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1. LCPOM: Precise Reconstruction of Polarized Optical Microscopy Images of Liquid Crystals

   Chen, Chuqiao; Palacio-Betancur, Viviana; Norouzi, Sepideh; Zubieta Rico, Pablo F.; Sadati, Monirosadat; Rowan, Stuart J. (July 2023, arXivorg)

   When viewed with a cross-polarized optical microscope (POM), liquid crystals display interference colors and complex patterns that depend on the material's microscopic orientation. That orientation can be manipulated by application of external fields, which provides the basis for applications in optical display and sensing technologies. The color patterns themselves have a high information content. Traditionally, however, calculations of the optical appearance of liquid crystals have been performed by assuming that a single-wavelength light source is employed, and reported in a monochromatic scale. In this work, the original Jones matrix method is extended to calculate the colored images that arise when a liquid crystal is exposed to a multi-wavelength source. By accounting for the material properties, the visible light spectrum and the CIE color matching functions, we demonstrate that the proposed approach produces colored POM images that are in quantitative agreement with experimental data. Results are presented for a variety of systems, including radial, bipolar, and cholesteric droplets, where results of simulations are compared to experimental microscopy images. The effects of droplet size, topological defect structure, and droplet orientation are examined systematically. The technique introduced here generates images that can be directly compared to experiments, thereby facilitating machine learning efforts aimed at interpreting LC microscopy images, and paving the way for the inverse design of materials capable of producing specific internal microstructures in response to external stimuli. 

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2. Efficient all-dielectric metasurface in-coupler for waveguide-based near-eye display

   https://doi.org/10.1117/12.2646494  

   Xiong, Pei; Goodsell, Jeremy; Nikolov, Daniel K; Rolland, Jannick P; Vamivakas, A Nick (March 2023, SPIE)

   Kress, Bernard C; Peroz, Christophe (Ed.)

   The creation of new see-through near-eye displays (NEDs) architectures is a topic of intense research focus. A fundamental problem that each design must address is the field of view (FOV) and eyebox of NEDs are limited by etendue conservation for a fixed display optics size. Waveguide architecture provides the solution to increasing the eyebox in NEDs without increasing the optics size through exit pupil expansion. Brightness and uniformity are two key features of waveguide architecture. In this work, we focus on the brightness of the waveguide since the image uniformity can be compensated by the display engine. We show that the geometry of the waveguide sets a fundamental limit on the in-coupling efficiency for a given FOV. This limit can be used as a tool for waveguide designers to benchmark the in-coupling efficiency of their incoupler gratings. With this derived limit, we designed and optimized a metasurface-based grating (metagrating) and a surface relief grating (SRG) as in-couplers. The diffractive efficiencies of the two types of in-couplers were then compared to the theoretical efficiency limit. The metagrating's 28% efficiency surpasses the SRG's 20% efficiency and nearly matches the geometry-based limit of 29% due to the superior angular response control of metasurfaces compared to SRGs. This work provides a new understanding of the brightness efficiency limit of waveguide-based combiners and paves a novel path toward implementing metasurfaces in efficient waveguide AR displays. 

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3. Thickness bound for nonlocal wide-field-of-view metalenses

   https://doi.org/10.1038/s41377-022-01038-6  

   Li, Shiyu; Hsu, Chia Wei (December 2022, Light: Science & Applications)

   Abstract Metalenses—flat lenses made with optical metasurfaces—promise to enable thinner, cheaper, and better imaging systems. Achieving a sufficient angular field of view (FOV) is crucial toward that goal and requires a tailored incident-angle-dependent response. Here, we show that there is an intrinsic trade-off between achieving a desired broad-angle response and reducing the thickness of the device. Like the memory effect in disordered media, this thickness bound originates from the Fourier transform duality between space and angle. One can write down the transmission matrix describing the desired angle-dependent response, convert it to the spatial basis where its degree of nonlocality can be quantified through a lateral spreading, and determine the minimal device thickness based on such a required lateral spreading. This approach is general. When applied to wide-FOV lenses, it predicts the minimal thickness as a function of the FOV, lens diameter, and numerical aperture. The bound is tight, as some inverse-designed multi-layer metasurfaces can approach the minimal thickness we found. This work offers guidance for the design of nonlocal metasurfaces, proposes a new framework for establishing bounds, and reveals the relation between angular diversity and spatial footprint in multi-channel systems. 

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4. Control of intense light with avalanche-ionization plasma gratings

   https://doi.org/10.1364/OPTICA.503283  

   Edwards, M_R; Waczynski, S.; Rockafellow, E.; Manzo, L.; Zingale, A.; Michel, P.; Milchberg, H_M (November 2023, Optica)

   High-peak-power lasers are fundamental to high-field science: increased laser intensity has enabled laboratory astrophysics, relativistic plasma physics, and compact laser-based particle accelerators. However, the meter-scale optics required for multi-petawatt lasers to avoid light-induced damage make further increases in power challenging. Plasma tolerates orders-of-magnitude higher light flux than glass, but previous efforts to miniaturize lasers by constructing plasma analogs for conventional optics were limited by low efficiency and poor optical quality. We describe a new approach to plasma optics based on avalanche ionization of atomic clusters that produces plasma volume transmission gratings with dramatically increased diffraction efficiency. We measure an average efficiency of up to 36% and a single-shot efficiency of up to 60%, which is comparable to key components of high-power laser beamlines, while maintaining high spatial quality and focusability. These results suggest that plasma diffraction gratings may be a viable component of future lasers with peak power beyond 10 PW. 

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5. Single‐ and Double‐Layer Embedded Metal Meshes for Flexible, Highly Transparent Electromagnetic Interference Shielding

   https://doi.org/10.1002/admt.202302057  

   Zarei, Mehdi; Li, Mingxuan; Papazekos, Ekaterini; Su, Yang‐Duan; Sinha, Sneh; Walker, S Brett; LeMieux, Melbs; Ohodnicki, Paul R; Leu, Paul W (May 2024, Advanced Materials Technologies)

   Abstract Simulation and experimental studies are carried out on single‐layer and double‐layer embedded metal meshes (SLEMM and DLEMM) to assess their performance as transparent electromagnetic interference (EMI) shielding. The structures consist of silver meshes embedded in polyethylene terephthalate (PET). As a transparent electrode, SLEMMs exhibit a transparency of 82.7% and a sheet resistance of 0.61 Ωsq⁻¹as well as 91.0% and 1.49 Ωsq⁻¹. This performance corresponds to figures of merit of 3101 and 2620, respectively. The SLEMMs achieve 48.0 dB EMI shielding efficiency (SE) in the frequency range of 8–18 GHz (X‐ and Ku‐bands) with 91% visible transmission and 56.2 dB EMI SE with 82.7% visible transmission. Samples exhibit stable performance after 1000 bending cycles with a radius of curvature of 4 mm and 60 tape test cycles. DLEMMs consist of fabricating SLEMM on opposite sides of the substrate where the distance can be varied using a spacer. Simulations are performed to investigate how varying spacer distance between two layers of metal meshes influences the EMI SE. DLEMMs are fabricated and achieved an EMI SE of 77.7 dB with 81.7% visible transmission. SLEMMs and DLEMMs may have a wide variety of applications in aerospace, medical, and military applications. 

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