Dual-layer achromatic metalens design with an effective Abbe number

Fluid-responsive tunable metasurfaces for high-fidelity optical wireless communication

Optical wireless communication (OWC), with its blazing data transfer speed and unparalleled security, is a futuristic technology for wireless connectivity. Despite the significant advancements in OWC, the realization of tunable devices for on-demand and versatile connectivity still needs to be explored. This presents a considerable limitation in utilizing adaptive technologies to improve signal directivity and optimize data transfer. This study proposes a unique platform that utilizes tunable, fluid-responsive multifunctional metasurfaces offering dynamic and unprecedented control over electromagnetic wave manipulation to enhance the performance of OWC networks. We have achieved real-time, on-demand beam steering with vary-focusing capability by integrating the fabricated metasurfaces with different isotropic fluids. Furthermore, the designed metasurfaces are capable of polarization-based switching of the diffracted light beams to enhance overall productivity. Our research has showcased the potential of fluid-responsive tunable metasurfaces in revolutionizing OWC networks by significantly improving transmission reliability and signal quality through real-time adjustments. The proposed methodology is verified by designing and fabricating an all-dielectric metasurface measuring 500 μm × 500 μm and experimentally investigating its fluid-responsive vary-focal capability. By incorporating fluid-responsive properties into spin-decoupled metasurfaces, we aim to develop advanced high-tech optical devices and systems to simplify beam-steering and improve performance, adaptability, and functionality, making the devices suitable for various practical applications.

Radial IR-GRIN lens prepared by multi-temperature fields manipulated gradient crystallization within chalcogenide glass

Chalcogenide glass has achieved great success in manufacturing axial-type infrared gradient refractive index (IR-GRIN) lenses. However, studies on radial-type IR-GRIN lenses, which are more ideal for optical design, remain rare. The present study introduces what we believe to be a new method for preparing radial IR-GRIN lens by creating high refractive index (n) In2S3 nanocrystals within a 65GeS2-25In2S3-10CsCl (GIC, in molar percentage) glass matrix. Upon introduction of multi-temperature field manipulation, we have successfully achieved central crystallization and simultaneous gradient attenuation spreading toward the edge within GIC glass, providing a radial GRIN profile with Δn over 0.1 while maintaining excellent IR transparency. In addition, the optical and structural properties of the GIC GRIN samples were characterized. The relationship between Raman intensity and the n of glass ceramics at different heat treatment temperatures was investigated, thereby enabling the indirect confirmation of the presence of radial gradient crystallization within the prepared GIC GRIN samples through Raman intensity. Multiple experimental results have shown that this approach has excellent reproducibility and potential for large-scale productions.

3D-printed multilayer structures for high-numerical aperture achromatic metalenses

Flat optics consisting of nanostructures of high-refractive index materials produce lenses with thin form factors that tend to operate only at specific wavelengths. Recent attempts to achieve achromatic lenses uncover a trade-off between the numerical aperture (NA) and bandwidth, which limits performance. Here, we propose a new approach to design high-NA, broadband, and polarization-insensitive multilayer achromatic metalenses (MAMs). We combine topology optimization and full-wave simulations to inversely design MAMs and fabricate the structures in low-refractive index materials by two-photon polymerization lithography. MAMs measuring 20 μm in diameter operating in the visible range of 400 to 800 nm with 0.5 and 0.7 NA were achieved with efficiencies of up to 42%. We demonstrate broadband imaging performance of the fabricated MAM under white light and RGB narrowband illuminations. These results highlight the potential of the 3D-printed multilayer structures for realizing broadband and multifunctional meta-devices with inverse design.

Theory and Fundamental Limit of Quasiachromatic Metalens by Phase Delay Extension

The periodic extension of phase difference is commonly applied in device design to obtain phase compensation beyond the system's original phase modulation capabilities. Based on this extension approach, we propose the application of quasiphase delay matching to extend the range of dispersion compensation for meta-atoms with limited height. Our theory expands the limit of frequency bandwidth coverage and relaxes the constraints of aperture, NA, and bandwidth for metalenses. By applying the uncertainty principle, we explain the fundamental limit of this achromatic bandwidth and obtain the achromatic spectrum using perturbation analysis. To demonstrate the effectiveness of this extended limit, we simulate a quasiachromatic metalens with a diameter of 2 mm and a NA of 0.55 in the range of 400-1500 nm. Our findings provide a novel theory for correcting chromatic aberration in large-diameter ultrawide bandwidth devices.

Chalcogenide GRIN glasses with high refractive index and large refractive index difference for LWIR imaging

Gradient refractive index (GRIN) materials utilize an internally tailored refractive index in combination with the designed curvature of the optical element surface, providing the optical designer with additional freedom for correcting chromatic and spherical aberrations. In this paper, new GRIN materials suitable for the second (3-5 µm) and third (8-12 µm) atmospheric windows were successfully developed by the thermal diffusion method based on Ge20As20Se60-xTex series high refractive index glasses, where the maximum refractive index difference (Δn) at 4 µm and 10.6 µm were 0.281 and 0.277, respectively. The diffusion characteristics and refractive index distribution of the GRIN glass were analyzed by Raman characterization. Furthermore, the performance of GRIN singlet and homogeneous singlet in the LWIR band (8 µm, 10.6 µm (primary wavelength), 12 µm) was compared, and the results showed that the GRIN singlet had better chromatic aberration correction and unique dispersion characteristics.

A Design Method of Diffraction Structure Based on Metasurface for High-Resolution Spectroscopy

In this paper, a design method of diffraction structure based on metasurface is proposed for light splitting and focusing simultaneously. In the method, firstly, the light field calculation model of the proposed structure is established based on Fresnel diffraction and the transmittance function is calculated. Then, the model structural parameter selection mechanism is determined, and the spectrum resolution equation of the structure is derived. Simulation results indicate that the proposed method can offer a broader working bandwidth and enhanced higher resolution compared to off-axis meta-lens. Moreover, this proposed method can be deployed in high-resolution, wide-band ultra-compact spectrometer systems potentially.

High Refractive Index GRIN Lens for IR Optics

Infrared gradient refractive index (GRIN) material lenses have attracted much attention due to their continuously varying refractive index as a function of spatial coordinates in the medium. Herein, a glass accumulation thermal diffusion method was used to fabricate a high refractive index GRIN lens. Six Ge_17.2As_17.2Se_xTe_(65-x) (x = 10.5-16) glasses with good thermal stability and high refractive index (n_{@10 μm} > 3.1) were selected for thermal diffusion. The refractive index span (∆n) of 0.12 was achieved in this GRIN lens. After thermal diffusion, the lens still had good transmittance (45%) in the range of 8-12 μm. Thermal imaging confirmed that this lens can be molded into the designed shape. The refractive index profile was indirectly characterized by the structure and composition changes. The structure and composition variation became linear with the increase in temperature from 260 °C to 270 °C for 12 h, indicating that the refractive index changed linearly along the axis. The GRIN lens with a high refractive index could find applications in infrared optical systems and infrared lenses for thermal imaging.

Multilayer dielectric metalens

We propose and analyze a metalens whose meta-atoms (nanoscatterers) are integrated by a stack of quarter-wave dielectric layers. Each multilayer meta-atom is a nanopillar, which consists of alternating layers of high- and low-refractive-index materials. We show that the nanopillars of a multilayer metalens may have a smaller aspect ratio than the nanopillars of a standard metalens and have similar optical properties (focusing efficiency and chromaticity).

Largest aperture metalens of high numerical aperture and polarization independence for long-wavelength infrared imaging

Because of unique superiorities of planar optical devices based on metalens on manipulating amplitude, phase, polarization, wavelength of incident light, metalenses have great prospects to replace traditional catadioptric optical components, especially in imaging and optoelectronic integration. However, the research of metalens has focused on visible or near-infrared wavelength in the past few years and little attention was paid to the long-wavelength infrared metalens. Here, we demonstrate the largest aperture, high numerical aperture, and polarization-independent metalens operating at long-wavelength infrared. The metalens has a numerical aperture of 0.45 at the center wavelength of10 µm. The aperture of the metalens is 80 mmwhich is much larger than the existing level we know. It has high-resolution imaging ability with focusing incident light down to a spot as small as ∼1.04λ. Ambient light imaging experiments are carried out to show the performance of the metalens. In addition, metalens is flimsy, large-scale and low-cost, which provides an effective solution for the development of ultra-lightweight and compact optical devices for LWIR technology.

Ultra-Thin, Short-Focus, and High-Aperture Metalens for Generating and Detecting Laser Optical Vortices

A combined high-aperture metalens in a thin silicon nitride film that consists of two tilted sectored metalenses is considered. Each sector of the metalens consists of a set of binary subwavelength gratings. The diameter of the metalens is 14 μm. Using a time-domain finite difference method, we show that the metalens can simultaneously detect optical vortices with two topological charges −1 and −2, almost over the entire spectrum of visible wavelengths. The metalens can distinguish several wavelengths that are focused at different points in the focal plane due to a 1-nm change in wavelength resulting in a focal spot shift of about 4 nm. When the metalens is illuminated by a Gaussian beam with left-handed circular polarization, two optical vortices with topological charges 1 and 2 are simultaneously formed 6-μm apart at the focal distance of 6 μm.

Dielectric metalens for miniaturized imaging systems: progress and challenges

Lightweight, miniaturized optical imaging systems are vastly anticipated in these fields of aerospace exploration, industrial vision, consumer electronics, and medical imaging. However, conventional optical techniques are intricate to downscale as refractive lenses mostly rely on phase accumulation. Metalens, composed of subwavelength nanostructures that locally control light waves, offers a disruptive path for small-scale imaging systems. Recent advances in the design and nanofabrication of dielectric metalenses have led to some high-performance practical optical systems. This review outlines the exciting developments in the aforementioned area whilst highlighting the challenges of using dielectric metalenses to replace conventional optics in miniature optical systems. After a brief introduction to the fundamental physics of dielectric metalenses, the progress and challenges in terms of the typical performances are introduced. The supplementary discussion on the common challenges hindering further development is also presented, including the limitations of the conventional design methods, difficulties in scaling up, and device integration. Furthermore, the potential approaches to address the existing challenges are also deliberated.

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.

High-efficiency one-dimensional metalens for 3D focusing

We demonstrate a high-efficiency on-chip one-dimensional metalens for three-dimensional (3D) light focusing. The metalens consists of a one-dimensional dielectric nano-antenna array, which scatters the evanescent wave of a nano-waveguide into free space and focuses this scattered light into a 3D ring. The corresponding phase profile of the metalens is controlled by the relative locations of antennas in the array. Through antenna-waveguide distance optimization, the designed metalens only scatters 1.5% of propagation light into free space and 55% of the scattered energy is focused into the 3D ring. When we use the antennas with an optimized shape, 50.18% of the focused energy is concentrated in a circular arc of the ring, which subtends an angle of 48°. This high-efficiency on-chip one-dimensional metalens is promising for non-invasive optical signal detection in photonic integrated chips.

Research on Fabrication Techniques and Focusing Characteristics of Metalens

Metalenses have recently attracted increased attention due to their remarkable characteristics. The fabrication technology of metalenses has also become an important research direction. In this study, we propose a metalens structure based on Au–MgF2–Au in infrared waveband. The preparation process of the metalens included magnetron sputtering, electron beam evaporation, and electron beam exposure. A dose test was performed during the exposure process, adjusting the exposure dose to minimize the proximity effect after exposure. Then, SEM was used to measure the processed metalens structure, and FDTD software was used to build a model based on the metalens, simulating and analyzing its focusing characteristics. The results show that the size deviation produced during the processing has little effect on the functionality of the metalens. The processed metalens can also focus different polarized light incidences at different spatial positions: The metalens can focus at 4.97 μm for x-polarized light and focus at 13.5 μm for y-polarized light. Additionally, the metalens has good focusing effects with different working wavelengths. We believe that the processing method of metalens proposed in this paper provides guidance for the preparation of subwavelength metasurface structures, and our findings are beneficial in developing new methods of near-infrared regime manipulation.

Spherical Aberration-Corrected Metalens for Polarization Multiplexed Imaging

We present a terahertz spherical aberration-corrected metalens that uses the dynamic phase to achieve polarization multiplexed imaging. The designed metalens has polarization-dependent imaging efficiencies and polarization extinction ratios that exceed 50% and 10:1, respectively. Furthermore, opposite gradient phases can be applied to orthogonal polarizations to shift the imaging of the two polarized sources in the longitudinal and transverse directions. Indeed, we find that the metalens has a smaller depth-of-focus than a traditional metalens when imaging point sources with limited objective lengths. These results provide a new approach for achieving multifunctional beam steering, tomographic imaging and chiroptical detection.

Transmissive mid-infrared achromatic bifocal metalens with polarization sensitivity

Metasurfaces have shown great potential in versatile areas such as vortex-beam generators, metalenses, holograms and so on. However, chromatic error hinders metasurfaces, especially metalenses, from wider applications. In this paper, we demonstrate a novel design for a transmissive mid-infrared achromatic bifocal metalens with polarization sensitivity. The compensation phase is used to eliminate the chromatic aberration. Simulation results show that, over a continuous waveband from 3.9 to 4.6µm, the focal length only changes by 2.26% with an average focusing efficiency of about 18%. This work can push the practical application of mid-infrared metasurfaces.