Symposium F.SF03—New Frontiers in the Design, Fabrication and Application of Metamaterials

Aluminum nanoparticles (ANPs) have been considered attractive additives for combustible applications because the ANPs possess a high energy density with an increased burning rate, owing to an increased surface area-to-volume ratio.¹ Unfortunately, use of the ANPs is limited by two factors: (1) ANPs can be readily sintered; and (2) ANPs can be easily oxidized, prior to the combustion process due to their high reactivity, degrading the combustion performance. To resolve this issue, synthesis of metamaterials, using ANPs and hydrocarbon has been suggested. For example, ANPs could be effectively coated by hydrocarbon precursors to improve its combustion performance.^2,3 The previous studies reported that the hydrocarbon-coated ANPs are less reactive at low temperatures and they became susceptible to the oxidation at higher temperatures, suggesting that the hydrocarbon coating is essential for the ANPs to be used as combustible materials.^4,5 However, a molecular-level understanding of thermal behaviors of the hydrocarbon-coated ANPs has yet to be achieved. Here we perform reactive molecular dynamics (RMD) simulations based on ReaxFF⁶ to investigate effects of hydrocarbon coating on the combustion process of the ANPs including sintering and oxidation processes. Our RMD simulations reveal molecular-level reaction steps for the combustion process of the bare/hydrocarbon coated ANPs. As such, our RMD simulations will help guide an experimental design of a novel energetic metamaterial using ANPs and hydrocarbon, providing a valuable input for the community of solid-fuel propellants.

S.H. and R.P. acknowledge a start-up funding from School of Natural Science, Mathematics and Engineering at California State University, Bakersfield (CSUB). A.M. acknowledges a funding support from Student Research Scholar (SRS) program at CSUB.

References

[1] Yetter, R. A.; Risha, G. A.; Son, S. F. Metal Particle Combustion and Nanotechnology. Proc. Combust. Inst. 2009, 32, 1819−1838
[2] Park, K.; Rai, A.; Zachariah, M. Characterizing the Coating and Size-Resolved Oxidative Stability of Carbon-Coated Aluminum Nanoparticles by Single-Particle Mass-Spectrometry. J. Nanopart. Res. 2006, 8, 455−464.
[3] Guo, L.; Song, W.; Hu, M.; Xie, C.; Chen, X. Preparation and Reactivity of Aluminum Nanopowders Coated by HydroxylTerminated Polybutadiene (Htpb). Appl. Surf. Sci. 2008, 254, 2413−2417.
[4] Park, K.; Rai, A.; Zachariah, M. Characterizing the Coating and Size-Resolved Oxidative Stability of Carbon-Coated Aluminum Nanoparticles by Single-Particle Mass-Spectrometry. J. Nanopart. Res. 2006, 8, 455−464.
[5] Hong, S. and van Duin A. Atomistic-Scale Analysis of Carbon Coating and Its Effect on the Oxidation of Aluminum Nanoparticles by ReaxFF-Molecular Dynamics Simulations. J. Phys. Chem. C. 2016, 120, 9464−9474
[6] Senftle, T. P.; Hong, S.; Islam, M. M.; Kylasa, S. B.; Zheng, Y.; Shin, Y. K.; Junkermeier, C.; Engel-Herbert, R.; Janik, M. J.; Aktulga, H. M. The ReaxFF Reactive Force-Field: Development, Applications and Future Directions. npj Comput. Mater. 2016, 2, 15011.

Aluminum nanoparticles (ANPs) have attracted a great amount of interest owing to their high energy density, high reaction rate, earth abundance, and low toxicity.¹ ANPs have been considered effective energetic materials. Unfortunately, ANPs are easily oxidized at room temperature, leading to oxide layers on their surfaces. Those oxide layers work as a dead weight. Therefore, it is vitally important to design a novel metamaterial that improves the combustion performance of ANPs. Recent studies show that adding graphene oxide and graphene fluoride improves the combustion performance of ANPs.^2.3 This is mainly due to a synergistic effect between graphene sheets and ANPs. For example, oxygen molecules could be easily dissociated when they get closer to both graphene sheets and ANPs. As such, we hypothesize that graphene-supported ANPs, which could be considered novel energetic metamaterials, provide a remarkable combustion performance. Here we simulate the oxidation of graphene-supported ANPs using reactive molecular dynamics (RMD) simulations based on ReaxFF.⁴ Our RMD simulations reveal atomic-level reaction mechanism for the improved oxidation process of graphene-supported ANPs. As such, the RMD results will help guide an experimental design of a novel energetic metamaterial using graphene sheets and ANPs, providing a valuable input for the community of energetic metamaterials.

S.H. and R.P. acknowledge a start-up funding from School of Natural Science, Mathematics and Engineering at California State University, Bakersfield (CSUB). A.M. acknowledges a funding support from Student Research Scholar (SRS) program at CSUB.

References
[1] Yetter, R. A.; Risha, G. A.; Son, S. F. Metal Particle Combustion and Nanotechnology. Proc. Combust. Inst. 2009, 32, 1819−1838
[2] Jiang, Y.; Deng, S.; Hong, S.; Zhao, J.; Huang, S.; Wu, C.-C.; Gottfried, J. L.; Nomura, K.-i.; Li, Y.; Tiwari, S.; Kalia, R. K.; Vashishta, P.; Nakano, A.; Zheng, X. Energetic Performance of Optically Activated Aluminum/Graphene Oxide Composites. ACS Nano 2018, 12, 11366−11375.
[3] Jiang Y.; Deng, S.; Hong, S.; Tiwari, S.; Chen, H.; Nomura, K.; Kalia, R. K.; Nakano, A.; Vashishta, P.; Zachariah, M.; Zheng, X. Synergistically Chemical and Thermal Coupling between Graphene Oxide and Graphene Fluoride for Enhancing Aluminum Combustion. ACS Appl. Mater. Interfaces 2020, 12, 7451−7458
[4] Senftle, T. P.; Hong, S.; Islam, M. M.; Kylasa, S. B.; Zheng, Y.; Shin, Y. K.; Junkermeier, C.; Engel-Herbert, R.; Janik, M. J.; Aktulga, H. M. The ReaxFF Reactive Force-Field: Development, Applications and Future Directions. npj Comput. Mater. 2016, 2, 15011.

Topological phases of matters, firstly investigated in condensed matter physics, have been extended to other wave physics such as photonics. In this poster presentation, we present spin-valley locking of edge states in a chiral photonic crystal in the two-dimensional honeycomb lattice. Using a coupled dipole method, we demonstrate bulk band structure with a band gap, which is spanned by the spin-valley locked edge states. The staggered chirality breaks the inversion symmetry and the chirality provides the spin-valley locking characteristics.

The use of aluminum nanoparticles (ANPs) in combustion applications has attracted a great amount of attention owing to their high energy density with an increased burning rate.¹ Unfortunately, due to the nature of ANPs’ high reactivity, the ANPs are easily sintered, leading to an increase in their particle size, and thus lowering the combustion efficiently. In order to tackle this issue, recent efforts have been made to synthesize novel metamaterials, using ANPs and hydrocarbon precursors. For example, ANPs could be effectively coated by hydrocarbon precursors to lower their reactivity, thus preventing sintering effects of ANPs. However, an effect of precursor types of hydrocarbon on sintering behaviors of ANPs has yet to be understood at the atomic level. Here we perform reactive molecular dynamics (RMD) simulations based on ReaxFF² to investigate computational synthesis of energetic metamaterials using ANPs and carbon precursors. In particular, we employed different types of carbon precursors to evaluate the sintering behaviors of ANPs. Our RMD simulations suggest detailed reaction steps for the sintering process of the bare/hydrocarbon coated ANPs. As such, our RMD simulations will help provide a valuable input for experimental synthesis of a novel metamaterial using ANPs and hydrocarbon for combustion application.

S.H. and R.E. acknowledge a start-up funding from School of Natural Science, Mathematics and Engineering at California State University, Bakersfield (CSUB).

References

[1] Yetter, R. A.; Risha, G. A.; Son, S. F. Metal Particle Combustion and Nanotechnology. Proc. Combust. Inst. 2009, 32, 1819−1838
[2] Senftle, T. P.; Hong, S.; Islam, M. M.; Kylasa, S. B.; Zheng, Y.; Shin, Y. K.; Junkermeier, C.; Engel-Herbert, R.; Janik, M. J.; Aktulga, H. M. The ReaxFF Reactive Force-Field: Development, Applications and Future Directions. npj Comput. Mater. 2016, 2, 15011.

Pyrolitic 3D-printed carbon nano-ceramics currently emerge as a class of lightweight materials with exceptional mechanical strength and stiffness, being characterized by feature-sizes well below the capabilities of the highest-resolution 3D additive manufacturing techniques alone. Despite the increasing applications, their use is hampered by the lack of knowledge on their mechanical reliability. Few studies have investigated their intrinsic mechanical properties: important characteristics, like hardness and fracture toughness, and their dependency on environmental conditions, which can be relevant for a thorough understanding of the complex mechanical behaviour of additively manufactured pyrolitic carbon structures, are unknown.
In this work, we present nanoindentation splitting of TPP-DLW-derived pyrolitic carbon micro-pillar specimens, characterized by a diameter of 5.3 µm and a hight-to-diameter ratio of two, as a straight-forward method to measure the fracture toughness and nanoindentation continuous stiffness measurements for the assessment of the elastic modulus and hardness. Micro-pillars with and without SEM detectable surface flaws, absent of STEM-detectable internal defects, were tested at two extremal intervals of the environmental relative humidity, humid air was used to adjust RH at values greater than 60% and dry nitrogen at room pressure ensured RH levels below 5%, to investigate the impact of pyrolysis-induced surface flaws and the interplayed effects with the testing environment on the mechanical reliability of the 3D printed nanoceramics. We show that TPP-derived pyrolytic carbon can achieve 500% increased fracture toughness over macroscopic forms of vitreous carbon, which typically lies in the range of 0.5-1.4 MPam^0.5, with values up to 3.1 MPam^0.5. However, experiments at RH > 60% revealed a detrimental effect of high relative humidity environments on the fracture toughness of the pyrolitic carbon nano-ceramic structures, causing embrittlement due to liquid diffusion ascribed to the presence of only a few, nanometer-sized, which probably promotes crack propagation under external loading due to chemically induced structural weakening of the glassy carbon structure. While comparable effects are less relevant in macro-size ceramics, this study demonstrates that reliability and durability of micro- and nano-architected ceramic metamaterials and devices require toughening design approaches which focus on size-dependent surface effects. The observed humidity effect can have a remarkable impact on the scale-up of such nano-architected metamaterial in a wide range of applications, especially biomedical and micro-fluidics fields.

Low-e glass panes are not well adapted for use in regions where the predominant heat that passes through windows has its origin indirect solar radiation. Almost 49% of solar energy is distributed in the spectrum from 0.7-2.5μm for which low-e glass-pane reflectivity is below 10%. This is the case with 14% of the contiguous U.S. according to NREL. The metamaterial that we propose is a robust, thin-film structure that has the ability to reflect near-infrared heat radiation (NIR) with λ 0.7-10μm while transmitting more than 90% of the visible light intensity. The ability to reflect NIR and transmit selected wavelength range is obtained through the field-enhanced electron concentration of moderately doped metal oxide thin films. Approximately 48% of the total energy produced in the U.S. or 18.6 billion kWh is spent on heating and cooling. Therefore, the ability to retrofit this technology onto existing buildings is of considerable interest. Electrical and optical characterization demonstrate the synthesis of state-of-the-art materials with properties suitable for the fabrication of smart glass devices.

We will use some examples to illustrate that quasi-1D systems which have very simple structures can give rise to interesting phenomena that are similar to those observed in topological materials.
A unique and very useful property of topological materials is the existence of topological edge modes which are guaranteed to exist by the topological invariants of the bulk through a bulk-edge correspondence. These topological edge modes or surface states support one-way transport that cannot be interrupted by disorder, and such robustness is probably the most sought after property of topological materials. The topological materials usually require a careful design and many have complex structures. Here, we give one example in which many topological properties of one-way edge modes can be realized in very simple quasi-1D photonic waveguides subjected to simple boundary conditions. These properties include the robustness of transport against imperfections and the ability to go around sharp corners. It appears that under some specific conditions, a certain boundary condition can emulate the existence of a material which carries a gap with some specific non-trivial topological characteristics. The same principle work for acoustic waves. The design principle also allows us to realize waveguides which have desirable characteristics such as being single-mode and have linear dispersion, in addition to being robust against disorder.
In the quest for topological materials, momentum space nodal points such as Weyl and Dirac points have attracted much attention. Realizing such nodal points require 3D/2D structures. However, some interesting nodal points can also be obtained using simple quasi-1D systems. For example, we will see that nexus points of nodal lines can be realized in 1D stacks of ordinary dielectric material layers with misaligned optical axes. Hidden symmetries give rise to interesting band crossings and new type of multifold nodal points. We will see that a fractional periodicity of different components of the permittivity tensor can manifest hidden symmetries of the Maxwell’s equations. The hidden symmetries cannot be described using the standard space group theory. Such systems carry a new kind of protected band crossings (Kramers-like nodal lines) which are topological features unique to photonic systems. Such systems carry triply degenerate nodal points as the nexuses of several nodal lines enforced by the hidden symmetry, which can be viewed as a new kind of magnetic monopoles terminating Berry flux strings in the momentum space. In the vicinity of the triple nexus points, the iso-frequency surfaces has spin-1 Dirac-like conical dispersion. This implies that the nexus point can serve as a new platform for probing spin-1 physics. Breaking the hidden symmetry give rise to Type-II and Type-III nodal rings in photonic systems.

Abstract
Offering enhanced SNR, contrast, and spatial resolution, ultra-high field (UHF, B₀ ≥ 7T) MRI has shown its potential for clinical diagnostics, such as evaluation of strokes, Alzheimer’s, and Parkinson’s disease [1-4]. However, at the Larmor frequency corresponding to B₀ above 7 Tesla, the effective wavelength of the B₁⁺ field becomes smaller than the dimension of the head, leading to the spatial phase variation and inhomogeneity of B₁⁺ fields inside. For the mitigation of this detrimental B₁⁺ inhomogeneity creating uneven contrast and the variation of SNR in the UHF MRI image, various approaches have been investigated, such as parallel transmission techniques (PTx), high permittivity material (HPM) structures, and metasurfaces. While PTx improves B₁⁺ homogeneity in the whole brain, hardware complexity, and the need for real-time SAR assessment prevents its clinical 7T practice [5,6]. Structures with metallic inclusions, such as metasurfaces and hybridized meta-atoms (HMAs) have also been reported to manipulate field profiles at subwavelength scales [7,8], but often resulted in deterioration of the global B1+ homogeneity over the ROI.
In this talk, we propose and present an inverse-design of metasurfaces addressing the global B₁⁺ homogeneity over the region of interest (ROI) in the whole axial planes. Approximating MRI environments as a 2D system governed by the Helmholtz equation, we inversely obtain permittivity distributions of cylindrical metasurfaces enclosing the head at proximity. With the metasurface constructed of cylindrical HPMs of different permittivity, stacked along the superior direction, it is numerically confirmed that the proposed metasurface excite the target fields near the surfaces and ROI in both approximated and real MRI environments. When tested with an actual anatomical head model (MIDA), it is found that the CV (coefficient of variation) and Max-to-min ratios of B₁⁺ in the ROI are reduced on average by 40% and 33% respectively, along with SAR reduction in the head by 34% (average) and 44% (peak). Since the B₁⁺ distribution itself is globally homogenized, the proposed structure is expected to be most useful in spin-echo based imaging in UHF MRI, such as T2-weighted imaging and diffusion-weighted imaging in the whole brain.

References
[1] G. Barisano, et al., The British Journal of Radiology, vol. 92, no. 1094, p. 20180492, 2019.
[2] P. Balchandani, et al., American Journal of Neuroradiology, vol. 36, no. 7, pp. 1204–1215, 2014.
[3] M. Inglese, et al., Expert Review of Neurotherapeutics, vol. 18, no. 3, pp. 221–230, 2018.
[4] B. R. Knowles, et al., Clinical and Translational Radiation Oncology, vol. 18, pp. 87-97, 2019.
[5] C. M. Deniz, et al., Topics in Magnetic Resonance Imaging, vol. 28, no. 3, pp. 159–171, 2019.
[6] O. Kraff, et al., Journal of Magnetic Resonance Imaging, vol. 46, no. 6, pp. 1573–1589, 2017.
[7] R. Schmidt, et al., Scientific Reports, Vol. 7, no. 1, 1678, 2017.
[8] M. Dubois, et al., Physical Review X, vol. 8, no. 3, 031083, 2018.

Nanoparticle supercrystals (NPSCs) are of great interest as materials with new emergent properties. The emergent properties of NPSCs through collective interaction between nanoparticles provide new opportunities for a wide range of applications such as plasmonics, photonics, electronics, magnetics, and catalysis. Different types of intermolecular interactions such as hydrogen bonding, van der Waal’s interaction, and electrostatic interaction have been used for fabricating NPSCs of various crystal symmetries and lattice parameters. However, the limited structural stability of NPSCs fabricated by the intermolecular forces still remains a major hurdle for realizing their potential applications.
The covalent bonding, which shares electrons to form stable organic molecules of various structures, is the most representative intramolecular force much stronger than the intermolecular forces such as van der Waals and hydrogen bonding interactions. Recently, the precision and versatility of covalent chemistry has been successfully established through covalent bonding of various large crystal structures such as metal-organic frameworks and covalent organic frameworks. Considering all these successes, covalent bonding interaction can be an excellent choice for fabricating more stable NPSCs. However, the covalent bonding interaction has not been successfully explored as a main driving force for the formation of highly ordered NPSCs yet.
Here, we report a new method to fabricate highly stable and highly ordered three-dimensional NPSCs by using covalent bonding interaction in conjunction with the slow solvent evaporation process, and demonstrated using isotropic and anisotropic building blocks. In this method, the NPSCs stabilized by covalent bonding are highly stable in solvents of different polarity ranging from water to toluene as well as dried states and at temperature up to 160 °C. Considering the diverse combined choices of covalent bonding reactions, ligand molecules with different chain lengths and physicochemical properties, and nanoparticles may allow the designed synthesis of NPSCs for potential applications. In addition to high strength and the large library of covalent chemistry, the low cost of organic ligands molecules used in this method makes the NPSCs induced by covalent bonding interaction highly versatile and economic, providing new opportunities for the fabrication and applications of NPSCs.

This work suggests an adjustable index of hydrogenated-amorphous silicon for obtaining high-efficiency metasurface in the visible by modulating chemical equilibrium of deposition. Hydrogenated-amorphous silicon has been widely used for metasurface since it has cost-effective, CMOS-compatible fabrication methods. Also, hydrogenated-amorphous silicon exhibit a high index ~3.0 at the visible frequency, which induces optical property modulation with geometrical structures on thin film. Based on these advantages, many practical metasurface have been reported such as metalenses, meta-hologram, and structural color filters. However, hydrogenated-amorphous silicon suffers from low-efficiency in high-frequency range because it has a bandgap tail in the visible. The bandgap tail is known to be the main obstacle to be transparent materials, so high-efficient metasurface is limited by inherent material properties of silicons. Here, this work explores atomic structure networks of hydrogenated amorphous silicon to modulate complex refractive index, which enables high-efficiency metasurface in the visible spectrum. The adjustability of the refractive index achieved with varying deposition conditions of plasma-enhanced chemical vapor deposition, which affect bonding states and their hydrogenation. It is revealed that substrate temperature and atmospheric pressure are the main factor to change bonding states and hydrogenation. By changing the deposition conditions, atomic structures were verified, and it is confirmed by X-ray diffraction and transmission electron microscopy. Substrate temperature and atmospheric pressure are the main factor to change bonding states and hydrogenation. Optimized pressure creates micro/nano crystallinity in hydrogenated silicon, which reduces the extinction coefficient. Another parameter affects hydrogen contents, which reduce dangling bonds of silicon. These phenomena change the bandgap tail of silicon, it induce high refractive index coverage when chemical equilibrium is manipulated. As a result, the refractive index can modify between 2.6 and 4.2; the extinction coefficient can be lower than 0.1 at the wavelength of 450 nm. Also, the adjusted hydrogenated-amorphous silicon has near-zero absorption at the wavelength of 532 nm and 635 nm, which is comparable to normal transparent materials (TiO2, GaN, etc). Furthermore, conversion efficiency for visible range metasurface was calculated with an optimized complex refractive index, resulting in 96.6%, 90.9%, and 64.7% at the wavelength of 635 nm, 532 nm, and 450 nm, respectively. Those results can alter materials of metasurface from conventional, transparent dielectrics (TiO2, GaN, etc) to hydrogenated amorphous silicon. Also, the high adjustability of the complex refractive index of silicon with simple steps can be useful for low-cost fabricatable, versatile metasurfaces. At last, the experimental demonstration was also conducted to verify high-efficiency metasurface composed of optimized materials. Optimized refractive index with suitable geometrical parameters achieves 64% at the wavelength of 635 nm, 65% at the one of 532 nm, and 42% at one of 450 nm, which have been not reported yet. Considering the simple fabrication steps with adjustability compared to other dielectrics, hydrogenated amorphous silicon will be a promising material for the practical application of metasurfaces.

The fast development of nanophotonics urges breakthrough concepts and design principles to create functional devices for diverse optical applications. In particular, the advent of metasurfaces has revolutionized nanophotonics, enabling a variety of opportunities. Metasurfaces, which comprise subwavelength optical antennas in ultrathin layers, identify a class of flat optical devices with exceptional control over the propagation of light.[1] Despite the exciting progress, a specific research focus - dynamic optical metasurfaces,[2] which probably also provide the most important link to the real-world optical applications, still awaits endeavors. They allow for active manipulation of light beams and enable fascinating optical functions, including dynamic beam steering, focusing and shaping, as well as optical vortex generations, imaging, and sensing. However, the reported schemes have been mostly limited to simultaneous tuning of the resonance frequencies or amplitudes of all the metasurface antennas.[3, 4] They also suffer from poor device control with low intensity modulation and narrow operating band. Thus, there is still plenty of room to advance this research focus, especially at visible frequencies.
We demonstrate a strategy to realize dynamic optical metasurfaces by tailoring their spatial frequencies via modulation of both the geometric and propagation phases at visible frequencies. In particular, we showcase electrically-controlled digital metasurface devices (DMSDs) for light projection displays. The DMSD consists of metasurface pixels in an M × N array. Each metasurface pixel contains gold nanorods arranged in a rectangular lattice. In some preselected (odd or even) columns, the nanorods are covered with a dielectric material. The sample is subsequently encapsulated in a thin liquid crystal (LC) cell. The dynamic function of the metasurface pixels is enabled by electrically controlling the relative phase between the neighboring odd and even columns via LCs on the millisecond time scale. In principle, each metasurface pixel can be designed to generate a specific dynamic holographic pattern in the far field. Particularly for light projection display applications, each metasurface pixel that can be electrically switched between ‘1’ and ‘0’ states is individually addressed by an electrode, forming an anomalous reflection spot that can be turned on and off with high contrast in the projection screen. As a result, programmable images are dynamically generated and displayed. [5]
References
Yu, N. et al. Light Propagation with phase discontinuities: generalized laws of reflection and refraction. Science 334, 333–337 (2011).
Shaltout, A. M., Shalaev, V. M. & Brongersma, M. L. Spatiotemporal light control with active metasurfaces. Science 364, eaat3100 (2019).
Li, S.-Q. et al. Phase-only transmissive spatial light modulator based on tunable dielectric metasurface. Science 364, 1087-1090 (2019).
Zubritskaya, I., Maccaferri, N., Inchausti Ezeiza, X., Vavassori, P. & Dmitriev, A. Magnetic Control of the Chiroptical Plasmonic Surfaces. Nano Lett. 18, 302-307 (2018).
J. X. Li, P. Yu, S. Zhang, and N. Liu, Electrically-controlled digital metasurface device for light projection displays. Nature Communications 11, 3574 (2020).

Plasmonic structural colors generated by metasurfaces have emerged as feasible alternatives to traditional pigment-based printing. Despite extensive studies, producing structural colors from nanostructured metasurfaces in metal-insulator-metal (MIM) configuration still presents some challenges. One major hurdle is pixel fabrication that does not rely on expensive lithographic methods but can still controllably produce saturated and bright colors. We report highly reflective cyan, magenta and yellow (CMY) color pixels based on silver metasurfaces, using a wide-area fabrication technique suitable for upscaling. Through numerical simulations we demonstrate that the colors achieved are as a result of resonances more characteristic of Fabry-Pérot than gap- surface plasmon modes due to the thick spacer layer.

Flat optics based on metasurfaces holds promise for the design of a new class of diffractive optical component that circumvents the limits of refractive as well as Fresnel optics, in terms of control of aberrations, compactness and multifunctionality. Here we highlight the enhanced functions enabled by metasurface flat optics in structuring light. Optical elements coupling the spin and orbital angular momentum (SAM/OAM) of light have found a range of applications in classical and quantum optics. Q-plate type devices based on dielectric metasurfaces have shown superior performance in terms of singularity definition and ability to generate fractional and high OAM beams. A far more general spin to orbital angular momentum scheme has been recently demonstrated based on novel metasurface optical elements (J-plates) not limited to conjugate converted states, which are used to generate new complex structured light. The J-plate, with J referring to the photon’s total angular momentum (TAM), is a metasurface device that imparts two arbitrary OAM states on an arbitrary orthogonal basis of spin states. When these J-plates are cascaded in series, they can generate several single quantum number beams and versatile superpositions thereof. Moreover, in contrast to previous spin-orbit-converters, the output polarization states of cascaded J-plates are not constrained to be the conjugate of the input states. Cascaded J-plates are also demonstrated to produce vector vortex beams and complex structured light, providing new ways to control TAM states of light
Metasurfaces hold major potential for multi-functionality, which may play a pivotal role in the next-generation compact nanodevices. OAM from lasers holds the promise for compact, at-the-source solutions to fuel applications from imaging to communications . However, conjugate symmetry between circular spin and opposite helicity OAM states from conventional spin-orbit (SO) approaches, as well as low purity modes, has meant that complete control of light’s angular momentum from lasers has remained elusive. Recent work on metasurface-enhanced lasers that overcomes this limitation will be discussed, including the demonstration of new high-purity OAM states with quantum numbers reaching l= 100, and non-symmetric vector vortex beams that lase simultaneously on independent OAM states as much as Δl = 90 apart. This laser conveniently emits in the visible, producing new OAM states of light as well as all previously reported OAM modes from lasers, offering a compact and power-scalable source that harnesses intra-cavity structured metasurfaces for the creation of arbitrary chiral states of structured light.
Finally, recent work on beams with light structured along the propagation direction will be presented, where the polarization or the OAM of the beam changes along the propagation direction.

Diffractive photonic devices manipulate light via local and nonlocal optical modes. Local devices, such as metasurfaces, can shape a wavefront at multiple selected wavelengths, but inevitably modify light across the spectrum; nonlocal devices, such as grating filters, offer great frequency selectivity but limited spatial control. In this talk, I will introduce a rational design paradigm using quasi-bound states in the continuum to realize multifunctional nonlocal devices: metasurfaces that produce narrowband spatially tailored wavefronts at multiple selected wavelengths and yet are otherwise transparent.
I will show an experimental demonstration of dielectric metalenses that focus light only for narrowband resonances and leave the rest of the spectrum unaffected. Our resonant metasurfaces can be cascaded to achieve multifunctional behavior with each metasurface independently shaping the wavefront for a distinct resonance resonant wavelength.
These devices may expand the capabilities of multifunctional meta-optics to include active or nonlinear wavefront shaping. Scaled to visible wavelengths, our resonant metalenses may prove useful for augmented reality applications as compact and highly transparent multi-color see-through lenses.

Structural requirements have pushed lightweight materials with advantageous material properties to the forefront of engineering practice. As a result, it has become necessary to enhance the breadth of design options with a new generation of lightweight materials offering high strength, high stiffness, and high energy absorption capacity. The accelerated progress of additive manufacturing has enabled the fabrication of intricate architectures which are used today to design new materials. These materials, also called architected materials, exhibit mechanical properties which are dictated primarily by their pattern rather than their composition and therefore the design of their architectures has received great attention. The fabrication capabilities gained from additive manufacturing has opened up a new space for design exploration based on these architectural forms. Until today, there have been numerous architected lattice materials with extraordinary properties such as negative Poisson’s ratio, negative stiffness, multi-stability, negative thermal expansion and others. Very recently, a new generation of architected materials has emerged in the form of plate-lattice materials in which plates are patterned in different configurations to create a cellular geometry. Recent findings have shown that plate-lattice materials are not only significantly stiffer and stronger than truss-lattices, but also able to attain the theoretical Hashin Shtrikman and Suquet bounds for isotropic materials. The objective of this work is to explore the imperfection sensitivity of a variety of isotropic and anisotropic plate-lattice architectures of cubic symmetry, such as Simple Cubic (SC), Body-Centered Cubic (BCC), Face-Centered Cubic (FCC) and their isotropic combinations (SC-BCC and SC-FCC). Three-dimensional shell computational models with periodic boundary conditions are used to study the buckling behavior. Short and long wavelength buckling modes are observed by increasing the tessellation size of plate-lattices under axial compression for a range of relative densities between 0.05% (ultra-thin) and 25%. Geometric imperfections are incorporated in the model by varying imperfections of modal shapes. The main goal of this work is to provide accurate knockdown factors for plate-lattice materials with geometric imperfections and to characterize fundamental behavior of these plate-lattice metamaterials with a potential use in new generation structural applications.

Metasurfaces are composed of arrays of subwavelength meta-atoms, which have demonstrated many exciting opportunities to control the amplitude, phase and polarization of incident light. Metasurfaces with circularly polarized sensitivity has attracted wide attention, because of the manifestation of the photonic spin Hall effect (PSHE)[1]. Pancharatnam-Berry (PB) phase is the frequently used method to control the phase of circularly polarized light. However, most metasurfaces based on the PB phase only work for circularly polarized light with one specific spin, hindering the simultaneous and independent control of left-handed circularly polarized (LCP) light and right-handed circularly polarized (RCP) light [2].
In this talk, we demonstrate that utilizing both the PB phase and the propagation phase, we can rationally control the phase for each spin state of light [3]. The PB phase provides opposite phase modulation for LCP and RCP light, and it is determined by the in-plane rotation angle of meta-atoms [4,5]. In contrast, the propagation phase provides the same phase modulation regardless of the polarization of incident light and rotation angle of meta-atoms. Therefore, combing the PB phase and propagation creates a flexible way to realize simultaneous phase control of LCP and RCP light. As proof-of-concept demonstrations, here we numerically and experimentally realize the independent focusing and manipulation of both spins of light by V-antenna metasurfaces. Our multidimensional metalens is able to focus light of different spins at designated positions along both transverse and longitudinal directions. It can be used as a polarization analyzer to distinguish the polarization state of incident light. In addition, our multifunctional metalens can act either as a convex lens or an axicon, depending on the spin of light. The demonstrated multidimensional and multifunctional metalens has versatile potentials in spin-dependent nanophotonics, ranging from optical imaging and micro/nano-object manipulation to optical sensing.

Metamaterials represent the concept of utilizing artificial material building blocks to create desirable properties derived from three-dimensional layout and compositions. While novel topologies can now be realized via additive manufacturing, the lack of processable materials, multi-material gradients, speed and scalabilities have stymied its further adoption. In this talk, I will outline a suite of new material design and manufacturing routes, enabled by additive manufacturing of topologies, multi-scale features and multi-material cues. Attention is focused on how additive manufacturing techniques will enable processing the unprocessable, from structural composites to functional and stimuli-responsive multi-materials. This unleashes new design freedoms for rapid material property discovery and product realizations; where electrical, thermo, mechanical behaviors and their couplings can be inversely designed and tailored by an end-user at will. I will present a few examples, where structural, electronic and energy transduction materials are 3D architected into a compact form factor, without requiring multiple processing stages such as printing, embedding or wiring. Next, we will present their new applications, such as ultralight and strong materials, self-sensing materials, microsystems for robotics, air and maritime sensing, transducers, wave guiding and telecommunications.

Lightweight materials with tailorable properties have attracted attention for decades, yet stiff materials that can resiliently tolerate extreme forces and deformation, while being manufactured at large scales, have remained a rare find. Bio-inspired designs such as hierarchical foams and atomic-lattice-mimicking trusses have achieved optimal combinations of mechanical parameters but suffer from limited mechanical tunability, limited long-term stability, and low throughput volumes. Most mechanical metamaterials have relied on symmetry, periodicity, and lack of defects to achieve the desired mechanical response, resulting in sub-optimal mechanical response under the presence of inevitable defects. Shell-type designs, which can mitigate stress concentrations and flaws, have come with high densities and limited recoverability.

We exploit non-periodic ceramic shell architectures of ultralow density (reaching 4 mg/cm³) whose engineered curvature distribution achieves close-to-optimal stiffness scaling and, moreover, whose careful combination of topology, geometry and base material results in mechanical resilience superior to previous mechanical metamaterials. We present a method for scalable fabrication of these materials, guided by natural spinodal decomposition, which results in shell-based ceramic architectures with thicknesses down to ∼10 nm and overall sample volumes of up to a few cubic centimeters. We show the capability of these architectures to maintain more than 50% of their original stiffness and strength after ten cycles of compression up to 30% while exhibiting no visible permanent deformation. Guided by simulations, we show how controlling the material morphology leads to the (an)isotropic material response whose directional stiffness distribution, impressively, remains constant over a wide range of shell thicknesses, as demonstrated experimentally. Our approach highlights pathways to harness self-assembly methods in the design and scalable fabrication of novel metamaterials with both tunable high stiffness and unsurpassed recoverability, simultaneously unachievable by previously reported mechanical metamaterials.

Photonic crystals have revolutionized the field of photonics with their unique capabilities of dispersion and energy band gap engineering, such as the demonstration of extreme group and phase velocities, topologically-protected photonic edge states, and the control of spontaneous emission of photons. On the other hand, time-variant media have shown their distinct functionalities including nonreciprocal propagation, frequency conversion, and amplification of light. However, these phenomena in time-variant media have mostly been investigated for the cases where spatiotemporal modulation is in the form of a simple harmonic wave. Here, we extend the type of spatiotemporal modulation so that time-variant and spatially-discrete photonic crystal structures, or spatiotemporal crystals, can be analyzed. The design of spatiotemporal crystals allows the engineering of momentum band gap, within which the parametric amplification occurs. As a potential platform for the construction of a parametric oscillator, a finite-sized spatiotemporal crystal is proposed and analyzed. The parametric oscillation is initiated by the energy and momentum conversion of an incident wave and the subsequent amplification by parametric gain within the momentum band gap. The oscillation process dominates over frequency mixing interactions above a transition threshold determined by the balance between gain and loss. Furthermore, the asymmetric formation of momentum band gaps can be realized by spatial phase control of temporal modulation, which leads to the directional radiation of oscillations at distinct frequencies. The proposed structure would enable simultaneous engineering of energy and momentum band gaps and provide a guideline to the implementation of advanced dispersion-engineered parametric oscillators.

The design of new chiral materials usually requires stereoselective organic synthesis to create molecules with chiral centers. Less commonly, achiral molecules can self-assemble into chiral materials, despite the absence of intrinsic molecular chirality. Here, we demonstrate the assembly of high-symmetry molecules into a chiral van der Waals structure by synthesizing crystals of C₆₀(SnI₄)₂ from icosahedral buckminsterfullerene (C₆₀) and tetrahedral SnI₄ molecules through spontaneous self-assembly. The SnI₄ tetrahedra template the Sn atoms into a chiral cubic three-connected net of the SrSi₂ type that is held together by van der Waals forces. Our results represent the remarkable emergence of a self-assembled chiral material from two of the most highly symmetric molecules, demonstrating that almost any molecular, nanocrystalline, or engineered precursor can be considered when designing chiral assemblies.

Metamaterials have been of great interest in the field of photonics due to their ability to realize intriguing new properties unachievable in natural materials. Advances in metamaterial design and characterization have led to advancing from the microwave region to terahertz and optical frequency to realize artificial magnetism, negative refractive index, and superlenses. One of the most common elements in metamaterial design is the split ring resonator. While much work has been devoted to optically modeling and characterizing planar plasmonic SRR arrays, there has yet to be a robust strategy for synthesizing 3 dimensional SRR structures. Enhanced functionality should be possible by extending into the third dimension. In this study, we illustrate a nanoscale synthesis process which utilizes a hybrid of direct-write 3D nanoprinting and thin film deposition to fabricate free-standing split ring resonators for the investigation of 3D metamaterials. Focused electron beam induced deposition is used to deposit non-plasmonic 3D scaffolds, which are subsequently isolated with a conformal SiO₂ layer and coated with a plasmonic materials, specifically Au, to create functional 3D metamaterials. A variety of single coupled SRR structures were fabricated and low-loss electron energy loss spectroscopy was utilized to characterize their full plasmonic spectra with nanoscale resolution. Complementary electron discrete dipole approximation simulations were performed to elucidate the resultant consequent electric and magnetic field distributions. This work demonstrates the flexibility FEBID scaffolds offer for the advancement of new 3D devices for applications and fundamental studies of metamaterials.

The valley degree of freedom (DOF) has recently been introduced into photonic crystals as a new way to manipulate electromagnetic waves, similar to how electrons are manipulated in valleytronic materials. Compared to the photonic pseudospin that requires complex construction such as ring resonators and bianisotropic metamaterials, the valley DOF originates from the underlying lattice symmetry, which does not rely on strong spin-orbit coupling and thus can be easily integrated in a photonic crystal setup. In this talk, I will introduce some of our recent works on valley manipulation.

First of all, the topological valley edge states have been realized in many photonic crystals. We firstly implemented a valley photonic crystal at microwave frequencies based on a four band model, and demonstrated the spin-valley locking feature. A very interesting phenomenon is the perfect out-coupling of valley edge states because of the valley conservation at the zigzag interface.

Secondly, we constructed an unpaired Dirac point at a single valley. It is well known that a photonic graphene exhibits two Dirac cones at two valleys. Therefore, in order to probe features of 2D Dirac particles, special precautions must be taken to ensure that only one valley is excited. Depending on the system geometry, this is not always possible, because defects and interfaces can give rise to strong intervalley scattering. Here we will discuss the construction of an unpaired Dirac point at only one valley. Some unique photonic phenomena such as the nonreciprocal reflection and angular selectivity are discussed. This unpaired single Dirac cone is inspired by the famous Haldane model, where the inversion symmetry breaking and time reversal symmetry breaking compete to open a topological bandgap.

Thirdly, we found that another mechanism that can be utilized to manipulate valley is via loss and gain, or non-Hermiticity. By modifying the Haldane model, it can be seen that the loss/gain can participate in the completion between inversion symmetry breaking and time reversal symmetry breaking, and drive the Haldane-type topological transition. In particular, the loss/gain can displace the Dirac points in momentum space. So the modulation of loss/gain can induce a uniform effective magnetic field and thus Landau levels, similar to the effect of strain engineering, but without lattice deformation.

Finally, these valley photonic crystals have found device applications such as the on-chip communication and electrically pumped lasers.

Symmetry and topology are fundamental notions existing in all kinds of natural systems, from spiral galaxies and hurricanes to amino acids in molecules and non-trivial topologically protected electronic states in condensed matter. A stream of linearly polarized photons is typically topologically trivial, nevertheless, its full-vector nature intrinsically endows light with full capability of creating and carrying unique symmetry and topology. Most notably, the intrinsic capability of photonics of superposing non-Hermitian eigenstates through optical gain and loss provides a powerful toolbox for exploring the symmetry paradigms that were deemed challenging in condensed matter systems. Such investigations of symmetry and topology in photonics are revolutionizing the design principles in optical sciences. We have discovered new synthetic photonic materials, geometries, and field profiles that systematically enable the exploration of symmetry and topology in photonics. As such, the elusive symmetries can simplify the design of photonic meta-atoms and unit cells to reshape the density of states of photons in a topological and ultraflexible manner, which leads to on-demand meta-control of light emission, transmission, scattering and detection [1], thereby sustaining the ever-expanding information explosion for information processing, communication and computing.
For example, we have demonstrated one of the most striking benefits: the creation of eigenstate degeneracies such as exceptional points intrinsically associated with non-Hermitian Hamiltonians. This breaks the natural symmetry between reciprocal counterpropagating resonant modes in a micro-cavity to conduct unidirectional light-matter interaction carrying the large orbital angular momentum for structured light emission. Such a symmetry-driven approach allowed us to create the first microlaser source carrying the desired spin and orbital chirality as unique information carriers [2-4], giving rise to a new dimension of storing and processing information in photon’s chiral degree of freedom. In addition to expanding the information dimensions to address the upcoming information explosion, flexible reconfiguration of topological pathways can offer a completely new paradigm for high-density photonics routing. However, the desired topological robustness unfortunately diminishes the hope of topological tunability. To overcome this fundamental limitation, we perform non-Hermitian symmetry to enable a new kind of topological states into the bulk of the topological insulator with a uniform topology, for the first time [5]. Without “battling” against topological protection, the new non-Hermitian topological states facilitate robust transmission links in the entire footprint to route optical signal to any desired output port. The ports-to-footprint ratio is at least 2 orders of magnitude higher than the state-of-the-art.
Hence, our research addresses grand challenges in fundamental quantum physics and also enables entirely new optical sciences and photonic technologies, which advances and benefits both fields simultaneously.

L. Feng, R. El-Ganainy, L. Ge, Non-Hermitian photonics based on parity-time symmetry. Nat. Photon. 11, 752-762 (2017).
P. Miao et al. Orbital angular momentum microlaser. Science 353, 464-467 (2016).
Z. Zhang et al. Tunable topological charge vortex microlaser. Science 368, 760-763 (2020).
Z. Ji et al. Photocurrent detection of the orbital angular momentum of light. Science 368, 763-767 (2020).
H. Zhao et al. Non-Hermitian topological light steering. Science 365, 1163-1166 (2019).

Topological photonics enables unusual and robust properties of electromagnetic waves, including reflections-free photon propagation along strongly curved interfaces and localization at the corners and other discontinuities of photonic structures. One of the key remaining challenges include bringing topological photonics to the nanoscale and making it active/reconfigurable. In this talk, I will describe our group’s progress on using two-dimensional materials such as electrostatically-gated graphene to accomplish these goals. The concept of a metagate – a structured metallic gate used to imprint topologically-complex chemical potential landscape onto graphene – will be introduced. Examples of lower- and higher-order topological structures that, respectively, confine graphene surface plasmon polaritons to edges and corners, will be presented, and the prospects for their experimental realization will be discussed. A novel regime of quantum landscaping, where periodic patterns of chemical potential simultaneously modify electronic and photonic propagation bands, will be introduced and exemplified using meta-gated graphene.

The generation of hot-carriers in plasmonic nanostructures, via plasmon decay, is of great current interest as hot-carriers play key roles in applications like photocatalysis, energy harvesting and in novel photodetection schemes that circumvent band-gap limitations. However, experimental quantification of steady-state energy distributions of hot-carriers in plasmonic nanostructures, which is critical for systematic progress, has not been possible. In this talk, we will describe our recent experimental advances that have enabled the direct measurement of hot-carrier energy distributions under steady-state conditions [1]. Specifically, we will explain how a scanning probe-based technique that records charge transport through single molecular junctions, when combined with nanoplasmonic experimental methods, can be leveraged to directly quantify hot-carrier energy distributions in a key model system—a thin gold film that supports propagating surface plasmon polaritons. Furthermore, key physical insights from our measurements on the role of Landau damping in producing hot-carriers and the contributions of different plasmonic modes towards hot-carrier generation will be discussed. Finally, we will outline how these experimental advances could potentially be leveraged to quantify hot-carrier distributions in plasmonic nanoparticles and other nanophotonic devices.

References:

[1] H. Reddy, K. Wang, Z. Kudyshev, L. Zhu, S. Yan, A. Vezzoli, S. J. Higgins, V. Gavini, A. Boltasseva, P. Reddy, V. M. Shalaev, and E. Meyhofer, “Determining plasmonic hot-carrier energy distributions via single molecule transport measurements,” Science, 10.1126/science.abb3457, 2020.

Topological photonics and mechanics aim to replicate fermionic symmetries as feats of precision engineering. Here I show how to enhance these systems via effects such as gain, loss and nonlinearities that do not have a direct electronic counterpart. This leads to a topological mechanism of mode selection [1,2,3], formation of compactons in flat band condensates [4] and sonic metamaterials [5], topological response of lasers [6] and robotic metamaterials [7], and practical applications such as for receiver protectors [8]. Common to them all are structured intensity distributions of the topological modes which are associated with topological anomalies and supersymmetry [9,10,11], normally abstract concepts that become directly visible in experiments.

[1] Topologically protected midgap states in complex photonic lattices, H. Schomerus, Opt. Lett. 38, 1912 (2013).
[2] Selective enhancement of topologically induced interface states in a dielectric resonator chain, C. Poli, M. Bellec, U. Kuhl, F. Mortessagne, H. Schomerus, Nat. Commun. 6, 6710 (2015).
[3] Topological Hybrid Silicon Microlasers, H. Zhao et al., Nat. Commun. 9, 981 (2018).
[4] Exciton-polaritons in a two-dimensional Lieb lattice with spin-orbit coupling, C. E. Whittaker et al., Phys. Rev. Lett. 120, 097401 (2018).
[5] Sonic Landau Levels and Synthetic Gauge Fields in Mechanical Metamaterials, H. Abbaszadeh, A. Souslov, J. Paulose, H. Schomerus, and V. Vitelli, Phys. Rev. Lett. 119, 195502 (2017).
[6] Topological dynamics and excitations in lasers and condensates with saturable gain or loss, S. Malzard, E. Cancellieri, and H. Schomerus, Opt. Express 26, 22506-22518 (2018).
[7] Nonreciprocal response theory of non-Hermitian mechanical metamaterials: Response phase transition from the skin effect of zero modes, H. Schomerus, Phys. Rev. Research 2, 013058 (2020)
[8] Self-Shielded Topological Receiver Protectors, M. Reisner et al., Phys. Rev. Applied 13, 034067 (2020).
[9] Topological tight-binding models from nontrivial square roots, J. Arkinstall, M. H. Teimourpour, L. Feng, R. El-Ganainy, and H. Schomerus, Phys. Rev. B 95, 165109 (2017).
[10] Supersymmetric Polarization Anomaly in Photonic Discrete-Time Quantum Walks, S. Barkhofen, L. Lorz, T. Nitsche, C. Silberhorn, and H. Schomerus, Phys. Rev. Lett. 121, 260501 (2018).
[11] Observation of supersymmetric pseudo-Landau levels in strained microwave graphene, M. Bellec, C. Poli, U. Kuhl, F. Mortessagne, and H. Schomerus, Light: Science and Applications (in press); arXiv:2001.10287.

Quantum mechanical (QM) tunneling and transit time effects are increasingly important to attosecond and nanoscale physics for which quasi-classical trajectory interpretations are useful [1] to characterize electron emission. Modeling electron transport with particle-based methods may possibly benefit from using a Wigner distribution function (WDF), which is a quantum distribution with a trajectory interpretation that has analogs to a classical distribution of particles. The WDF is therefore potentially useful to describing tunneling and other purely quantum mechanical behaviors like resonant transmission [2, 3]. An exploration of the trajectory interpretation, however, is made difficult because there are few analytically solvable cases on which to test the methods, investigate the trajectory behavior, and relate to dielectric structure experiments examining the relationship between QM tunneling and electromagnetic wave propagation through a barrier that are progressing in tandem and described by N. M. Riga, et al. (this conference).
In this work, the WDF for an exactly solvable system for closed boundary conditions [4] gives both analytic trajectories and time-dependent behavior, and is described. Changes in the eigenstates associated with a step-potential lead to trajectories whose tunneling behavior is examined. Open boundary conditions are required to consider current flow and are required for the emission models, but require numerical approaches to the time evolution behavior for even simple barriers (in contrast to exact solutions to Schrodinger's Eq. [5]). Our methods and findings are presented and compared to alternate methods for thin barriers using analytic models [6].
1. M.F. Ciappina, J.A. Perez-Hernandez, A.S. Landsman, W.A. Okell, et al., Attosecond physics at the nanoscale, Rep. Prog. Phys. 80, 054401 (2017).
2. F. A. Buot, K.L. Jensen, Lattice Weyl-wigner Formulation of Exact Many-body Quantum-transport Theory and Applications to Novel Solid-state Quantum-based Devices, Phys. Rev. B 42, 9429 (1990).
3. S. Dries, B. Fons, M. Wim, Classical trajectories: A powerful tool for solving tunneling problems, Physica A: Statistical Mechanics and its Applications 391, 78 -81 (2012).
4. K.L. Jensen, D.A. Shiffler, N.M. Riga, et al., Analytic Wigner distribution function for a split potential well, J. Appl. Phys. 126, 144301 (2019); K.L. Jensen, D.A. Shiffler, J.L. Lebowitz, et al., Analytic Wigner distribution function for tunneling and trajectory models, J. Appl. Phys. 125, 114303 (2019).
5. O. Costin, R. Costin, I. Jauslin, J.L. Lebowitz, Solution of the time dependent Schrodinger equation leading to Fowler-Nordheim field emission, J. Appl. Phys. 124, (2018).
6. K. L. Jensen, A. Shabaev, S. G. Lambrakos, et al., Analytic Model of Electron Transport Through and Over Non-Linear Barriers, (accepted for publication, J. Appl. Phys, 2020).

In recent years, there has been extensive research on photonic topological insulators (PTIs), motivated by the prospect of backscattering-immune wave propagation. One way to realize PTIs is by altering the crystalline symmetry of the lattice to emulate the quantum valley-Hall effect. This relies on the valley degree of freedom (DOF) that is readily available in photonic crystals (PhCs) with C6v symmetry. Typically, however, the lattice is reduced to C3v symmetry, causing a transition of the system to the valley topological phase (characterized by half-integer Chern numbers of opposite signs associated with the positive and negative halves of the Brillouin zone, respectively). This enables the occurrence of valley-polarized modes exhibiting backscattering-immune wave propagation in the absence of inter-valley scattering disorder. Here, we propose a local-valley approach that maintains C6v symmetry yet supports robust valley-polarized modes.
To differentiate our approach form the typical valley-based approach, consider the example of a hexagonal lattice of dielectric rods. In the typical approach, the rods are usually deformed into triangular shape and the edge modes are observed at the interface between two PhCs, where the triangles in one PhC are facing the opposite way with respect to those in the other PhC. In comparison, in the proposed approach, the rods have cylindrical shape and the edge modes are observed along one row of rods that are misaligned with respect to the original lattice like in common line defect waveguides. Note that the preserved rotational symmetry of the unit cell means that the edge states in the later approach must reside in an intrinsic bandgap of the PhC that is different than that associated with C3v symmetry.
In both approaches, the edge modes are the results of linking opposite orbital angular momentum vortices across the waveguide region. In addition, the proposed edge modes exhibit the expected unidirectional wave excitation and robust wave transmission through sharp zigzag-shaped bends. Moreover, we show that our proof-of-concept platform is in fact an analogue of bilayer graphene system with AB/BA stacking regions. Hence, we reveal that the sole criterion for supporting valley-polarized states is breaking the mirror inversion symmetry locally, i.e. only near the waveguide region. Attractively, the proposed approach leads to simple implementation, benefiting practical applications and easier fabrication at optical frequencies compared to existing valley-type PTIs.

In this talk, we discuss our recent progress in the area of twistronics with light based on twisted metasurfaces or twisted van-der-Waals bilayers, which support topological transitions at photonic magic angles. The unusual interplay between coupled hyperbolic polaritons and the rotation angle between them enables unusual transitions from hyperbolic to elliptical propagation, which can be predicted by counting the number of intersections between the polariton bands of the two individual surfaces when isolated. During the talk, we discuss the unusual phenomena supported by these structures, and their opportunities for nano-imaging and information transport.

Combining state of the art optimization and machine learning techniques with high speed electromagnetic solvers offers a new approach to "inverse" design and implement classical and quantum photonic circuits with superior properties, including robustness to errors in fabrication and environment, compact footprints, novel functionalities, and high efficiencies. We illustrate this with a number of demonstrated devices in silicon, diamond, and silicon carbide, including wavelength and polarization splitters and converters, power splitters, couplers, nonreciprocal switches and routers, on chip laser driven particle accelerators, and efficient quantum emitter-photon interfaces.

Over the past decades, the study of metamaterials and metasurfaces has brought enormous success in modern photonics. With rationally designed sub-wavelength units, known as meta-atoms, one can manipulate light in a desired and prescribed manner. Although modern numerical methods can accurately simulate the optical responses of complex structures, the full-wave simulation methods such as FEM and FDTD still require a lot of time and computational resources, especially when parameter sweeping in a huge design space is required to inversely retrieve the optimal structure according to certain optical responses. In the traditional research approach, researchers need to adjust the geometric/material parameters through experience repeatedly until the simulated optical responses meet the requirements to the maximum. This process still takes enormous time even for experienced researchers when the design problem is challenging.

Very recently, data-driven approaches, such as deep learning, have been applied to design various photonic structures, including metamaterials, metasurfaces, photonic crystals and plasmonic nanostructures^1-4. To better reveal the implicit structure-property relationship and thus facilitate metasurface designs, we proposed to represent metasurfaces and model the inverse design problem in a probabilistically generative manner with the help of variational autoencoders (VAE), which not only enable to efficiently and elegantly investigate the complex structure–optical response relationship in an interpretable way, but solve the one-to-many mapping issue that is tough in a deterministic model². Moreover, to avoid vast full-wave simulation when collecting data, a semi-supervised learning strategy is developed that allows the model to utilize unlabeled data in addition to labeled data in an end-to-end training network⁵. We demonstrate that the self-supervised deep learning model can effectively utilize randomly generated unlabeled data during training, with the total test loss and prediction accuracy improved by about 15% compared with the fully supervised counterpart. On a data-driven basis, the proposed deep generative model can serve as a comprehensive and efficient tool that prompts the design, characterization, and even novel discovery in the research domain of metasurface, and photonics in general.


References
1 Ma, W., Cheng, F. & Liu, Y. Deep-Learning-Enabled On-Demand Design of Chiral Metamaterials. ACS Nano 12, 6326-6334, doi:10.1021/acsnano.8b03569 (2018).
2 Ma, W., Cheng, F., Xu, Y., Wen, Q. & Liu, Y. Probabilistic Representation and Inverse Design of Metamaterials Based on a Deep Generative Model with Semi-Supervised Learning Strategy. Adv. Mater. 31, e1901111, doi:10.1002/adma.201901111 (2019).
3 Liu, Z., Zhu, D., Rodrigues, S. P., Lee, K. T. & Cai, W. Generative Model for the Inverse Design of Metasurfaces. Nano Lett. 18, 6570-6576, doi:10.1021/acs.nanolett.8b03171 (2018).
4 Asano, T. & Noda, S. Optimization of photonic crystal nanocavities based on deep learning. Opt. Express 26, 32704-32717, doi:10.1364/OE.26.032704 (2018).
5 Wei, M. & Yongmin, L. A data-efficient self-supervised deep learning model for design and characterization of nanophotonic structures. SCIENCE CHINA Physics, Mechanics & Astronomy.

3D-printing and additive manufacturing enable much larger design spaces than conventional manufacturing methods (e.g. subtractive manufacturing, injection molding). This applies to the geometry as well as to the materials, with some 3D-printers offering multi-material printing with continuous variation in composition. Without the bounds from “design for manufacturing”, more complex designs become possible and are not penalized by manufacturability. In fact, complex designs are often favored over geometrically simple designs (e.g. structured infill vs. solid infill). This freedom of design leads to a vast design space which can be hard to navigate. Even simple objects with 5 to 10 design parameters, resulting in a design space of an equally large number of dimensions, require sophisticated methods to fully take advantage of the design freedom and find the design with optimum performance. Typically, the optimal design is found by searching the design space using simulation and the optimal simulated outcome is manufactured.
In this work, we add an additional layer of optimization. We use simulation to narrow down the design candidates to a set which is Pareto-optimal. Then, we take these designs as seeds for experimental optimization. The simulation as well as the experimental optimization use active learning based on multi-objective Gaussian processes Bayesian optimization. For the experimental optimization, we perform active learning in batches, filling the print volume with a set of Pareto-optimal candidate designs, which maximize the acquisition function, enabling efficient experimental iteration over candidate designs.
This approach of combining simulation and experiments is particularly useful because often simulations do not accurately represent the real-life object performance, e.g. due to simplifying assumptions, difficult to characterize material parameters etc.
In this paper, we use the outlined approach to optimize the sound transmission loss (STL) spectrum of an acoustic metamaterial. The objectives are to maximize STL at certain frequencies, while keeping the material thickness in the deep subwavelength regime (e.g. 20 mm thickness, vs. 343 mm wavelength for f = 1000 Hz). We achieve the optimal design by starting with a conceptual idea, e.g. a combination of membranes with Helmholtz resonators, or using an array of multi-membrane cells producing phase shifts that lead to a desirable interference in the outgoing wave field. This conceptual idea has 5 to 10 design variables (e.g. membrane size, resonator size). These design variables are then optimized using active learning with multi-objective Gaussian processes Bayesian optimization. Our framework incorporates NSGA-II (Non-dominated Sorting Genetic Algorithm II) optimization implemented in Matlab and Python. The workflow seamlessly integrates with COMSOL simulation and creates STL files for printing. Experimental measurements are performed on an impedance tube, which is a fairly quick and simple measurement (~ 3 min per sample), such that a large number of samples can be measured.
Ultimately, we optimized acoustic metamaterials for various applications. We obtained and experimentally validated superior properties compared to state-of-the-art acoustic insulators. This optimization framework combining simulations and experiments can be adapted to various design problems in materials science.

We present recent studies in which engineering the interaction between light and nanoscale materials has been pursued for applications in photodetectors and in optical tweezers.

We review our recent work that demonstrates that the absorption spectrum of a photodetector can be controlled via waveguide resonances in semiconductor nanowires [1-6]. We discuss the physical interpretation for this phenomenon [1]. We review work in which p-i-n photodiodes were incorporated into vertically oriented silicon nanowires, and then used for colour imaging [2-5]. We also review recent work on extending this concept to the short-wave infrared via Ge nanowires [6]. We describe recent work in which we demonstrated chip-scale microspectrometers based on arrays of nanowire photodetector pixels [7] and an interleaved design with a “fishnet” pattern [8].

We also describe our work on optical trapping with plasmonic nanoapertures [9] and with all-dielectric (silicon) nanoantennas [10,11]. Fluorescence microscopy is used to track the position of, and emission from, individual trapped nanoparticles. The nanoparticles consist of fluorescent polystyrene nanospheres with diameters of 20 nm and 100 nm and streptavidin-coated CdSe/ZnS quantum dots. We observe the Brownian motion of the trapped nanoparticles using fluorescence imaging. Other experiments reported include the two-photon excitation of fluorescence from trapped nanoparticles, and the trapping of multiple nanoparticles simultaneously. We also discuss our recent work on an algorithmic approach to the design of plasmonic nanoapertures for optical tweezers [12].

References
1. K. Seo, M. Wober, P. Steinvurzel, E. Schonbrun, Y. Dan, T. Ellenbogen, K.B. Crozier, Nano Letters 11, 1851 (2011)
2 H. Park, Y. Dan, K. Seo, Y.J. Yu, P.K. Duane, M. Wober, K.B. Crozier, Nano Letters 14, 1804 (2014)
3. H. Park, K. Seo, K.B. Crozier, Applied Physics Letters 101, 193107 (2012)
4. H. Park and K.B. Crozier, Sci. Rep. 3, 2460 (2013)
5. H. Park and K. B. Crozier, ACS Photonics 2, 544 (2015)
6. A. Solanki, S. Li, H. Park, and K.B. Crozier, ACS Photonics 5, 520 (2018)
7. J. Meng, J.J. Cadusch, and K.B. Crozier, Nano Letters 20, 320 (2020)
8. J.J. Cadusch, J. Meng, B. Craig, and K.B. Crozier, Optica 6, 1171 (2019)
9. Z. Xu, W. Song, K. B. Crozier, ACS Photonics 5, 2850 (2018)
10. Z. Xu, W. Song, K. B. Crozier, ACS Photonics 5, 4993 (2018)
11. Z. Xu, K. B. Crozier, Opt. Express 27, 4034 (2019)
12. N. Li, J. Cadusch, and K. Crozier, Optics Letters 44, 5250-5253 (2019)

Solar-driven interfacial steam generation for desalination has attracted broad attention. However, a significant challenge still exists for achieving a fast evaporation rate and high quality of water together with the low-cost and simple-to-manufacture device to provide a feasible solar-driven steam generation system. In this study, a novel "blackest" paint Black 3.0 serving as a perfect solar absorber is introduced into the hot-pressed melamine foam networks, constructing the blackest (99% absorptance in the solar region) and self-floating evaporation device. The features of effective solar absorptance and salt-rejection capability contribute to a high evaporation rate at 2.48 kg m⁻² h⁻¹ under one sun (1 kW m⁻²). This feature shines remarkable lights on a facilely fabricated, robust, high-efficient, and cost-effective solar steam generation system.

Here, we report on anisotropic nanostructure assemblies at the vapor/aqueous interface and in the bulk solution. Gold nano-triangles and octahedra were synthesized and functionalized with poly(ethylene) glycol (both polymer-grafted nanostructures are referred to as PEG-AuNSts). Synchrotron-based grazing incidence small angle X-ray scattering and liquid surface X-ray reflectivity showed that PEG-AuNSts were populated at the vapor/aqueous interface with preferred orientation at the surface by introducing salts or polyelectrolytes which can induce interpolymer complexation between grafted-polymer and polyelectrolytes at low pH level. The resulting assemblies of PEG-AuNSts were further investigated by regulating the concentration of electrolytes and solution pH. Similar strategies were also used to create the three-dimensional assemblies of PEG-AuNSts in the bulk solution. Solution small angle X-ray scattering revealed that controlled assemblies could be achieved by the interplay of temperature, salt concentration, and interpolymer complexation.

Metamaterials, a major type of artificially engineered materials, exhibits unprecedented and controllable effective properties, including electric permittivity and magnetic permeability, and have boosted the development of optical and photonic devices. Metamaterials are composed of arrays of subwavelength unit cells. The effective properties of metamaterials are mainly determined by the structural design of the constituting subwavelength unit cells rather than their chemical composition. The design-driven properties enable versatile designs of their electromagnetic properties. Recent research has shifted to construct reconfigurable, tunable, and nonlinear metamaterials towards the development of metamaterial devices via integrating actuation mechanisms and quantum materials with meta-atoms. Microelectromechanical systems (MEMS) provide a powerful platform for manipulating the effective properties of metamaterials and the integration of multiple functions with metamaterials. In this talk, I will introduce the fundamentals of metamaterials, approaches to integrate MEMS with metamaterials, functional metamaterial devices from the synergy, and outlooks for metamaterial-enabled devices. Specifically, I will talk about terahertz devices based on the tunable metamaterials for dynamic control of the magnitude, phase, wave front, and polarization by MEMS-driven metamaterial unit cells. Terahertz detectors based on nonlinear terahertz metamaterials will be introduced. In addition, I will discuss the recent advances in the metamaterials for magnetic resonance imaging (MRI) based on the controllable nonlinearity in metamaterials. The implementation of intelligent metamaterials to boost the signal to noise ratio of clinical MRI, which will be translated to improvement in image quality and efficiency. The implementation of nonreciprocal devices based on nonlinearity will also be presented. Besides electromagnetic metamaterials, I will also present the construction of acoustic metamaterials for sound wave manipulation. The acoustic lens to focus sound waves based on the horn-like space coiling structures will be discussed. The ultra-open metamaterial for noise mitigation will also be presented. The future of metamaterials will be outlooked. Based on the MEMS and advanced manufacture technology, multifunctional metamaterial in electromagnetic and acoustic domains will be developed to yield functional devices to achieve functions that are not achievable by nature materials.

Optical nanomaterials offer the ability to bend, twist, guide, and confine light in nanoscale dimensions. Among these materials, chiral nanostructures particularly show promise for applications ranging from polarization manipulation to 3D displays, sensing, and spin-selective transport. Compared to their molecular counterparts, chiral nanomaterials exhibit orders of magnitude stronger dissymmetry factors, but have only been realized in a limited set of materials systems. We have recently demonstrated different strategies to tune and manipulate the polarization response of chiral metamaterials that combine metallic nanostructures, dielectric materials, and semiconductor nanocrystals. By controlling the refractive index of the dielectric components in different architectures, we show that the sign of the circular dichroism can be reversed, and the polarization and directionality of the outcoupled luminescence from nearby nanocrystals can be controlled. To create light-emitting metamaterials, it is additionally useful to create nanostructured elements comprised entirely of photoluminescent materials. We have recently developed patterning methods to transform semiconductor nanocrystals into patterned nanocrystal solids, realizing lateral feature sizes as small as 30 nm and heights in excess of 100 nm without degradation of the photoluminescence. We show that by designing the shape of the nanocrystal solid at this length scale and controlling connectivity between the nanocrystals, the refractive index and nanostructure absorptivity can be tailored. This work points to new strategies to design dynamically tunable metamaterials and control nanoscale light-matter interactions.

The radiative cooling phenomenon relies on the atmospheric transparency window to dissipate heat from the earth into outer space, which is an energy-saving cooling technique. This work demonstrates a highly effective aluminized Polymethylpentene (PMP) thin-film thermal structure. The emissivity of aluminized PMP thin films matches perfectly to the atmospheric transparency window so as to minimize parasitic heat losses. This photon-to-cooling structure yields a highest-to-date temperature drop of 8.5 K in comparison to the ambient temperature and a corresponding radiative cooling power of 193 W/m²during a one-day cycle. The easy-to-manufacture feature of an aluminized PMP thin film makes it a practically scalable radiative cooling method.

Passive daytime radiative cooling involves solar reflection and thermal emission into the cold outer space through the atmospheric transmission windows. Due to its passive nature and net cooling effect, it is a promising alternative or complement to electrical cooling.^1-2 For efficient radiative cooling of objects, an unimpeded view of the sky is ideal. However, the view of the sky is often limited - for instance, the walls of buildings have > 50% of their field of view subtended by the earth. Moreover, objects on earth become sources of heat under sunlight. Therefore, vertical building facades with hot terrestrial objects in view experience reduced cooling or heating, even with materials optimized for heat loss into the sky.^2-3

We show that by using materials with selective long-wavelength infrared (LWIR) emittances, walls and roofs can radiatively cool to considerably lower temperatures than achievable by using broadband thermal emitters like paints and recent designs.^2-3 Promisingly, selective LWIR emitters can be scalably manufactured from common polymers, ceramics and composites, with some being sufficiently ubiquitous to be sourced from waste. The materials are suitable for use on both opaque and transparent building envelopes, making them applicable on both walls and windows. Cooling enhancements (theoretical and demonstrated) achieved by such commonplace materials offer a remarkable opportunity for untapped energy savings in buildings.

[1] A. Raman et. al., Nature 515, 541 (2014)
[2] J. Mandal et. al., Science 362, 315 (2018)
[3] Y. Zhai et. al. Science 355, 1062–1066 (2017)

Acknowledgments: Jyotirmoy Mandal was supported by Schmidt Science Fellows, in partnership with the Rhodes Trust.

Additive manufacturing (AM), or 3D printing, represents a set of processes that enable layer-by-layer fabrication of complex structures using a wide range of materials that include ceramics, polymers, and metals. AM has allowed exploiting novel material properties, especially those that arise at the nano-scale, that do not occur in conventional materials, revolutionizing the production of complex parts for aerospace, military, automotive, optial, and medical applications. Facilitating these technologies requires a fabrication process to create a variety of functional materials in 3D, however the material choice for AM at the nano- and micro-scale is limited. A conspicuous example is a lack of AM processes for high refractive index (n), low absorption materials with nano-sized dimensions, which are typically required for microoptics and device applications. To harness the beneficial properties of 3D nano-architected meta-materials, it is critical to assess their properties at each relevant scale while capturing over­all structural complexity. We present the fabrication and synthesis of nano- and micro-architected materials using 3D (two-photon and interference) lithography, as well as characterization of their properties as a function of architecture, constituent materials, and microstructural detail. We focus on additive manufacturing of complex, multi-component and multi-material architectures comprised of metal oxides, metals, and hydrogels that are derived from (metal) salt-based, aqueous photoresin synthesis. The discussion will focus on each representative materials system (metals, oxides, and hydrogels), with a detailed overview of titanium dioxide, with critical feature dimensions between 150 and 600 nm and <1% porosity. Using the described methodology, we created 3D dielectric photonic crystals (PhCs) out of rutile TiO2 patterned into woodpile face-centered tetragonal (FCT) architectures with beam dimensions of 300-600nm and lateral periods of 0.8-1.5 μm. We use Plane Wave Expansion (PWE) simulations and Fourier Transform Infrared Spectroscopy (FTIR) to demonstrate the full photonic bandgaps centered at 1.8-2.9 μm. This fundamental research provides insights into the range of possible, often transient attainable chemical compositions, microstructures and architectures of chemically-derived, 3D-archtiected ceramics and metals, enabling advances in multiple technologies, i.e. 3D MEMS, micro-optics, and prototyping of 3D PhCs.

Flat optics based on metasurfaces holds promise for the design of a new class of diffractive optical component that circumvents the limits of refractive as well as Fresnel optics, in terms of control of aberrations, compactness and multifunctionality. Here we highlight the enhanced functions enabled by metasurface flat optics in structuring light. Optical elements coupling the spin and orbital angular momentum (SAM/OAM) of light have found a range of applications in classical and quantum optics. Q-plate type devices based on dielectric metasurfaces have shown superior performance in terms of singularity definition and ability to generate fractional and high OAM beams. A far more general spin to orbital angular momentum scheme has been recently demonstrated based on novel metasurface optical elements (J-plates) not limited to conjugate converted states, which are used to generate new complex structured light. The J-plate, with J referring to the photon’s total angular momentum (TAM), is a metasurface device that imparts two arbitrary OAM states on an arbitrary orthogonal basis of spin states. When these J-plates are cascaded in series, they can generate several single quantum number beams and versatile superpositions thereof. Moreover, in contrast to previous spin-orbit-converters, the output polarization states of cascaded J-plates are not constrained to be the conjugate of the input states. Cascaded J-plates are also demonstrated to produce vector vortex beams and complex structured light, providing new ways to control TAM states of light
Metasurfaces hold major potential for multi-functionality, which may play a pivotal role in the next-generation compact nanodevices. OAM from lasers holds the promise for compact, at-the-source solutions to fuel applications from imaging to communications . However, conjugate symmetry between circular spin and opposite helicity OAM states from conventional spin-orbit (SO) approaches, as well as low purity modes, has meant that complete control of light’s angular momentum from lasers has remained elusive. Recent work on metasurface-enhanced lasers that overcomes this limitation will be discussed, including the demonstration of new high-purity OAM states with quantum numbers reaching l= 100, and non-symmetric vector vortex beams that lase simultaneously on independent OAM states as much as Δl = 90 apart. This laser conveniently emits in the visible, producing new OAM states of light as well as all previously reported OAM modes from lasers, offering a compact and power-scalable source that harnesses intra-cavity structured metasurfaces for the creation of arbitrary chiral states of structured light.
Finally, recent work on beams with light structured along the propagation direction will be presented, where the polarization or the OAM of the beam changes along the propagation direction.

Diffractive photonic devices manipulate light via local and nonlocal optical modes. Local devices, such as metasurfaces, can shape a wavefront at multiple selected wavelengths, but inevitably modify light across the spectrum; nonlocal devices, such as grating filters, offer great frequency selectivity but limited spatial control. In this talk, I will introduce a rational design paradigm using quasi-bound states in the continuum to realize multifunctional nonlocal devices: metasurfaces that produce narrowband spatially tailored wavefronts at multiple selected wavelengths and yet are otherwise transparent.
I will show an experimental demonstration of dielectric metalenses that focus light only for narrowband resonances and leave the rest of the spectrum unaffected. Our resonant metasurfaces can be cascaded to achieve multifunctional behavior with each metasurface independently shaping the wavefront for a distinct resonance resonant wavelength.
These devices may expand the capabilities of multifunctional meta-optics to include active or nonlinear wavefront shaping. Scaled to visible wavelengths, our resonant metalenses may prove useful for augmented reality applications as compact and highly transparent multi-color see-through lenses.

Additive manufacturing (AM), or 3D printing, represents a set of processes that enable layer-by-layer fabrication of complex structures using a wide range of materials that include ceramics, polymers, and metals. AM has allowed exploiting novel material properties, especially those that arise at the nano-scale, that do not occur in conventional materials, revolutionizing the production of complex parts for aerospace, military, automotive, optial, and medical applications. Facilitating these technologies requires a fabrication process to create a variety of functional materials in 3D, however the material choice for AM at the nano- and micro-scale is limited. A conspicuous example is a lack of AM processes for high refractive index (n), low absorption materials with nano-sized dimensions, which are typically required for microoptics and device applications. To harness the beneficial properties of 3D nano-architected meta-materials, it is critical to assess their properties at each relevant scale while capturing over­all structural complexity. We present the fabrication and synthesis of nano- and micro-architected materials using 3D (two-photon and interference) lithography, as well as characterization of their properties as a function of architecture, constituent materials, and microstructural detail. We focus on additive manufacturing of complex, multi-component and multi-material architectures comprised of metal oxides, metals, and hydrogels that are derived from (metal) salt-based, aqueous photoresin synthesis. The discussion will focus on each representative materials system (metals, oxides, and hydrogels), with a detailed overview of titanium dioxide, with critical feature dimensions between 150 and 600 nm and <1% porosity. Using the described methodology, we created 3D dielectric photonic crystals (PhCs) out of rutile TiO2 patterned into woodpile face-centered tetragonal (FCT) architectures with beam dimensions of 300-600nm and lateral periods of 0.8-1.5 μm. We use Plane Wave Expansion (PWE) simulations and Fourier Transform Infrared Spectroscopy (FTIR) to demonstrate the full photonic bandgaps centered at 1.8-2.9 μm. This fundamental research provides insights into the range of possible, often transient attainable chemical compositions, microstructures and architectures of chemically-derived, 3D-archtiected ceramics and metals, enabling advances in multiple technologies, i.e. 3D MEMS, micro-optics, and prototyping of 3D PhCs.

Metamaterials represent the concept of utilizing artificial material building blocks to create desirable properties derived from three-dimensional layout and compositions. While novel topologies can now be realized via additive manufacturing, the lack of processable materials, multi-material gradients, speed and scalabilities have stymied its further adoption. In this talk, I will outline a suite of new material design and manufacturing routes, enabled by additive manufacturing of topologies, multi-scale features and multi-material cues. Attention is focused on how additive manufacturing techniques will enable processing the unprocessable, from structural composites to functional and stimuli-responsive multi-materials. This unleashes new design freedoms for rapid material property discovery and product realizations; where electrical, thermo, mechanical behaviors and their couplings can be inversely designed and tailored by an end-user at will. I will present a few examples, where structural, electronic and energy transduction materials are 3D architected into a compact form factor, without requiring multiple processing stages such as printing, embedding or wiring. Next, we will present their new applications, such as ultralight and strong materials, self-sensing materials, microsystems for robotics, air and maritime sensing, transducers, wave guiding and telecommunications.

We present recent studies in which engineering the interaction between light and nanoscale materials has been pursued for applications in photodetectors and in optical tweezers.

We review our recent work that demonstrates that the absorption spectrum of a photodetector can be controlled via waveguide resonances in semiconductor nanowires [1-6]. We discuss the physical interpretation for this phenomenon [1]. We review work in which p-i-n photodiodes were incorporated into vertically oriented silicon nanowires, and then used for colour imaging [2-5]. We also review recent work on extending this concept to the short-wave infrared via Ge nanowires [6]. We describe recent work in which we demonstrated chip-scale microspectrometers based on arrays of nanowire photodetector pixels [7] and an interleaved design with a “fishnet” pattern [8].

We also describe our work on optical trapping with plasmonic nanoapertures [9] and with all-dielectric (silicon) nanoantennas [10,11]. Fluorescence microscopy is used to track the position of, and emission from, individual trapped nanoparticles. The nanoparticles consist of fluorescent polystyrene nanospheres with diameters of 20 nm and 100 nm and streptavidin-coated CdSe/ZnS quantum dots. We observe the Brownian motion of the trapped nanoparticles using fluorescence imaging. Other experiments reported include the two-photon excitation of fluorescence from trapped nanoparticles, and the trapping of multiple nanoparticles simultaneously. We also discuss our recent work on an algorithmic approach to the design of plasmonic nanoapertures for optical tweezers [12].

References
1. K. Seo, M. Wober, P. Steinvurzel, E. Schonbrun, Y. Dan, T. Ellenbogen, K.B. Crozier, Nano Letters 11, 1851 (2011)
2 H. Park, Y. Dan, K. Seo, Y.J. Yu, P.K. Duane, M. Wober, K.B. Crozier, Nano Letters 14, 1804 (2014)
3. H. Park, K. Seo, K.B. Crozier, Applied Physics Letters 101, 193107 (2012)
4. H. Park and K.B. Crozier, Sci. Rep. 3, 2460 (2013)
5. H. Park and K. B. Crozier, ACS Photonics 2, 544 (2015)
6. A. Solanki, S. Li, H. Park, and K.B. Crozier, ACS Photonics 5, 520 (2018)
7. J. Meng, J.J. Cadusch, and K.B. Crozier, Nano Letters 20, 320 (2020)
8. J.J. Cadusch, J. Meng, B. Craig, and K.B. Crozier, Optica 6, 1171 (2019)
9. Z. Xu, W. Song, K. B. Crozier, ACS Photonics 5, 2850 (2018)
10. Z. Xu, W. Song, K. B. Crozier, ACS Photonics 5, 4993 (2018)
11. Z. Xu, K. B. Crozier, Opt. Express 27, 4034 (2019)
12. N. Li, J. Cadusch, and K. Crozier, Optics Letters 44, 5250-5253 (2019)

Photonic crystals have revolutionized the field of photonics with their unique capabilities of dispersion and energy band gap engineering, such as the demonstration of extreme group and phase velocities, topologically-protected photonic edge states, and the control of spontaneous emission of photons. On the other hand, time-variant media have shown their distinct functionalities including nonreciprocal propagation, frequency conversion, and amplification of light. However, these phenomena in time-variant media have mostly been investigated for the cases where spatiotemporal modulation is in the form of a simple harmonic wave. Here, we extend the type of spatiotemporal modulation so that time-variant and spatially-discrete photonic crystal structures, or spatiotemporal crystals, can be analyzed. The design of spatiotemporal crystals allows the engineering of momentum band gap, within which the parametric amplification occurs. As a potential platform for the construction of a parametric oscillator, a finite-sized spatiotemporal crystal is proposed and analyzed. The parametric oscillation is initiated by the energy and momentum conversion of an incident wave and the subsequent amplification by parametric gain within the momentum band gap. The oscillation process dominates over frequency mixing interactions above a transition threshold determined by the balance between gain and loss. Furthermore, the asymmetric formation of momentum band gaps can be realized by spatial phase control of temporal modulation, which leads to the directional radiation of oscillations at distinct frequencies. The proposed structure would enable simultaneous engineering of energy and momentum band gaps and provide a guideline to the implementation of advanced dispersion-engineered parametric oscillators.

Metamaterials, a major type of artificially engineered materials, exhibits unprecedented and controllable effective properties, including electric permittivity and magnetic permeability, and have boosted the development of optical and photonic devices. Metamaterials are composed of arrays of subwavelength unit cells. The effective properties of metamaterials are mainly determined by the structural design of the constituting subwavelength unit cells rather than their chemical composition. The design-driven properties enable versatile designs of their electromagnetic properties. Recent research has shifted to construct reconfigurable, tunable, and nonlinear metamaterials towards the development of metamaterial devices via integrating actuation mechanisms and quantum materials with meta-atoms. Microelectromechanical systems (MEMS) provide a powerful platform for manipulating the effective properties of metamaterials and the integration of multiple functions with metamaterials. In this talk, I will introduce the fundamentals of metamaterials, approaches to integrate MEMS with metamaterials, functional metamaterial devices from the synergy, and outlooks for metamaterial-enabled devices. Specifically, I will talk about terahertz devices based on the tunable metamaterials for dynamic control of the magnitude, phase, wave front, and polarization by MEMS-driven metamaterial unit cells. Terahertz detectors based on nonlinear terahertz metamaterials will be introduced. In addition, I will discuss the recent advances in the metamaterials for magnetic resonance imaging (MRI) based on the controllable nonlinearity in metamaterials. The implementation of intelligent metamaterials to boost the signal to noise ratio of clinical MRI, which will be translated to improvement in image quality and efficiency. The implementation of nonreciprocal devices based on nonlinearity will also be presented. Besides electromagnetic metamaterials, I will also present the construction of acoustic metamaterials for sound wave manipulation. The acoustic lens to focus sound waves based on the horn-like space coiling structures will be discussed. The ultra-open metamaterial for noise mitigation will also be presented. The future of metamaterials will be outlooked. Based on the MEMS and advanced manufacture technology, multifunctional metamaterial in electromagnetic and acoustic domains will be developed to yield functional devices to achieve functions that are not achievable by nature materials.

Symmetry and topology are fundamental notions existing in all kinds of natural systems, from spiral galaxies and hurricanes to amino acids in molecules and non-trivial topologically protected electronic states in condensed matter. A stream of linearly polarized photons is typically topologically trivial, nevertheless, its full-vector nature intrinsically endows light with full capability of creating and carrying unique symmetry and topology. Most notably, the intrinsic capability of photonics of superposing non-Hermitian eigenstates through optical gain and loss provides a powerful toolbox for exploring the symmetry paradigms that were deemed challenging in condensed matter systems. Such investigations of symmetry and topology in photonics are revolutionizing the design principles in optical sciences. We have discovered new synthetic photonic materials, geometries, and field profiles that systematically enable the exploration of symmetry and topology in photonics. As such, the elusive symmetries can simplify the design of photonic meta-atoms and unit cells to reshape the density of states of photons in a topological and ultraflexible manner, which leads to on-demand meta-control of light emission, transmission, scattering and detection [1], thereby sustaining the ever-expanding information explosion for information processing, communication and computing.
For example, we have demonstrated one of the most striking benefits: the creation of eigenstate degeneracies such as exceptional points intrinsically associated with non-Hermitian Hamiltonians. This breaks the natural symmetry between reciprocal counterpropagating resonant modes in a micro-cavity to conduct unidirectional light-matter interaction carrying the large orbital angular momentum for structured light emission. Such a symmetry-driven approach allowed us to create the first microlaser source carrying the desired spin and orbital chirality as unique information carriers [2-4], giving rise to a new dimension of storing and processing information in photon’s chiral degree of freedom. In addition to expanding the information dimensions to address the upcoming information explosion, flexible reconfiguration of topological pathways can offer a completely new paradigm for high-density photonics routing. However, the desired topological robustness unfortunately diminishes the hope of topological tunability. To overcome this fundamental limitation, we perform non-Hermitian symmetry to enable a new kind of topological states into the bulk of the topological insulator with a uniform topology, for the first time [5]. Without “battling” against topological protection, the new non-Hermitian topological states facilitate robust transmission links in the entire footprint to route optical signal to any desired output port. The ports-to-footprint ratio is at least 2 orders of magnitude higher than the state-of-the-art.
Hence, our research addresses grand challenges in fundamental quantum physics and also enables entirely new optical sciences and photonic technologies, which advances and benefits both fields simultaneously.

L. Feng, R. El-Ganainy, L. Ge, Non-Hermitian photonics based on parity-time symmetry. Nat. Photon. 11, 752-762 (2017).
P. Miao et al. Orbital angular momentum microlaser. Science 353, 464-467 (2016).
Z. Zhang et al. Tunable topological charge vortex microlaser. Science 368, 760-763 (2020).
Z. Ji et al. Photocurrent detection of the orbital angular momentum of light. Science 368, 763-767 (2020).
H. Zhao et al. Non-Hermitian topological light steering. Science 365, 1163-1166 (2019).

In this talk, we discuss our recent progress in the area of twistronics with light based on twisted metasurfaces or twisted van-der-Waals bilayers, which support topological transitions at photonic magic angles. The unusual interplay between coupled hyperbolic polaritons and the rotation angle between them enables unusual transitions from hyperbolic to elliptical propagation, which can be predicted by counting the number of intersections between the polariton bands of the two individual surfaces when isolated. During the talk, we discuss the unusual phenomena supported by these structures, and their opportunities for nano-imaging and information transport.

Topological photonics and mechanics aim to replicate fermionic symmetries as feats of precision engineering. Here I show how to enhance these systems via effects such as gain, loss and nonlinearities that do not have a direct electronic counterpart. This leads to a topological mechanism of mode selection [1,2,3], formation of compactons in flat band condensates [4] and sonic metamaterials [5], topological response of lasers [6] and robotic metamaterials [7], and practical applications such as for receiver protectors [8]. Common to them all are structured intensity distributions of the topological modes which are associated with topological anomalies and supersymmetry [9,10,11], normally abstract concepts that become directly visible in experiments.

[1] Topologically protected midgap states in complex photonic lattices, H. Schomerus, Opt. Lett. 38, 1912 (2013).
[2] Selective enhancement of topologically induced interface states in a dielectric resonator chain, C. Poli, M. Bellec, U. Kuhl, F. Mortessagne, H. Schomerus, Nat. Commun. 6, 6710 (2015).
[3] Topological Hybrid Silicon Microlasers, H. Zhao et al., Nat. Commun. 9, 981 (2018).
[4] Exciton-polaritons in a two-dimensional Lieb lattice with spin-orbit coupling, C. E. Whittaker et al., Phys. Rev. Lett. 120, 097401 (2018).
[5] Sonic Landau Levels and Synthetic Gauge Fields in Mechanical Metamaterials, H. Abbaszadeh, A. Souslov, J. Paulose, H. Schomerus, and V. Vitelli, Phys. Rev. Lett. 119, 195502 (2017).
[6] Topological dynamics and excitations in lasers and condensates with saturable gain or loss, S. Malzard, E. Cancellieri, and H. Schomerus, Opt. Express 26, 22506-22518 (2018).
[7] Nonreciprocal response theory of non-Hermitian mechanical metamaterials: Response phase transition from the skin effect of zero modes, H. Schomerus, Phys. Rev. Research 2, 013058 (2020)
[8] Self-Shielded Topological Receiver Protectors, M. Reisner et al., Phys. Rev. Applied 13, 034067 (2020).
[9] Topological tight-binding models from nontrivial square roots, J. Arkinstall, M. H. Teimourpour, L. Feng, R. El-Ganainy, and H. Schomerus, Phys. Rev. B 95, 165109 (2017).
[10] Supersymmetric Polarization Anomaly in Photonic Discrete-Time Quantum Walks, S. Barkhofen, L. Lorz, T. Nitsche, C. Silberhorn, and H. Schomerus, Phys. Rev. Lett. 121, 260501 (2018).
[11] Observation of supersymmetric pseudo-Landau levels in strained microwave graphene, M. Bellec, C. Poli, U. Kuhl, F. Mortessagne, and H. Schomerus, Light: Science and Applications (in press); arXiv:2001.10287.

Topological photonics enables unusual and robust properties of electromagnetic waves, including reflections-free photon propagation along strongly curved interfaces and localization at the corners and other discontinuities of photonic structures. One of the key remaining challenges include bringing topological photonics to the nanoscale and making it active/reconfigurable. In this talk, I will describe our group’s progress on using two-dimensional materials such as electrostatically-gated graphene to accomplish these goals. The concept of a metagate – a structured metallic gate used to imprint topologically-complex chemical potential landscape onto graphene – will be introduced. Examples of lower- and higher-order topological structures that, respectively, confine graphene surface plasmon polaritons to edges and corners, will be presented, and the prospects for their experimental realization will be discussed. A novel regime of quantum landscaping, where periodic patterns of chemical potential simultaneously modify electronic and photonic propagation bands, will be introduced and exemplified using meta-gated graphene.

Combining state of the art optimization and machine learning techniques with high speed electromagnetic solvers offers a new approach to "inverse" design and implement classical and quantum photonic circuits with superior properties, including robustness to errors in fabrication and environment, compact footprints, novel functionalities, and high efficiencies. We illustrate this with a number of demonstrated devices in silicon, diamond, and silicon carbide, including wavelength and polarization splitters and converters, power splitters, couplers, nonreciprocal switches and routers, on chip laser driven particle accelerators, and efficient quantum emitter-photon interfaces.

The generation of hot-carriers in plasmonic nanostructures, via plasmon decay, is of great current interest as hot-carriers play key roles in applications like photocatalysis, energy harvesting and in novel photodetection schemes that circumvent band-gap limitations. However, experimental quantification of steady-state energy distributions of hot-carriers in plasmonic nanostructures, which is critical for systematic progress, has not been possible. In this talk, we will describe our recent experimental advances that have enabled the direct measurement of hot-carrier energy distributions under steady-state conditions [1]. Specifically, we will explain how a scanning probe-based technique that records charge transport through single molecular junctions, when combined with nanoplasmonic experimental methods, can be leveraged to directly quantify hot-carrier energy distributions in a key model system—a thin gold film that supports propagating surface plasmon polaritons. Furthermore, key physical insights from our measurements on the role of Landau damping in producing hot-carriers and the contributions of different plasmonic modes towards hot-carrier generation will be discussed. Finally, we will outline how these experimental advances could potentially be leveraged to quantify hot-carrier distributions in plasmonic nanoparticles and other nanophotonic devices.

References:

[1] H. Reddy, K. Wang, Z. Kudyshev, L. Zhu, S. Yan, A. Vezzoli, S. J. Higgins, V. Gavini, A. Boltasseva, P. Reddy, V. M. Shalaev, and E. Meyhofer, “Determining plasmonic hot-carrier energy distributions via single molecule transport measurements,” Science, 10.1126/science.abb3457, 2020.

Optical nanomaterials offer the ability to bend, twist, guide, and confine light in nanoscale dimensions. Among these materials, chiral nanostructures particularly show promise for applications ranging from polarization manipulation to 3D displays, sensing, and spin-selective transport. Compared to their molecular counterparts, chiral nanomaterials exhibit orders of magnitude stronger dissymmetry factors, but have only been realized in a limited set of materials systems. We have recently demonstrated different strategies to tune and manipulate the polarization response of chiral metamaterials that combine metallic nanostructures, dielectric materials, and semiconductor nanocrystals. By controlling the refractive index of the dielectric components in different architectures, we show that the sign of the circular dichroism can be reversed, and the polarization and directionality of the outcoupled luminescence from nearby nanocrystals can be controlled. To create light-emitting metamaterials, it is additionally useful to create nanostructured elements comprised entirely of photoluminescent materials. We have recently developed patterning methods to transform semiconductor nanocrystals into patterned nanocrystal solids, realizing lateral feature sizes as small as 30 nm and heights in excess of 100 nm without degradation of the photoluminescence. We show that by designing the shape of the nanocrystal solid at this length scale and controlling connectivity between the nanocrystals, the refractive index and nanostructure absorptivity can be tailored. This work points to new strategies to design dynamically tunable metamaterials and control nanoscale light-matter interactions.