Characteristic Analysis of Compact Spectrometer Based on Off-Axis Meta-Lens

Review of Miniaturized Computational Spectrometers

Spectrometers are key instruments in diverse fields, notably in medical and biosensing applications. Recent advancements in nanophotonics and computational techniques have contributed to new spectrometer designs characterized by miniaturization and enhanced performance. This paper presents a comprehensive review of miniaturized computational spectrometers (MCS). We examine major MCS designs based on waveguides, random structures, nanowires, photonic crystals, and more. Additionally, we delve into computational methodologies that facilitate their operation, including compressive sensing and deep learning. We also compare various structural models and highlight their unique features. This review also emphasizes the growing applications of MCS in biosensing and consumer electronics and provides a thoughtful perspective on their future potential. Lastly, we discuss potential avenues for future research and applications.

High-efficiency metalens-based compact multispectral variable spectrometer

Conventional spectrometers are bulky, and researchers have continuously made efforts in their miniaturization and integration in recent years. Among these studies, metalenses have attracted immense interest because of their merits of a flat shape and flexible regulation. Herein, we introduce a design of a polarization-insensitive metalens-based spectrometer that utilizes an off-axis high-efficiency broadband metalens in the wavelength range of 500-1000 nm. The demonstrated metalens consisting of nanopillars employs propagation phase and phase function optimization methods and can achieve spectral resolutions of 0.6 nm with efficiency as high as 77%. By stitching metalenses with different focal lengths, the functionality of the spectrometer can be expanded. Hence, a compact variable design with favorable focusing and dispersive properties can be achieved by one single component instead of traditional cascading optics, thus shrinking the volume to the millimeter scale and reducing cost. This research proves the potential for applications of metalenses in spectrometers as well as other consumer and industry products.

Microwave Metamaterial Absorbers with Controllable Luminescence Features

Traditional microwave absorbers can hide target objects from the detection of radar waves without tackling optical waves. However, in actual scenarios, the objects might still be observed by a hyperspectral remote sensor or an imaging system from comparison with environmental optical features, in particular for variable backgrounds. For this aspect, a comprehensive stealthing technique able to deal with microwaves and optical features simultaneously adapting to the variable environment is highly desired. In this work, an ultrathin flexible metamaterial that can simultaneously realize wideband microwave absorption and controllable visible and near-infrared luminescence spectra is proposed. The designed artificial coat can absorb more than 80% of the incident energy in the X-band (8-12 GHz) within a large incidence angle range up to 54° at low polarization sensitivity, while its real-time visible and near-infrared luminescence spectrum can be electrically adjusted through an integrated emission system. The method proposed here can be extended to broader wave bands and find important applications in multifunctional stealthing technologies.

Ultrawide-angle and high-efficiency metalens in hexagonal arrangement

Wide-angle optical systems play a vital role in imaging applications and have been researched for many years. In traditional lenses, attaining a wide field of view (FOV) by using a single optical component is difficult because these lenses have crucial aberrations. In this study, we developed a wide-angle metalens with a numerical aperture of 0.25 that provided a diffraction-limited FOV of over 170° for a wavelength of 532 nm without the need for image stitching or multiple lenses. The designed wide-angle metalens is free of aberration and polarization, and its full width of half maximum is close to the diffraction limit at all angles. Moreover, the metalens which is designed through a hexagonal arrangement exhibits higher focusing efficiency at all angles than most-seen square arrangement. The focusing efficiencies are as high as 82% at a normal incident and 45% at an incident of 85°. Compared with traditional optical components, the proposed metalens exhibits higher FOV and provides a more satisfactory image quality because of aberration correction. Because of the advantages of the proposed metalens, which are difficult to achieve for a traditional single lens, it has the potential to be applied in camera systems and virtual and augmented reality.

Spin-dependent switchable metasurfaces using phase change materials

Metasurfaces have been widely investigated for various applications enabled by their strong light manipulation capabilities. Their monolithic designs offer the convenience to incorporate novel natural materials in order to realize advanced electromagnetic (EM) functionalities. Here, based on the usage of the phase change material vanadium dioxide (VO₂), a switchable metasurface that could work at two different working states is proposed. With insulating VO₂, we show that helicity-dependent metasurface could be rigorously designed by adopting two phase variables, i.e., initial phase and Pancharatnam-Berry (P-B) phase, which is verified by showing an asymmetric photonic spin Hall effect (APSHE). When VO₂ goes into the metallic phase (e.g., by raising the operating temperature above ~341K), the loss factor of the unit cell will be enhanced, and in this case with the assistance of multi-mode resonances, the metasurface will turn into a perfect broadband circular-polarization-insensitive EM absorber. Based on these, switchable beam splitters and focus-lenses have been designed and discussed in the paper. The method proposed here may pave a new way to pursue active and multifunctional optical devices.

Special Issue on “Metasurfaces: Physics and Applications”

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