Design of Polarization-Independent Reflective Metalens in the Ultraviolet-Visible Wavelength Region

Dual-wavelength UV-visible metalens for multispectral photoacoustic microscopy: A simulation study

Photoacoustic microscopy is advancing with research on utilizing ultraviolet and visible light. Dual-wavelength approaches are sought for observing DNA/RNA- and vascular-related disorders. However, the availability of high numerical aperture lenses covering both ultraviolet and visible wavelengths is severely limited due to challenges such as chromatic aberration in the optics. Herein, we present a groundbreaking proposal as a pioneering simulation study for incorporating multilayer metalenses into ultraviolet-visible photoacoustic microscopy. The proposed metalens has a thickness of 1.4 µm and high numerical aperture of 0.8. By arranging cylindrical hafnium oxide nanopillars, we design an achromatic transmissive lens for 266 and 532 nm wavelengths. The metalens achieves a diffraction-limited focal spot, surpassing commercially available objective lenses. Through three-dimensional photoacoustic simulation, we demonstrate high-resolution imaging with superior endogenous contrast of targets with ultraviolet and visible optical absorption bands. This metalens will open new possibilities for downsized multispectral photoacoustic microscopy in clinical and preclinical applications.

Reflective metalens with an enhanced off-axis focusing performance

Metalenses are one of the most promising metasurface applications. However, all-dielectric reflective metalenses are rarely studied, especially regarding their off-axis focusing performance. After experimentally studying the material optical properties in this work, we propose reflective metalens based on titanium dioxide (TiO₂) and silicon dioxide (SiO₂), which operate at a visible wavelength of 0.633 µm. Unlike conventional reflective metalenses based on metallic mirrors, the proposed device was designed based on a modified parabolic phase profile and was integrated onto a dielectric distributed Bragg reflector periodic structure to achieve high reflectivity with five dielectric pairs. The focusing efficiency characteristics of the metalens were experimentally studied for beam angles of incidence between 0^∘ and 30^∘. The results reveal that the focusing efficiency for the modified metalens design remains higher than 54%, which is higher than 50%, making it promising for photonic miniaturization and integration.

Individually tunable array reflector for amplitude and phase modulation

Based on graphene's phase modulation property and vanadium dioxide's amplitude modulation property, we developed an array reflector for terahertz frequencies that is individually adjustable. Starting with a theoretical analysis, we look into the effects of voltage on the Fermi level of graphene and temperature on the conductivity of vanadium dioxide, analyze the beam focusing characteristics, and finally link the controllable quantities with the reflected beam characteristics to independently regulate each cell in the array. The simulation findings demonstrate that the suggested array structure can precisely manage the focus point's position, intensity, and scattering degree and that, with phase compensation, it can control the wide-angle incident light. The array structure offers a novel concept for adjustable devices and focusing lenses, which has excellent potential for study and application.

Multifunctional tunable visible light metalens based on double-layer barium titanate

A tunable metalens plays an indispensable role in the development of integrated optics, multi-imaging systems, etc. We propose a multifunctional tunable metalens that combines a double-layer barium titanate (BTO) structure and geometric phase in the visible light band. The refractive indices of the upper and lower layer BTO nanorods can be tuned continuously and independently by applying external voltage (0-60 V), and the lower layer can be converted between a full-wave plate and normal scattering unit, while the scatterers of the upper layer can be switched between a half-wave plate and full-wave plate. The voltages of the upper and lower layers can be adjusted to achieve different functions such as optical switches, conversion between monofocal and bifocal metalenses, adjustment of bifocal intensity, and broadband focusing (585-690 nm). Simulation results show that the multifunctional tunable metalens has a good focusing effect. A metalens with high focusing efficiency, dynamic reconfigurability, and a switching function has tremendous application potential in the fields of multifunctional devices, biomedicine, optical communication, imaging, and so on.

Computational design and optimization of nanostructured AlN deep-UV grating reflectors

Deep-ultraviolet (DUV) optoelectronics require innovative light collimation and extraction schemes for wall-plug efficiency improvements. In this work, we computationally survey material limitations and opportunities for intense, wavelength-tunable DUV reflection using AlN-based periodic hole and pillar arrays. Refractive-index limitations for underlayer materials supporting reflection were identified, and MgF₂ was chosen as a suitable low-index underlayer for further study. Optical resonances giving rise to intense reflection were then analyzed in AlN/MgF₂ nanostructures by varying film thickness, duty cycle, and illumination incidence angle, and were categorized by the emergence of Fano modes sustained by guided mode resonances (holes) or Mie-like dipole resonances (pillars). The phase-offset conditions between complementary modes that sustain high reflectance (%R) were related to a thickness-to-pitch ratio (TPR) parameter, which depended on the geometry-specific resonant mechanism involved (e.g., guided mode vs. Mie dipole resonances) and yielded nearly wavelength-invariant behavior. A rational design space was constructed by pointwise TPR optimization for the entire DUV range (200-320 nm). As a proof of concept, this optimized phase space was used to design reflectors for key DUV wavelengths and achieved corresponding maximum %R of 85% at λ = 211 nm to >97% at λ = 320 nm.

Design and Numerical Analysis of an Infrared Cassegrain Telescope Based on Reflective Metasurfaces

Reflective imaging systems such as Cassegrain-type telescopes are widely utilized in astronomical observations. However, curved mirrors in traditional Cassegrain telescopes unavoidably make the imaging system bulky and costly. Recent developments in the field of metasurfaces provide an alternative way to construct optical systems, possessing the potential to make the whole system flat, compact and lightweight. In this work, we propose a design for a miniaturized Cassegrain telescope by replacing the curved primary and secondary mirrors with flat and ultrathin metasurfaces. The meta-atoms, consisting of SiO₂ stripes on an Al film, provide high reflectance (>95%) and a complete phase coverage of 0~2π at the operational wavelength of 4 μm. The optical functionality of the metasurface Cassegrain telescope built with these meta-atoms was confirmed and studied with numerical simulations. Moreover, fabrication errors were mimicked by introducing random width errors to each meta-atom; their influence on the optical performance of the metasurface device was studied numerically. The concept of the metasurface Cassegrain telescope operating in the infrared wavelength range can be extended to terahertz (THz), microwave and even radio frequencies for real-world applications, where metasurfaces with a large aperture size are more easily obtained.

Silicon Metalens Fabrication from Electron Beam to UV-Nanoimprint Lithography

This study presents the design and manufacture of metasurface lenses optimized for focusing light with 1.55 µm wavelength. The lenses are fabricated on silicon substrates using electron beam lithography, ultraviolet-nanoimprint lithography and cryogenic deep reactive-ion etching techniques. The designed metasurface makes use of the geometrical phase principle and consists of rectangular pillars with target dimensions of height h = 1200 nm, width w = 230 nm, length l = 354 nm and periodicity p = 835 nm. The simulated efficiency of the lens is 60%, while the master lenses obtained by using electron beam lithography are found to have an efficiency of 45%. The lenses subsequently fabricated via nanoimprint are characterized by an efficiency of 6%; the low efficiency is mainly attributed to the rounding of the rectangular nanostructures during the pattern transfer processes from the resist to silicon due to the presence of a thicker residual layer.