High-Efficiency, Broadband, Near Diffraction-Limited, Dielectric Metalens in Ultraviolet Spectrum

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.

Ultraviolet metalens for photoacoustic microscopy with an elongated depth of focus

Ultraviolet photoacoustic microscopy (UV-PAM) can achieve in vivo imaging without exogenous markers and play an important role in pathological diagnosis. However, traditional UV-PAM is unable to detect enough photoacoustic signals due to the very limited depth of focus (DOF) of excited light and the sharp decrease in energy with increasing sample depth. Here, we design a millimeter-scale UV metalens based on the extended Nijboer-Zernike wavefront-shaping theory which can effectively extend the DOF of a UV-PAM system to about 220 μm while maintaining a good lateral resolution of 1.063 μm. To experimentally verify the performance of the UV metalens, a UV-PAM system is built to achieve the volume imaging of a series of tungsten filaments at different depths. This work demonstrates the great potential of the proposed metalens-based UV-PAM in the detection of accurate diagnostic information for clinicopathologic imaging.

Wavelength-Independent Excitation Bessel Beams for High-Resolution and Deep Focus Imaging

Bessel beams are attaining keen interest in the current era considering their unique non-diffractive, self-healing nature and their diverse applications spanning over a broad spectral range of microwave to optical frequencies. However, conventional generators are not only bulky and complex but are also limited in terms of numerical aperture (NA) and efficiency. In this study, we experimentally develop a wavelength-independent Bessel beam generator through custom-designed metasurfaces to accomplish high resolution and large depth-of-focus imaging. These meta-axicons exhibit a high NA of up to 0.7 with an ability to generate Bessel beams with a full width at half maximum (FWHM) of 300 nm (~λ/2) and a depth of focus (DOF) of 153 μm (~261λ) in a broad spectral range of 500-700 nm. This excitation approach can provide a promising avenue for cutting-edge technology and applications related to Bessel beams for imaging along with a high axial resolution and an ultra-large depth of focus.

Investigations on Grating-Enhanced Waveguides for Wide-Angle Light Couplings

As a universal physical scheme, effective light couplings to waveguides favor numerous applications. However, the low coupling efficiency at wide angles prohibits this fundamental functionality and thus lowers the performance levels of photonic systems. As previously found, the transmission gratings patterned on waveguide facets could significantly improve the large-angle-inputted efficiency to the order of 10-1. Here, we continue this study with a focus on a common scenario, i.e., a grating-modified waveguide excited by the Gaussian beam. A simplified 2D theoretical model is firstly introduced, proving that the efficiency lineshape could be well flattened by elaborately arranged diffractive gratings. For demonstration, subsequent explorations for proper grating geometries were conducted, and four structural configurations were selected for later full-wave numerical simulations. The last comparison studies showcase that the analytical method approximates the finite element method-based modelings. Both methods highlight grating-empowered coupling efficiencies, being 2.5 bigger than the counterparts of the previously reported seven-ring structure. All in all, our research provides instructions to simulate grating effects on the waveguide's light-gathering abilities. Together with algorithm-designed coupling structures, it would be of great interest to further benefit real applications, such as bioanalytical instrumentation and quantum photon probes.

Micro-dimensional oscillation-based optimization for a dielectric metalens in the mid-infrared

In the past few decades, there has been significant progress made in metasurfaces and integrated and miniaturized optical devices. As one of the most prominent applications of metasurfaces, the metalens is the subject of significant research. In this paper, for achieving better focusing performance of the initial metalens designed by the Pancharatnam-Berry (PB) phase, a concept of micro-dimensional oscillation is proposed to optimize the geometric parameters of nanopillars. A strategy of grouping iteration is proposed to reduce the loss rate and computational effort in a holistic way. Its essence is to divide an extremely large-scale optimization space into many overlapping groups. Meanwhile, an improved genetic-simulated annealing (IGSA) algorithm is presented for the optimal solution of each group. By introducing the adaptive crossover and mutation probabilities in traditional genetic algorithms, the IGSA algorithm has both strong global searching capability and excellent local searching capability. After optimization, the maximum field intensity of the central hot spot can be increased by about 8% compared to the initial metalens. Moreover, the field intensity of the side lobes around the hot spot is almost constant, and the central hot spot increases, which provides a potential for the realization of high imaging contrast.

Focusing Characteristics and Widefield Imaging Performance of the Silicon Metalens in the Visible Range

Conventional optical high numerical aperture lenses are essential for high-resolution imaging, but bulky and expensive. In comparison, metalens-based optical components are the subjects of intensive investigation for their flexible manipulation of light. Methods of detecting and characterizing focal spots and scanning imaging produced by metalenses are well established. However, widefield imaging by metalenses is experimentally challenging. This study demonstrates the design and realization of silicon-based metalenses with numerical apertures of 0.447 and 0.204 in the broadband spectrum of 580–780 nm for microscopic widefield imaging. The optimized aspect ratio of the single nanorod is 5.1:1, which reduces the fabrication difficulty compared to other, more complicated designs and fabrication. Furthermore, we successfully demonstrate widefield imaging by the designed metalens and compare the simulated and the experimentally extracted modulation transfer function curves of the metalens.

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.

Generating ultraviolet perfect vortex beams using a high-efficiency broadband dielectric metasurface

Due to the topological charge-independent doughnut spatial structure as well as the association of orbital angular momentums, perfect vortex beams promise significant advances in fiber communication, optical manipulation and quantum optics. Inspired by the development of planar photonics, several plasmonic and dielectric metasurfaces have been constructed to generate perfect vortex beams, instead of conventional bulky configuration. However, owing to the intrinsic Ohmic losses and interband electron transitions in materials, these metasurface-based vortex beam generators only work at optical frequencies up to the visible range. Herein, using silicon nitride nanopillars as high-efficiency half-wave plates, broadband and high-performance metasurfaces are designed and demonstrated numerically to directly produce perfect vortex beams in the ultraviolet region, by combining the phase profiles of spiral phase plate, axicon and Fourier transformation lens based on geometric phase. The conversion efficiency of the metasurface is up to 86.6% at the design wavelength. Moreover, the influence of several control parameters on perfect vortex beam structures is discussed. We believe that this ultraviolet dielectric generator of perfect vortex beams will find many significant applications, such as high-resolution spectroscopy, optical tweezer and on-chip communication.

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.

Broadband achromatic metalens for linearly polarized light from 450 to 800 nm

Metalens is a planar optical component that uses nanostructures with a thickness on the order of the wavelength to manipulate the wavefront of the incident light. A key problem, especially in color imaging and display applications, is the correction of chromatic aberration, which is an inherent effect caused by the dispersion of periodic lattices and resonance modes. However, the current achromatic metalenses either use the PB phase method that is only valid for circularly polarized light or nanostructures with complex cross sections that are difficult to manufacture. Here, we designed a broadband achromatic metalens for linearly polarized light from 450 to 800 nm. Rectangular titanium dioxide nanofins of various lengths and widths were applied to modulate the phase and dispersion of the incident light. The metalens can fulfill three target phases simultaneously by using an optimization method. The designed metalens has a stable focus from 450 to 800 nm with an average focusing efficiency of 64%. It can be potentially applied in microscopes, lithography machines, sensors, and displays.

Time-Effective Simulation Methodology for Broadband Achromatic Metalens Using Deep Neural Networks

Metasurface has demonstrated potential and novel optical properties in previous research. The prevailing method of designing a macroscale metasurface is based on the local periodic approximation. Such a method relies on the pre-calculated data library, including phase delay and transmittance of the nanostructure, which is rigorously calculated by the electromagnetic simulation. However, it is usually time-consuming to design a complex metasurface such as broadband achromatic metalens due the required huge data library. This paper combined different numbers of nanofins and used deep neural networks to train our data library, and the well-trained model predicted approximately ten times more data points, which show a higher transmission for designing a broadband achromatic metalens. The results showed that the focusing efficiency of designed metalens using the augmented library is up to 45%, which is higher than that using the original library over the visible spectrum. We demonstrated that the proposed method is time-effective and accurate enough to design complex electromagnetic problems.

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

Flat lens or metalens, as one of the most important application branches of metasurfaces, has recently been attracting significant research interest. Various reflective and transmissive metalenses have been demonstrated in the terathertz, infrared and visible wavelength range. However, metalens operating in the ultraviolet (UV) wavelength range is rare. Moreover, the development of reflective UV metalens, the important counterpart of transmissive ones, falls far behind. In this work, with thorough investigation of material properties, we propose a reflective metalens based on silicon dioxide (SiO₂) and aluminum (Al) that operates in the vacuum ultraviolet (VUV) to visible wavelength region. Four reflective metalenses were designed and optimized for wavelengths of 193, 441, 532 and 633 nm, and prominent focusing capability was observed, especially for the VUV wavelength of 193 nm. Dispersion characteristics of the metalenses were also studied within ±50 nm of the design wavelength, and negative dispersion was found for all cases. In addition, the SiO₂ + Al platform can be, in principle, extended to the mid-infrared (IR) wavelength range. The reflective VUV metalens proposed in this work is expected to propel miniaturization and integration of UV optics.

A hybrid broadband metalens operating at ultraviolet frequencies

The investigation on metalenses have been rapidly developing, aiming to bring compact optical devices with superior properties to the market. Realizing miniature optics at the UV frequency range in particular has been challenging as the available transparent materials have limited range of dielectric constants. In this work we introduce a low absorption loss and low refractive index dielectric material magnesium oxide, MgO, as an ideal candidate for metalenses operating at UV frequencies. We theoretically investigate metalens designs capable of efficient focusing over a broad UV frequency range (200-400 nm). The presented metalenses are composed of sub-wavelength MgO nanoblocks, and characterized according to the geometric Pancharatnam-Berry phase method using FDTD method. The presented broadband metalenses can focus the incident UV light on tight focal spots (182 nm) with high numerical aperture ([Formula: see text]). The polarization conversion efficiency of the metalens unit cell and focusing efficiency of the total metalens are calculated to be as high as 94%, the best value reported in UV range so far. In addition, the metalens unit cell can be hybridized to enable lensing at multiple polarization states. The presented highly efficient MgO metalenses can play a vital role in the development of UV nanophotonic systems and could pave the way towards the world of miniaturization.

Polarization Insensitive, Broadband, Near Diffraction-Limited Metalens in Ultraviolet Region

Metasurfaces in the ultraviolet spectrum have stirred up prevalent research interest due to the increasing demand for ultra-compact and wearable UV optical systems. The limitations of conventional plasmonic metasurfaces operating in transmission mode can be overcome by using a suitable dielectric material. A metalens holds promising wavefront engineering for various applications. Metalenses have developed a breakthrough technology in the advancement of integrated and miniaturized optical devices. However, metalenses utilizing the Pancharatnam–Berry (PB) phase or resonance tuning methodology are restricted to polarization dependence and for various applications, polarization-insensitive metalenses are highly desirable. We propose the design of a high-efficiency dielectric polarization-insensitive UV metalens utilizing cylindrical nanopillars with strong focusing ability, providing full phase delay in a broadband range of Ultraviolet light (270–380 nm). The designed metalens comprises Silicon nitride cylindrical nanopillars with spatially varying radii and offers outstanding polarization-insensitive operation in the broadband UV spectrum. It will significantly promote and boost the integration and miniaturization of the UV photonic devices by overcoming the use of Plasmonics structures that are vulnerable to the absorption and ohmic losses of the metals. The focusing efficiency of the designed metalens is as high as 40%.