Broadband achromatic metasurfaces for sub-diffraction focusing in the visible

Far-field sub-diffraction focusing and controlled focus shaping of circularly polarized light using a dielectric phase plate

In recent years, sub-diffraction focusing has received substantial attention due to its versatility. However, achieving a flexible sub-diffraction focusing in the far field remains stimulating. Existing techniques either require complex fabrication facilities or are limited to the short focal length and high numerical aperture (NA) of the imaging system. Here, we introduce an optimization method for sub-diffraction focusing of a circularly polarized beam in the far field with a lens of large focal length. A cost-effective dielectric phase plate serves the purpose. By employing a phase plate composed of a thin layer of dielectric S i 3 N 4, the phase of the propagating beam is modulated in the beam's cross-section, which is divided into two regions of the opposite phase by the plate. A sub-diffraction focusing is achieved for a proper tunning between the two regions. In addition to sub-diffraction focusing, the phase plate is also capable of shaping the focus into a doughnut-shaped and a flat-top profile in the far field. This design provides a simple solution for sub-diffraction focusing and focus shaping that will find potential applications in optical imaging, optical trapping, and material processing.

Computational hyperspectral devices based on quasi-random metasurface supercells

Computational hyperspectral devices that use artificial filters have shown promise as compact spectral devices. However, the current designs are restricted by limited types and geometric parameters of unit cells, resulting in a high cross-correlation between the transmission spectra. This limitation prevents the fulfillment of the requirement for compressed-sensing-based spectral reconstruction. To address this challenge, we proposed and simulated a novel design for computational hyperspectral devices based on quasi-random metasurface supercells. The size of the quasi-random metasurface supercell was extended above the wavelength, which enables the exploration of a larger variety of symmetrical supercell structures. Consequently, more quasi-random supercells with lower polarization sensitivity and their spectra with low cross-correlation were obtained. Devices for narrowband spectral reconstruction and broadband hyperspectral single-shot imaging were designed and fabricated. Combined with the genetic algorithm with compressed sensing, the narrowband spectral reconstruction device reconstructs the complex narrowband hyperspectral signal with 6 nm spectral resolution and ultralow errors. The broadband hyperspectral device reconstructs a broadband hyperspectral image (λ/λ ∼ 0.001) with a high average signal fidelity of 92%. This device has the potential to be integrated into a complementary metal-oxide-semiconductor (CMOS) chip for single-shot imaging.

Rapid deep-learning-assisted design method for 2-bit coding metasurfaces

This paper proposes a deep-learning-assisted design method for 2-bit coding metasurfaces. This method uses a skip connection module and the idea of an attention mechanism in squeeze-and-excitation networks based on a fully connected network and a convolutional neural network. The accuracy limit of the basic model is further improved. The convergence ability of the model increased nearly 10 times, and the mean-square error loss function converges to 0.000168. The forward prediction accuracy of the deep-learning-assisted model is 98%, and the accuracy of inverse design results is 97%. This approach offers the advantages of an automatic design process, high efficiency, and low computational cost. It can serve users who lack metasurface design experience.

Miniaturized solar-blind ultraviolet imaging system enabled by a diffractive/refractive hybrid

In this paper, we demonstrated a miniaturized diffractive/refractive hybrid system based on a diffractive optical element and three refractive lenses to achieve solar-blind ultraviolet imaging within a range of 240-280 nm. We experimentally demonstrate the optical system has both outstanding resolution and excellent imaging capability. The experiments demonstrate that the system could distinguish the smallest line pair with a width of 16.7 µm. The modulation transfer function (MTF) at the target maximum frequency (77 lines pair/mm) is great than 0.76. The strategy provides significant guidance for the mass production of solar-blind ultraviolet imaging systems towards miniaturization and lightweight.

Polarization-modulated broadband achromatic bifunctional metasurface in the visible light

Achromatic bifunctional metasurface is of great significance in optical path miniaturization among advanced integrated optical systems. However, the reported achromatic metalenses mostly utilize a phase compensate scheme, which uses geometric phase to realize the functionality and uses transmission phase to compensate the chromatic aberration. In the phase compensation scheme, all the modulation freedoms of a nanofin are driven at the same time. This makes most of the broadband achromatic metalenses restricted to realizing single function. Also, the phase compensate scheme is always addressed with circularly polarized (CP) incidence, leading to a limitation in efficiency and optical path miniaturization. Moreover, for a bifunctional or multifunctional achromatic metalens, not all the nanofins will work at the same time. Owing to this, achromatic metalenses using a phase compensate scheme are usually of low focusing efficiencies. To this end, based on the pure transmission phase in the x-/y- axis provided by the birefringent nanofins structure, we proposed an all-dielectric polarization-modulated broadband achromatic bifunctional metalens (BABM) in the visible light. Applying two independent phases on one metalens at the same time, the proposed BABM realizes achromatism in a bifunctional metasurface. Releasing the freedom of nanofin's angular orientation, the proposed BABM breaks the dependence on CP incidence. As an achromatic bifunctional metalens, all the nanofins on the proposed BABM can work at the same time. Simulation results show that the designed BABM is capable of achromatically focusing the incident beam to a single focal spot and an optical vortex (OV) under the illumination of x- and y-polarization, respectively. In the designed waveband 500 nm (green) to 630 nm (red), the focal planes stay unchanged at the sampled wavelengths. Simulation results prove that the proposed metalens not only realized bifunctional achromatically, but also breaks the dependence of CP incidence. The proposed metalens has a numerical aperture of 0.34 and efficiencies of 33.6% and 34.6%. The proposed metalens has advantages of being flexible, single layer, convenient in manufacturing, and optical path miniaturization friendly, and will open a new page in advanced integrated optical systems.

Thermally reconfigurable metalens

Reconfigurable metalenses are compact optical components composed by arrays of meta-atoms that offer unique opportunities for advanced optical systems, from microscopy to augmented reality platforms. Although poorly explored in the context of reconfigurable metalenses, thermo-optical effects in resonant silicon nanoresonators have recently emerged as a viable strategy to realize tunable meta-atoms. In this work, we report the proof-of-concept design of an ultrathin (300 nm thick) and thermo-optically reconfigurable silicon metalens operating at a fixed, visible wavelength (632 nm). Importantly, we demonstrate continuous, linear modulation of the focal-length up to 21% (from 165 μm at 20 °C to 135 μm at 260 °C). Operating under right-circularly polarized light, our metalens exhibits an average conversion efficiency of 26%, close to mechanically modulated devices, and has a diffraction-limited performance. Overall, we envision that, combined with machine-learning algorithms for further optimization of the meta-atoms, thermally reconfigurable metalenses with improved performance will be possible. Also, the generality of this approach could offer inspiration for the realization of active metasurfaces with other emerging materials within field of thermo-nanophotonics.

All-Dielectric Metasurface Lenses for Achromatic Imaging Applications

Metasurface can use artificial microstructures to manipulate electromagnetic waves more accurately and flexibly. All-dielectric metalens have a wide range of materials and low cost so it has a wide application prospect. Herein, we propose a all-dielectric achromatic metalens built with Si as the structural unit that can operate over a broadband of wavelengths in the visible region. It controls the wavefront of light through the Pancharatnam-Berry phase and propagation phase to eliminate the chromatic aberration. Meanwhile, we also use Gerchberg-Saxton algorithm and its improved algorithm to iterate over multiple design wavelengths and obtain holographic phases suitable for broadband. Thus, both the metalenses and holographic metasurfaces can achieve achromatic broadband in the visible light range, which provides a new method for the development of meta-optical imaging devices.

Robust Achromatic All-Dielectric Metalens for Infrared Detection in Intelligent Inspection

Metalens has the advantages of high design freedom, light weight and easy integration, thus provides a powerful platform for infrared detection. Here, we numerically demonstrated a broadband achromatic infrared all-dielectric metalens over a continuous 800 nm bandwidth, with strong environmental adaptability in air, water and oil. By building a database with multiple 2π phase coverage and anomalous dispersions, optimizing the corrected required phase profiles and designing the sizes and spatial distributions of silicon nanopillars, we numerically realized the design of broadband achromatic metalens. The simulation results of the designed metalens show nearly constant focal lengths and diffraction-limited focal spots over the continuous range of wavelengths from 4.0 to 4.8 μm, indicating the ability of the designed metalens to detect thermal signals over a temperature range from various fault points. Further simulation results show that the metalens maintains good focusing performance under the environment of water or oil. This work may facilitate the application of metalens in ultra-compact infrared detectors for power grid faults detection.

Design of a vortex metalens with high focusing efficiency using propagation phase

Three vortex-focused beams are produced with linearly polarized light along the x or y axis at a wavelength of 1550 nm. First, a polarization-independent vortex metalens with a topological charge of three and focal length of 3000 nm is designed by selecting cylindrical-shaped elements. This design has a focusing efficiency of 83%. Second, vortex beams with different focal lengths and topological charges are achieved by combining various shapes of structures. Both designs have a focusing efficiency of greater than 92%. The designed metasurface is of great significance to optical communication and radar detection.

RGB Achromatic Metalens Doublet for Digital Imaging

Chromatic aberration is a major challenge faced by metalenses. Current methods to achieve broadband achromatic operation in metalenses usually suffer from limited size, numerical aperture, and working bandwidth due to the finite group delay of meta-atoms, thus restricting the range of practical applications. Multiwavelength achromatic metalenses can overcome those limitations, making it possible to realize larger numerical aperture (NA) and sizes simultaneously. However, they usually require three layers, which increases their fabrication complexity, and have only been demonstrated in small sizes, with low numerical aperture and modest efficiencies. Here, we demonstrate a 1 mm diameter red-green-blue achromatic metalens doublet with a designed NA of 0.8 and successfully apply the metalens in a digital imaging system. This work shows the potential of the doublet metasurfaces, extending their applications to digital imaging systems such as digital projectors, virtual reality glasses, high resolution microscopies, etc.

Band-tunable achromatic metalens based on phase change material

Achromatic metalens have the potential to significantly reduce the size and complexity of broadband imaging systems. A large variety of achromatic metalens has been proposed and most of them have the fixed achromatic band that cannot be actively modified. However, band-tunable is an important function in practical applications such as fluorescence microscopic imaging and optical detection. Here, we propose a bilayer metalens that can switch achromatic bands by taking the advantage of the high refractive index contrast of Sb₂S₃ between amorphous and crystalline state. By switching the state of Sb₂S₃, the achromatic band can be reversibly switched between the red region of visible spectrum (650-830 nm) and the near-infrared spectrum (830-1100 nm). This band-tunable design indicates a novel (to our knowledge) method to solve the problem of achromatic focusing in an ultrabroad band. The metalens have an average focusing efficiency of over 35% and 55% in two bands while maintaining diffraction-limited performance. Moreover, through proper design, we can combine different functionalities in two bands such as combining achromatic focusing and diffractive focusing. The proposed metalens have numerous potential applications in tunable displaying, detecting devices and multifunctional devices.

Broadband achromatic longitudinal bifocal metalens in the visible range based on a single nanofin unit cell

Metasurfaces provide a remarkable platform to manipulate over phase, amplitude, and polarization flexibly and precisely. Bifocal metalens draws great research interest due to its ability of converging wavefronts to different focal positions horizontally and longitudinally. However, as wavelength of incident light changes, chromatic aberration will cause the focal lengths reliance on the incident wavelength, which will affect the performance of metasurface, especially for longitudinal bifocal metalens. In this work, a broadband achromatic longitudinal bifocal metalens (BALBM) based on single nanofin unit cell is demonstrated. Pancharatnam-Berry (PB) phase is used to converge the incident light. Cross commixed sequence distribution (CCSD) is introduced to control the positions of focal points F_Land F_Rwhen left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) incident. Propagation phase is used to compensate the phase difference caused by chromatic aberration. Simulation results show that in the continuous wavelength range from 500 nm to 700 nm, the focal point shifts caused by chromatic dispersion are reduced 81% for F_L and 83% for F_R, respectively. The focal length variations are stabilized to 6.21% for F_Land 4.8% for F_Rcomparing with the focal lengths at the initial wavelength 500 nm. The proposed BALBM brings advances to bifocal metasurfaces in versatile application areas including machine vision, optical computed tomography and microimaging.

Broadband tunable metasurface platform enabled by dynamic phase compensation

Broadband metasurfaces have attracted significant attention for a variety of applications in imaging and communication systems. Here, a method to alleviate the chromatic aberrations issue is proposed in the microwave region using dynamic phase compensation enabled by a reconfigurable metasurface. The dispersion characteristic of the meta-atom implemented with varactor diodes can be flexibly manipulated electronically, such that the dispersion-induced phase distortions over a wide frequency band can be compensated dynamically to achieve broadband performances. Various aberration-free functionalities can be realized with the proposed active metasurface. Near-field measurements are performed on a fabricated prototype to demonstrate aberration-free beam bending and hologram imaging, showing good agreement with simulation results. Such an active metasurface platform paves the way to efficient devices for wireless power transfer, sensors, and communication and antenna systems at radio or much higher frequencies.

An Extended k-Surface Framework for Electromagnetic Fields in Artificial Media

The complete understanding of the electromagnetic field characteristics in artificially created bulk or thin media is essential to the efficient harnessing of the multitude of linear and nonlinear effects resulting from it. Due to the fact that recently developed artificial metastructures exhibit controllable electric and magnetic properties that are completely different from natural ones, the spectrum of behavior resulting from subjecting such media to electromagnetic fields has to be revisited. In this paper, we introduce a k-surface framework that offers complete information on the dispersion properties of media with designer electric and magnetic responses with positive and negative values, as well as for the coupling between the two. The extension from the classic k-surface case resides in the consideration of magnetic and bianisotropic materials with positive and negative permittivity and permeability values, as well as the introduction of the chirality coefficient.To illustrate the applicability of our framework, we have investigated the conditions to obtain collinear second harmonic generation in the case of artificial media with positively and negatively valued electric and magnetic responses. As expected, the phase matching tuning curves, defined as the intersections between the k-surfaces at both frequencies, are significantly modified with respect to the classic ones.

Environmentally robust immersion supercritical lens with an invariable sub-diffraction-limited focal spot

Planar metalenses provide an effective way to break the diffraction barrier in the far field. Their physical mechanism and applications have been intensively studied in the past decade. These investigations on sub-diffraction-limited light modulations have only been applied to specified single immersion environments; however, changing immersion environments can severely degrade their focusing performance, limiting their application potential. In this work, we propose and experimentally demonstrate an environmentally robust immersion supercritical lens (SCL) that can work in various immersion environments. The design of such a lens is based on the vectorial Rayleigh-Sommerfeld diffraction theory combined with a multi-objective optimization algorithm. The sub-diffraction-limited focusing effect has been experimentally demonstrated in commonly used media, including air, water, and oil, with refractive indices of 1.0, 1.33, and 1.51, respectively. Moreover, such a lens can maintain its effective numerical aperture at a fixed value, bringing a unique advantage in that the lateral size of the focal spots exhibits a similar value of ${{317}}\;{{\pm}}\;{{7}}\;{\rm{nm}}$ in all three media. Our demonstration provides the feasibility of SCLs in various application scenarios with multi-immersion environments, such as bioimaging, light trapping, and optical storage.

Reflective Quasi-Continuous Metasurface with Continuous Phase Control for Light Focusing

Benefitting from the arbitrary and flexible light modulation, metasurface has attracted extensive attention and been demonstrated in different applications. However, most reported metasurface-based devices were normally composed of discrete micro or nano structures, therefore the discretization precision limited the performance of the device, including the focusing efficiency, stray light suppression, and broadband performance. In this work, an all-metallic reflective metasurface consisting of numerous quasi-continuous nanostructures is proposed to realize high-efficiency and broadband focusing. The constructed quasi-continuous metasurface (QCMS) is then verified numerically through electromagnetic simulation, and the numerical results show a higher focusing efficiency and a better stray light suppression effect, compared to a binary-phase-based metalens. Through the same design strategy, a QCMS with the ability to overcome the diffraction limit can also be constructed, and a focal spot with the size of 0.8 times the diffraction limit can be achieved. We expect that this quasi-continuous structure could be utilized to construct other high-performance devices that require continuous phase controls, such as the beam deflector, orbital angle momentum generator, and self-accelerating beam generator.