All-Silicon Broadband Ultraviolet Metasurfaces

Design and simulation of an extreme ultraviolet metalens based on the Pancharatnam-Berry phase

Extreme ultraviolet (EUV) radiation plays a key role in the fields of material science, attosecond metrology, and lithography. However, the reflective optical components typically used in EUV systems contribute to their bulky size, weight, and increased costs for fabrication. In this paper, we theoretically investigate transmissive metalens designs capable of focusing the EUV light based on the Pancharatnam-Berry phase. The designed metalens is composed of nanoscale elliptical holes, which can guide and manipulate EUV light due to the higher refractive index of the vacuum holes compared to that of the surrounding material. We designed an EUV metalens with a diameter of 10 µm, which supports a focal length of 24 µm and a numerical aperture of up to 0.2. It can focus 55-nm EUV incident light to a diffraction-limited spot, and the focusing efficiency is calculated to be as high as about 7% over a broad EUV frequency range (50-65 nm). This study reveals the possibility of applying a dielectric metalens in the EUV region without a transmissive optical material.

Spin-isolated ultraviolet-visible dynamic meta-holographic displays with liquid crystal modulators

Wearable displays or head-mounted displays (HMDs) have the ability to create a virtual image in the field of view of one or both eyes. Such displays constitute the main platform for numerous virtual reality (VR)- and augmented reality (AR)-based applications. Meta-holographic displays integrated with AR technology have potential applications in the advertising, media, and healthcare sectors. In the previous decade, dielectric metasurfaces emerged as a suitable choice for designing compact devices for highly efficient displays. However, the small conversion efficiency, narrow bandwidth, and costly fabrication procedures limit the device's functionalities. Here, we proposed a spin-isolated dielectric multi-functional metasurface operating at broadband optical wavelengths with high transmission efficiency in the ultraviolet (UV) and visible (Vis) regimes. The proposed metasurface comprised silicon nitride (Si₃N₄)-based meta-atoms with high bandgap, i.e., ∼ 5.9 eV, and encoded two holographic phase profiles. Previously, the multiple pieces of holographic information incorporated in the metasurfaces using interleaved and layer stacking techniques resulted in noisy and low-efficiency outputs. A single planar metasurface integrated with a liquid crystal was demonstrated numerically and experimentally in the current work to validate the spin-isolated dynamic UV-Vis holographic information at broadband wavelengths. In our opinion, the proposed metasurface can have promising applications in healthcare, optical security encryption, anti-counterfeiting, and UV-Vis nanophotonics.

Extreme ultraviolet metalens by vacuum guiding

Extreme ultraviolet (EUV) radiation is a key technology for material science, attosecond metrology, and lithography. Here, we experimentally demonstrate metasurfaces as a superior way to focus EUV light. These devices exploit the fact that holes in a silicon membrane have a considerably larger refractive index than the surrounding material and efficiently vacuum-guide light with a wavelength of ~50 nanometers. This allows the transmission phase at the nanoscale to be controlled by the hole diameter. We fabricated an EUV metalens with a 10-millimeter focal length that supports numerical apertures of up to 0.05 and used it to focus ultrashort EUV light bursts generated by high-harmonic generation down to a 0.7-micrometer waist. Our approach introduces the vast light-shaping possibilities provided by dielectric metasurfaces to a spectral regime that lacks materials for transmissive optics.

One-step printable platform for high-efficiency metasurfaces down to the deep-ultraviolet region

A single-step printable platform for ultraviolet (UV) metasurfaces is introduced to overcome both the scarcity of low-loss UV materials and manufacturing limitations of high cost and low throughput. By dispersing zirconium dioxide (ZrO₂) nanoparticles in a UV-curable resin, ZrO₂ nanoparticle-embedded-resin (nano-PER) is developed as a printable material which has a high refractive index and low extinction coefficient from near-UV to deep-UV. In ZrO₂ nano-PER, the UV-curable resin enables direct pattern transfer and ZrO₂ nanoparticles increase the refractive index of the composite while maintaining a large bandgap. With this concept, UV metasurfaces can be fabricated in a single step by nanoimprint lithography. As a proof of concept, UV metaholograms operating in near-UV and deep-UV are experimentally demonstrated with vivid and clear holographic images. The proposed method enables repeat and rapid manufacturing of UV metasurfaces, and thus will bring UV metasurfaces more close to real life.

Ultraviolet-Visible Multifunctional Vortex Metaplates by Breaking Conventional Rotational Symmetry

Metasurfaces have shown remarkable potential to manipulate many of light's intrinsic properties, such as phase, amplitude, and polarization. Recent advancements in nanofabrication technologies and persistent efforts from the research community result in the realization of highly efficient, broadband, and multifunctional metasurfaces. Simultaneous control of these characteristics in a single-layered metasurface will be an apparent technological extension. Here, we demonstrate a broadband multifunctional metasurface platform with the unprecedented ability to independently control the phase profile for two orthogonal polarization states of incident light over dual-wavelength spectra (ultraviolet to visible). In this work, multiple single-layered metasurfaces composed of bandgap-engineered silicon nitride nanoantennas are designed, fabricated, and optically characterized to demonstrate broadband multifunctional light manipulation ability, including structured beam generation and meta-interferometer implementation. We envision the presented metasurface platform opening new avenues for broadband multifunctional applications including ultraviolet-visible spectroscopy, spatially modulated illumination microscopy, optical data storage, and information encoding.

Highly Efficient Perfect Vortex Beams Generation Based on All-Dielectric Metasurface for Ultraviolet Light

Featuring shorter wavelengths and high photon energy, ultraviolet (UV) light enables many exciting applications including photolithography, sensing, high-resolution imaging, and optical communication. The conventional methods of UV light manipulation through bulky optical components limit their integration in fast-growing on-chip systems. The advent of metasurfaces promised unprecedented control of electromagnetic waves from microwaves to visible spectrums. However, the availability of suitable and lossless dielectric material for the UV domain hindered the realization of highly efficient UV metasurfaces. Here, a bandgap-engineered silicon nitride (Si3N4) material is used as a best-suited candidate for all-dielectric highly efficient UV metasurfaces. To demonstrate the wavefront manipulation capability of the Si3N4 for the UV spectrum, we design and numerically simulate multiple all-dielectric metasurfaces for the perfect vortex beam generation by combing multiple phase profiles into a single device. For different numerical apertures (NA =0.3 and 0.7), it is concluded that the diffracted light from the metasurfaces with different topological charges results in an annular intensity profile with the same ring radius. It is believed that the presented Si3N4 materials and proposed design methodology for PV beam-generating metasurfaces will be applicable in various integrated optical and nanophotonic applications such as information processing, high-resolution spectroscopy, and on-chip optical communication.

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.

Broadband polarization-insensitive metalens integrated with a charge-coupled device in the short-wave near-infrared range

The performance of a charge-coupled device is important in detection accuracy for terminal sensitivity of a short-wave near-infrared spectrometer. The sizes of pixel pitch and pixel itself are reducing with the development of CCD technologies. However, the fill factor of CCD pixels has not been significantly improved due to the limits of the shift registers, which makes a lower utilization rate of incident light of CCD. In recent years, the advance of metasurface optics provides a new solution for solving this problem. In this paper, a polarization-insensitive metalens is experimentally demonstrated to increase the fill factor of short-wave near-infrared CCD pixels by 4 times, and the simulated results show that the designed metalens has an excellent optical crosstalk (≤0.8%). It proves that the fill factor of CCD pixels can be further improved by the proposed approach which would pave the way for the overall integration of metalens array and photodetectors, as well as the development of CCD miniaturization and lightweight.

Photonic Encryption Platform via Dual-Band Vectorial Metaholograms in the Ultraviolet and Visible

Metasurface-driven optical encryption devices have attracted much attention. Here, we propose a dual-band vectorial metahologram in the visible and ultraviolet (UV) regimes for optical encryption. Nine polarization-encoded vectorial holograms are observed under UV laser illumination, while another independent hologram appears under visible laser illumination. The proposed engineered silicon nitride, which is transparent in UV, is employed to demonstrate the UV hologram. Nine holographic images for different polarization states are encoded using a pixelated metasurface. The dual-band metahologram is experimentally implemented by stacking the individual metasurfaces that operate in the UV and visible. The visible hologram can be decrypted to provide the first key, a polarization state, which is used to decode the password hidden in the UV vectorial hologram through the use of an analyzer. Considering the property of UV to be invisible to the naked eye, the multiple polarization channels of the vectorial hologram, and the dual-band decoupling, the demonstrated dual-band vectorial hologram device could be applied in various high-security and anticounterfeiting applications.

A review of dielectric optical metasurfaces for spatial differentiation and edge detection

Dielectric metasurfaces-based planar optical spatial differentiator and edge detection have recently been proposed to play an important role in the parallel and fast image processing technology. With the development of dielectric metasurfaces of different geometries and resonance mechanisms, diverse on-chip spatial differentiators have been proposed by tailoring the dispersion characteristics of subwavelength structures. This review focuses on the basic principles and characteristic parameters of dielectric metasurfaces as first- and second-order spatial differentiators realized via the Green's function approach. The spatial bandwidth and polarization dependence are emphasized as key properties by comparing the optical transfer functions of metasurfaces for different incident wavevectors and polarizations. To present the operational capabilities of a two-dimensional spatial differentiator in image information acquisition, edge detection is described to illustrate the practicability of the device. As an application example, experimental demonstrations of edge detection for different biological cells and a flower mold are discussed, in which a spatial differentiator and objective lens or camera are integrated in three optical pathway configurations. The realization of spatial differentiators and edge detection with dielectric metasurfaces provides new opportunities for ultrafast information identification in biological imaging and machine vision.

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.

Diamond step-index nanowaveguide to structure light efficiently in near and deep ultraviolet regimes

Two-dimensional metamaterials, consisting of an array of ultrathin building blocks, offer a versatile and compact platform for tailoring the properties of the electromagnetic waves. Such flat metasurfaces provide a unique solution to circumvent the limitations imposed by their three-dimensional counterparts. Albeit several successful demonstrations of metasurfaces have been presented in the visible, infrared, and terahertz regimes, etc., there is hardly any demonstration for ultraviolet wavelengths due to the unavailability of the appropriate lossless materials. Here, we present diamond as an ultra-low loss material for the near and deep ultraviolet (UV) light and engineer diamond step-index nanowaveguides (DSINs) to achieve full control over the phase and amplitude of the incident wave. A comprehensive analytical solution of step-index nanowaveguides (supported by the numerical study) is provided to describe the underlying mechanism of such controlled wavefront shaping. Due to the ultra-low loss nature of diamond in near and deep UV regimes, our DSINs and metasurfaces designed (from them) exhibit a decent efficiency of ≈ 84% over the entire spectrum of interest. To verify this high efficiency and absolute control over wavefront, we have designed polarization-insensitive meta-holograms through optimized DSINs for operational wavelength λ = 250 nm.

Symmetric accelerating beam generation via all-dielectric metasurfaces

Traditionally, symmetric accelerating beam (SAB) generation requires bulky optical components, which hinder the miniaturization of optical systems. Recently, metasurfaces, which are composed of sub-wavelength features, have provided a captivating boulevard for the realization of ultra-thin and flat optical devices. Therefore, for the first time, we design and simulate all-dielectric metasurfaces based on an optical caustic approach to generate highly efficient SABs by tailoring the phase of an incident wave. The designed metasurface utilizes spatial distribution of optimized Nb₂O₅ nano-rods on SiO₂ substrate to perform the phase modulation. In contrast with conventional accelerating beams, the generated SABs can follow any predefined propagation trajectory with unique features, such as symmetric intensity profile, autofocusing, and thin needle-like structure in their intensity profile. In addition to this, these beams have also shown the ability to avoid obstacles, placed in the direction of propagation of main lobes. We believe that these beams can be useful in applications, including Raman spectroscopy and fluorescent imaging, and multiparticle manipulation.

Ultrawide bandgap AlN metasurfaces for ultraviolet focusing and routing

All-dielectric metasurfaces offer a promising way to control amplitude, polarization, and phase of light. However, ultraviolet (UV) component metasurfaces are rarely reported due to significant absorption loss for most dielectric materials and the required smaller footprint or feature size. Here, we demonstrate broadband UV focusing and routing in both transmission and reflection modes in simulations by adopting aluminum nitride (AlN) with ultrawide bandgap and a waveplate metasurface structure. As for experiments, the on-axis, off-axis focusing characteristics in transmission mode have been investigated at representative UVA (375 nm) wavelength for the first time, to the best of our knowledge. Furthermore, we fabricated a UV transmission router for monowavelength, guiding UV light to the designated different spatial positions of the same or different focal planes. Our work is meaningful for the development of UV photonics components and devices and would facilitate the integration and miniaturization of UV nanophotonics.

Design of AlN ultraviolet metasurface for single-/multi-plane holography

The metasurface promises an unprecedented way for manipulating wavefronts and has strengths in large information capacity for the hologram. However, strong absorption loss for most dielectric materials hinders the realization of such a metasurface operating in the ultraviolet (UV) spectrum. Herein, aluminum nitride (AlN) with an ultrawide bandgap has been utilized as the material of the UV metasurface for multi-plane holography, increasing the information capacity and security level of information storage simultaneously. The metasurface for multi-plane holography achieving a correlation coefficient of over 0.8 with three reconstructed images has been investigated, and also the single-plane holography at an efficiency of 34.05%. Our work might provide potential application in UV nanophotonics.

Low-loss metasurface optics down to the deep ultraviolet region

Shrinking conventional optical systems to chip-scale dimensions will benefit custom applications in imaging, displaying, sensing, spectroscopy, and metrology. Towards this goal, metasurfaces-planar arrays of subwavelength electromagnetic structures that collectively mimic the functionality of thicker conventional optical elements-have been exploited at frequencies ranging from the microwave range up to the visible range. Here, we demonstrate high-performance metasurface optical components that operate at ultraviolet wavelengths, including wavelengths down to the record-short deep ultraviolet range, and perform representative wavefront shaping functions, namely, high-numerical-aperture lensing, accelerating beam generation, and hologram projection. The constituent nanostructured elements of the metasurfaces are formed of hafnium oxide-a loss-less, high-refractive-index dielectric material deposited using low-temperature atomic layer deposition and patterned using high-aspect-ratio Damascene lithography. This study opens the way towards low-form factor, multifunctional ultraviolet nanophotonic platforms based on flat optical components, enabling diverse applications including lithography, imaging, spectroscopy, and quantum information processing.

Omnidirectional Photodetectors Based on Spatial Resonance Asymmetric Facade via a 3D Self-Standing Strategy

Integration of photovoltaic materials directly into 3D light-matter resonance architectures can extend their functionality beyond traditional optoelectronics. Semiconductor structures at subwavelength scale naturally possess optical resonances, which provides the possibility to manipulate light-matter interactions. In this work, a structure and function integrated printing method to remodel 2D film to 3D self-standing facade between predesigned gold electrodes, realizing the advancement of structure and function from 2D to 3D, is demonstrated. Due to the enlarged cross section in the 3D asymmetric rectangular structure, the facade photodetectors possess sensitive light-matter interaction. The single 3D facade photodetectors can measure the incident angle of light in 3D space with a 10° angular resolution. The resonance interaction of the incident light at different illumination angles and the 3D subwavelength photosensitive facade is analyzed by the simulated light flow in the facade. The 3D facade structure enhances the manipulation of the light-matter interaction and extends metasurface nanophotonics to a wider range of materials. The monitoring of dynamic variation is achieved in a single facade photodetector. Together with the flexibility of structure and function integrated printing strategy, three and four branched photodetectors extend the angle detection to omnidirectional ranges, which will be significant for the development of 3D angle-sensing devices.

All-Dielectric Terahertz Plasmonic Metamaterial Absorbers and High-Sensitivity Sensing

Two types of plasmonic metamaterial absorbers (PMAs) formed from patterned all-dielectric resonators are designed and demonstrated experimentally in the terahertz (THz) range. Both PMAs use a simple grating design on highly N-doped silicon. The first shows broadband absorption with near-perfect peak absorbance at 1.45 THz and a bandwidth of 1.05 THz for 90% absorbance, while the second is a dual-band absorber. Experiments show that the second absorber has two distinct absorption peaks at 0.96 and 1.92 THz with absorption rates of 99.7 and 99.9%, respectively. A fundamental cavity mode coupled to coaxial surface plasmon polaritons is responsible for the characteristics of both PMAs. Additionally, the optically tunable responses of these all-dielectric absorbers demonstrate that the absorption behavior can be modified. The quality factor (Q) values of the dual-band resonances are 4.6 and 7.8 times larger than those of the broadband PMAs, respectively, which leads to a better sensing performance. As an example, the two proposed PMAs act as high-sensitivity sensors and demonstrate considerable potential for chlorpyrifos detection. These results show that these PMAs can be used as sensors that can detect the presence of trace pesticides in adsorption analyses, among other practical applications.

Resonance-free ultraviolet metaoptics via photon nanosieves

Ultraviolet (UV) light with high-energy photons is widely used in various areas such as nano-lithography, biology, and photoemission spectroscopy. The flexible control over its amplitude and phase is a longstanding problem due to the strong absorption from most materials. Here, we propose a nano-aperture platform to control the amplitude and phase of UV light and experimentally demonstrate amplitude- and phase-type holograms at a wavelength of 355 nm. In principle, nano-apertures etched on a metal film can be filled in vacuum, so that the material issue about optical absorption is not involved in this configuration, allowing us to manipulate UV light through the geometry of nano-apertures even when plasmonic resonances are absent. A binary-amplitude nanosieve is used to reconstruct three holographic images at different cut-planes by tuning the constructive interference elaborately. Meanwhile, rectangular nano-apertures are employed to demonstrate UV holograms with geometric phase that is controlled by the orientation of the nano-apertures. This platform could be extended to other UV regions.

Polarization-independent infrared micro-lens array based on all-silicon metasurfaces

The long-wavelength infrared (LWIR) micro-lens arrays, as one of the important components in wafer level thermal optics, have been applied for wavefront sensing, beam shaping, integral imaging, and other thermal optical applications. Recently, electromagnetic metasurfaces provide a promising platform for designing high-performance, lightweight and ultracompact optical elements. Here, we experimentally demonstrate a 60 × 60 transmissive type, polarization-independent LWIR micro-lens array based on all-silicon metasurfaces with a fill factor approaching 100%. Each single micro-metalens with a pitch of 100 μm and a focal length of 100 μm operating at λ = 10.6 μm, can focus the light to a spot with a full-width at half-maximum (FWHM) of 12.7 μm (~1.2λ) at the focal plane. Considering the fact of single-step photolithography and standard integrated circuit (IC) compatible fabrication processes, these metasurface-based micro-lens arrays may have great potentials in compact thermal imaging systems.

Broadband c-Si metasurfaces with polarization control at visible wavelengths: applications to 3D stereoscopic holography

Visual arts and entertainment related industries are continuously looking at promising innovative technologies to improve users' experience with state-of-the-art visualization platforms. This requires further developments on pixel resolution and device miniaturization which can be achieved, for instance, with high contrast materials, such as crystalline silicon (c-Si). Here, a new broadband stereoscopic hologram metasurface is introduced, where independent phase control is achieved for two orthogonal polarizations in the visible spectrum. The holograms are fabricated with a birefringent metasurface consisting of elliptical c-Si nanoposts on Sapphire substrate. Two holograms are combined on the same metasurface (one for each polarization) where each is encoded with four phase levels. The theoretical bandwidth is 110 nm with a signal to noise ratio (SNR) >15 dB. The stereoscopic view is obtained with a pair of cross-polarized filters in front of the observers' eyes. The measured transmission and diffraction efficiencies are about 70% and 15%, respectively, at 532 nm (the design wavelength). The metasurfaces are also investigated at 444.9 nm and 635 nm to experimentally assess their bandwidth performance. The stereoscopic effect is surprisingly good at 444.9 nm (and less so at 635 nm) with transmission and diffraction efficiencies around 70% and 18%, respectively.