the generation of reactive oxygen species (ROS), but current methods utilize ultraviolet (UV) light—only 5% of the solar spectrum—and make use of particulate suspensions which must be separated from the water post-disinfection. Visible-light-driven ROS generation is a promising alternative, but immobilized photocatalysts with strong visible light absorption, high quantum efficiency, and proper redox energy positions are necessary.

Here, we report on the fabrication and characterization of metal-semiconductor-metal plasmonic photocatalysts for visible-light-driven ROS generation. Our plasmonic metamaterials are supported on glass and composed of Au nanoparticles deposited on ZnO and TiO2 films, with an Al sub-layer for optical reflection and plasmonic coupling. These structures are broadband absorbers which capture 30% of the solar spectrum centered around 600 nm, while simulations show 90% absorbance is achievable through improved fabrication. We discuss the enhancement of visible-light absorbance and hot-carrier excitation through our design.

We test the ability of our plasmonic photocatalysts to generate ROS in 20 mL batch systems under UV filtered AM1.5G illumination. We will present potential wavelength-dependent reaction pathways for ROS generation and explore the impact of aqueous redox conditions on the production of individual ROS (i.e., H₂O₂, ●OH, and O₂^-). Hot-carrier dynamics are inherent to the redox chemistry responsible for ROS generation. We use ultrafast transient absorption spectroscopy to probe hot-electron transfer and show that our Al-backed photocatalysts produce longer exciton lifetimes than those without Al. Lastly, we will discuss the performance and stability of our plasmonic photocatalysts, and the improvements necessary to realize real-world application.

References:
1) World Health Organization. 2012. Progress on Drinking Water and Sanitation. Geneva, Switzerland: WHO Press.

Optically excited plasmonic nanostructures display remarkable electron dynamics in the form of coherent electron displacement motion, as well as generation of non-thermal ‘hot electrons’ with kinetic energy substantially greater than kT. In contrast to traditional methods of photothermalization, photo-excited non-thermal ‘hot electrons’ can be efficiently utilized through thermionic power conversion or quantum tunneling (Wu, S. and M. T. Sheldon (2018). "Optical Power Conversion via Tunneling of Plasmonic Hot Carriers." ACS Photonics 5(6): 2516-2523.)

Here, a thermionic power convertor fabricated from gold nanostructures is systematically tested. Periodic patterned gold nanostructures are fabricated through electron beam lithography. With laser excitation at different incident power and wavelength, the corresponding thermionic emission current is collected at a paired ITO anode. Short circuit current J_sc and open circuit voltage V_OC provide a unique way to pinpoint the effective electronic temperature of the ‘hot electron’ distribution as well as its relative population compared to thermalized electrons. The wavelength and geometry dependence study reveal detailed insight into the light-matter interaction and electron-phonon coupling dynamics. Additionally, the experimentally observed electronic temperature of the non-thermal electron gas is also supported by anti-Stokes Raman thermometry recently developed in our group.

Nanoscale assembly coupled with competing anisotropic interactions can produce complex structures and provide additional degrees of freedom for tailoring their collective properties. However, the on-going researches are highly limited by the poor quality of building blocks, inaccessibility of direct observation of crystal structures and lack of reliable numerical methods for analyzing their dynamic anisotropic interactions. In this presentation, we report magnetic assembly of super uniform magnetic nanorods into responsive body-centered tetragonal (bct) photonic crystals. The assembled structures are fixed during assembly by silica coating and directly characterized under electron microscopy, which suggests a body-centered tetragonal (bct) crystal structure. Finite-element calculations show that two magnetic nanorods approach thermodynamic equilibrium when the tangent components (orthogonal to the surface normal) of their magnetic forces is zero, showing good agreement with structures from TEM images of the assembled crystals. This method provides reliable analytical approach towards understanding the anisotropic interactions between building blocks. Thanks to the pretty good orders of the crystals, well-defined diffraction peaks are observed during SAXS measurements, which is consistent with the bct structures obtained by TEM images and provides detailed information about assembly kinetics. In combination with high packing density, the assembled photonic crystals exhibit extremely bright structural colors, whose intensity overwhelms our other tunable photonic crystals. In addition, the diffraction peaks and structural colors can be continuously tuned across visible range by varying the directions of magnetic field. We also extend the assembly strategy to other morphologies, like nanocubes and nanoplates. In the case of nanocubes, they tend to assemble along [110] direction (edge to edge) as determined by the interplay of the shape and magnetic anisotropic interactions. In case of nanoplates, however, 2D photonic crystals are more favorable due to the symmetry breaking along the directions orthogonal to assembly axis (direction of external magnetic fields).

Plasmonic materials have been extensively studied because of their broad applications in areas such as chemical and biological sensing, bioimaging, and solar energy conversion. So far major research efforts have been limited to noble metals particularly gold and silver due to their considerably large scattering and absorbance cross-sections as well as high chemical stability. However, these noble metals have limited earth abundance, making their large scale application impossible. Although much desired, little progress has been made to the development of non-noble-metal-based plasmonic materials due to their weak plasmonic response and/or low chemical stability against oxidation. In this presentation, we first analyze theoretically the plasmonic characteristics of copper using FDTD solution, and demonstrate that copper nanostructures can be made highly active in surface plasmon resonance if their resonance mode can be shifted to infrared to avoid the overlap with the intrinsic interband transition. Practically, this can be achieved by adopting an anisotropic shape such as plate and rod. The challenge however is the synthesis of copper nanostructures with well-defined anisotropic shapes using wet chemical approaches. In the second part of the presentation, we report a seed-mediated templating growth method for the preparation of copper nanorods with highly controllable dimension and excellent size/shape uniformity. The resulting copper nanorods show surface plasmon resonance in infrared with intensity comparable to or better than silver and gold counterparts. With the protection of a crosslinked polymer layer, the nanorods also display considerably high stability against oxidation. Lastly, by taking advantage of their excellent photothermal conversion efficiency and high stability, we integrate the copper nanorods into the fabrication of shape-memory polymer nanocomposite and further demonstrate the construction of smart microrobots whose mechanical movement can be fully controlled by the illumination of light.

We have developed digital assembly of a variety of hybrid nanomaterials. With their complex architectures, enhanced light-matter interactions and novel functionalities, in combination with computer control, low-power consumption and simple setup of our digital assembly, these designer nanomaterials are finding a wide range of applications in photonics, electronics and medicine.

We report significantly improved UV-VIS photodetection properties from n-silicon nanowires (SiNWs)-amorphous Titanium dioxide (TiO₂) based hybrid heterojunction fabricated in the ambient environmental conditions. Our UV-VIS photodetectors have various ranges of applications in commercial, research and military fields. We observe fast rise/decay time of 0.23µs/0.17µs and high responsivity of 7-25 A/W in the 300-700nm wavelength range with very small external bias. The large surface area due to the nanowires leads up to 2.5 orders of magnitude enhancement of photosensitivity. The device fabrication part was carried out through low-cost, all solution-based methods. Our SiNWs/TiO₂ heterojunction photodetectors could be potential for weak signal detection in the UV-VIS range in various fields.

The use of quantum dots, QDs, in optoelectronic devices holds great promise towards the development of novel optoelectronic devices.¹ These colloidal nanocrystals benefit from inexpensive synthetic and device fabrication techniques, such as roll-to-roll processing, that greatly decrease the device cost compared to traditional epitaxial and lithographic methods. Furthermore, the unique photophysics of QDs can enable solar cells with efficiencies beyond the Shockley-Queisser limit and photodetectors.^2,3 However, the efficiency of these devices essentially relies on the light absorption properties of QDs. As sub-wavelength dielectric nanoparticles, QDs intrinsically feature low absorption cross sections. Increasing this figure of merit is therefore desirable to enable new applications.
Here, we demonstrate a general approach to increase the absorption efficiency of QDs. We have recently shown how to drive the assembly of QDs into close-packed, spherical superstructures exhibiting face-centered cubic order, QD supercrystals.⁴ By combining electron microscopy, absorption and scattering measurements of single supercrystals with optical simulations, we show that these supercrystals exhibit Mie resonances both in scattering and absorption. This photonic behavior leads to absorption efficiencies of QD supercrystals greater than unity in a wide spectral range in the visible. Finally, we investigate QD coupling in supercrystals via ultrafast spectroscopy, finding that QD supercrystals feature a transition from bound to free biexciton as the interparticle distance decreases.

(1) Kagan, C. R.; Lifshitz, E.; Sargent, E. H.; Talapin, D. V. Science 2016, 353, aac5523.
(2) Saran, R.; Curry, R. J. Nature Photonics 2016, 10, 81.
(3) Shockley, W.; Queisser, H. J. Journal of applied physics 1961, 32, 510.
(4) Marino, E.; Kodger, T. E.; Wegdam, G. H.; Schall, P. Advanced Materials 2018, 30, 1803433.