Abrupt autofocusing beam from a phase-only mask.

Airy beams have become an important beam shape for structured light beams because of their interesting self-accelerating and parabolic propagation properties. Many variants of Airy beams have been proposed, among which the Airy beam with cylindrical symmetry [also known as the circular Airy beam or abrupt autofocusing (AAF) beam] is particularly peculiar and has attracted special attention due to its shape transformation during propagation. Much effort has been devoted to understanding the properties of the AAF beam. In this work, we present simulation results for generating the AAF beam using a phase-only mask. A cubic chirp-modulated axicon phase is used to create the mask. We found an optimal value for the axiconic phase, and the cubic phase is essential for controlling the AAF beam's shape. We demonstrate that a phase-only mask is an effective and simple method for generating high contrast between the initial and AAF plane. We present the results for beam formation and propagation dynamics of the AAF beam using the control parameters of the phase mask. We also discuss the design parameters and their influence on the AAF beam shapes. Our results pave the way for a deeper understanding of the beam formation and propagation dynamics of the AAF beam.

Varifocal Metalenses: Harnessing Polarization-Dependent Superposition for Continuous Focal Length Control.

Traditional varifocal lenses are bulky and mechanically complex. Emerging active metalenses promise compactness and design flexibility but face issues like mechanical tuning reliability and nonlinear focal length tuning due to additional medium requirements. In this work, we propose a varifocal metalens design based on superimposing light intensity distributions from two orthogonal polarization states. This approach enables continuous and precise focal length control within the visible spectrum, while maintaining relatively high focusing efficiencies (∼41% in simulation and ∼28% in measurement) and quality. In experimental validation, the metalens exhibited flexible tunability, with the focal length continuously adjustable between two spatial positions upon variation of the incident polarization angle. The MTF results showed high contrast reproduction and sharp imaging, with a Strehl ratio of >0.7 for all polarization angles. With compactness, design flexibility, and high focusing quality, the proposed varifocal metalens holds potential for diverse applications, advancing adaptive and versatile optical devices.

High-Resolution Metalens Imaging with Sequential Artificial Intelligence Models.

An analysis of the optical response of a GaN-based metalens was conducted alongside the utilization of two sequential artificial intelligence (AI) models in addressing the occasional issues of blurriness and color cast in captured images. The optical loss of the metalens in the blue spectral range was found to have resulted in the color cast of images. Autoencoder and CodeFormer sequential models were employed in order to correct the color cast and reconstruct image details, respectively. Said sequential models successfully addressed the color cast and reconstructed details for all of the allocated face image categories. Subsequently, the CIE 1931 chromaticity diagrams and peak signal-to-noise ratio analysis provided numerical evidence of the AI models' effectiveness in image reconstruction. Furthermore, the AI models can still repair the image without blue information. Overall, the integration of metalens and artificial intelligence models marks a breakthrough in enhancing the performance of full-color metalens-based imaging systems.

Intelligent Phase Contrast Meta-Microscope System.

Phase contrast imaging techniques enable the visualization of disparities in the refractive index among various materials. However, these techniques usually come with a cost: the need for bulky, inflexible, and complicated configurations. Here, we propose and experimentally demonstrate an ultracompact meta-microscope, a novel imaging platform designed to accomplish both optical and digital phase contrast imaging. The optical phase contrast imaging system is composed of a pair of metalenses and an intermediate spiral phase metasurface located at the Fourier plane. The performance of the system in generating edge-enhanced images is validated by imaging a variety of human cells, including lung cell lines BEAS-2B, CLY1, and H1299 and other types. Additionally, we integrate the ResNet deep learning model into the meta-microscope to transform bright-field images into edge-enhanced images with high contrast accuracy. This technology promises to aid in the development of innovative miniature optical systems for biomedical and clinical applications.

Varifocal Metalens for Optical Sectioning Fluorescence Microscopy.

Fluorescence microscopy with optical sectioning capabilities is extensively utilized in biological research to obtain three-dimensional structural images of volumetric samples. Tunable lenses have been applied in microscopy for axial scanning to acquire multiplane images. However, images acquired by conventional tunable lenses suffer from spherical aberration and distortions. Here, we design, fabricate, and implement a dielectric Moiré metalens for fluorescence imaging. The Moiré metalens consists of two complementary phase metasurfaces, with variable focal length, ranging from ∼10 to ∼125 mm at 532 nm by tuning mutual angles. In addition, a telecentric configuration using the Moiré metalens is designed for high-contrast multiplane fluorescence imaging. The performance of our system is evaluated by optically sectioned images obtained from HiLo illumination of fluorescently labeled beads, as well as ex vivo mice intestine tissue samples. The compact design of the varifocal metalens may find important applications in fluorescence microscopy and endoscopy for clinical purposes.

Ultrathin Planar Cavity Metasurfaces.

An ultrathin planar cavity metasurface is proposed based on ultrathin film interference and its practicability for light manipulation in visible region is experimentally demonstrated. Phase of reflected light is modulated by finely adjusting the thickness of amorphous silicon (a-Si) by a few nanometers on an aluminum (Al) substrate via nontrivial phase shifts at the interfaces and interference of multireflections generated from the planar cavity. A phase shift of π, the basic requirement for two-level phase metasurface systems, can be accomplished with an 8 nm thick difference. For proof of concept, gradient metasurfaces for beam deflection, Fresnel zone plate metalens for light focusing, and metaholograms for image reconstruction are presented, demonstrating polarization-independent and broadband characteristics. This novel mechanism for phase modulation with ultrathin planar cavity provides diverse routes to construct advanced flat optical devices with versatile applications.

Advances in optical metasurfaces: fabrication and applications [Invited].

The research and development of optical metasurfaces has been primarily driven by the curiosity for novel optical phenomena that are unattainable from materials that exist in nature and by the desire for miniaturization of optical devices. Metasurfaces constructed of artificial patterns of subwavelength depth make it possible to achieve flat, ultrathin optical devices of high performance. A wide variety of fabrication techniques have been developed to explore their unconventional functionalities which in many ways have revolutionized the means with which we control and manipulate electromagnetic waves. The relevant research community could benefit from an overview on recent progress in the fabrication and applications of the metasurfaces. This review article is intended to serve that purpose by reviewing the state-of-the-art fabrication methods and surveying their cutting-edge applications.

Versatile Polarization Generation with an Aluminum Plasmonic Metasurface.

All forms of light manipulation rely on light-matter interaction, the primary mechanism of which is the modulation of its electromagnetic fields by the localized electromagnetic fields of atoms. One of the important factors that influence the strength of interaction is the polarization of the electromagnetic field. The generation and manipulation of light polarization have been traditionally accomplished with bulky optical components such as waveplates, polarizers, and polarization beam splitters that are optically thick. The miniaturization of these devices is highly desirable for the development of a new class of compact, flat, and broadband optical components that can be integrated together on a single photonics chip. Here we demonstrate, for the first time, a reflective metasurface polarization generator (MPG) capable of producing light beams of any polarizations all from a linearly polarized light source with a single optically thin chip. Six polarization light beams are achieved simultaneously including four linear polarizations along different directions and two circular polarizations, all conveniently separated into different reflection angles. With the Pancharatnam-Berry phase-modulation method, the MPG sample was fabricated with aluminum as the plasmonic metal instead of the conventional gold or silver, which allowed for its broadband operation covering the entire visible spectrum. The versatility and compactness of the MPG capable of transforming any incident wave into light beams of arbitrary polarizations over a broad spectral range are an important step forward in achieving a complete set of flat optics for integrated photonics with far-reaching applications.

GaN Metalens for Pixel-Level Full-Color Routing at Visible Light.

Metasurface-based components are known to be one of the promising candidates for developing flat optical systems. However, their low working efficiency highly limits the use of such flat components for feasible applications. Although the introduction of the metallic mirror has been demonstrated to successfully enhance the efficiency, it is still somehow limited for imaging and sensing applications because they are only available for devices operating in a reflection fashion. Here, we demonstrate three individual GaN-based metalenses working in a transmission window with extremely high operation efficiency at visible light (87%, 91.6%, and 50.6% for blue, green, and red light, respectively). For the proof of concept, a multiplex color router with dielectric metalens, which is capable of guiding individual primary colors into different spatial positions, is experimentally verified based on the design of out-of-plane focusing metalens. Our approach with low-cost, semiconductor fabrication compatibility and high working efficiency characteristics offers a way for establishing a complete set of flat optical components for a wide range of applications such as compact imaging sensors, optical spectroscopy, and high-resolution lithography, just named a few.

In Vivo Intelligent Fluorescence Endo-Microscopy by Varifocal Meta-Device and Deep Learning.

Endo-microscopy is crucial for real-time 3D visualization of internal tissues and subcellular structures. Conventional methods rely on axial movement of optical components for precise focus adjustment, limiting miniaturization and complicating procedures. Meta-device, composed of artificial nanostructures, is an emerging optical flat device that can freely manipulate the phase and amplitude of light. Here, an intelligent fluorescence endo-microscope is developed based on varifocal meta-lens and deep learning (DL). The breakthrough enables in vivo 3D imaging of mouse brains, where varifocal meta-lens focal length adjusts through relative rotation angle. The system offers key advantages such as invariant magnification, a large field-of-view, and optical sectioning at a maximum focal length tuning range of ≈2 mm with 3 µm lateral resolution. Using a DL network, image acquisition time and system complexity are significantly reduced, and in vivo high-resolution brain images of detailed vessels and surrounding perivascular space are clearly observed within 0.1 s (≈50 times faster). The approach will benefit various surgical procedures, such as gastrointestinal biopsies, neural imaging, brain surgery, etc.

Fabrication of plasmonic devices using femtosecond laser-induced forward transfer technique.

Using femtosecond laser-induced forward transfer techniques we have fabricated gold dots and nanoparticles on glass substrates, as well as nanobumps on gold thin film. The surface morphologies of these structures with different laser fluences and film thicknesses are investigated. We also study the focusing and defocusing properties of the nanofence-an arranged nanobump pattern-by the total-internal reflection microscope. Observations reveal that surface plasmon waves can be highly directed and focused via this nanofence pattern. Results are in good agreement with the simulation results using the finite-element method and demonstrate the potential applications of these nanophotonic devices. Furthermore, we utilize high laser energy to fabricate plasmonic waveguides, and also succeed in transferring the waveguides to another substrate. The attenuation rates of the light propagating in the waveguides are observed to achieve 0.31 dB µm(-1) and 0.48 dB µm(-1) on the target and receiver sides, respectively.

Vacuum ultraviolet nonlinear metalens.

Vacuum ultraviolet (VUV) light plays an essential role across science and technology, from molecular spectroscopy to nanolithography and biomedical procedures. Realizing nanoscale devices for VUV light generation and control is critical for next-generation VUV sources and systems, but the scarcity of low-loss VUV materials creates a substantial challenge. We demonstrate a metalens that both generates-by second-harmonic generation-and simultaneously focuses the generated VUV light. The metalens consists of 150-nm-thick zinc oxide (ZnO) nanoresonators that convert 394 nm (~3.15 eV) light into focused 197-nm (~6.29 eV) radiation, producing a spot 1.7 µm in diameter with a 21-fold power density enhancement as compared to the wavefront at the metalens surface. The reported metalens is ultracompact and phase-matching free, allowing substantial streamlining of VUV system design and facilitating more advanced applications. This work provides a useful platform for developing low-loss VUV components and increasing the accessibility of the VUV regime.

Metasurface-Based Abrupt Autofocusing Beam for Biomedical Applications.

Manipulation and precise delivery of optical energies in the regions of interest within specimens require different strategies. Hence, proper control of input beam parameters is a prerequisite. One of the prominent methods is metasurface optics, capable of crafting properties of light at nanoscales. Here, the generation of an abrupt autofocusing (AAF) beam by a nanophotonic metasurface for biomedical applications is demonstrated. Fluorescence guided laser microprofiling of mouse cardiac samples is experimentally investigated, using the AAF beam to deliver optical energy selectively to specific locations. In addition, photocoagulation of ex vivo swine skin tissue is performed and observed through optical coherence tomography. The results show great potentials for integrating metasurface optics to realize miniature laser surgery instruments for wide applications in biomedicine.

Local electrical characterization of laser-recorded phase-change marks on amorphous Ge2Sb2Te5 thin films.

Amorphous thin films of Ge(2)Sb(2)Te(5), sputter-deposited on a thin-film gold electrode, are investigated for the purpose of understanding the local electrical conductivity of recorded marks under the influence of focused laser beam. Being amorphous, the as-deposited chalcogenide films have negligible electrical conductivity. With the aid of a focused laser beam, however, we have written on these films micron-sized crystalline marks, ablated holes surrounded by crystalline rings, and other multi-ring structures containing both amorphous and crystalline zones. Within these structures, nano-scale regions of superior local conductivity have been mapped and probed using our high-resolution, high-sensitivity conductive-tip atomic force microscope (C-AFM). Scanning electron microscopy and energy-dispersive spectrometry have also been used to clarify the origins of high conductivity in and around the recorded marks. When the Ge(2)Sb(2)Te(5) layer is sufficiently thin, and when laser crystallization/ablation is used to define long isolated crystalline stripes on the samples, we find the C-AFM-based method of extracting information from the recorded marks to be superior to other forms of microscopy for this particular class of materials. Given the tremendous potential of chalcogenides as the leading media candidates for high-density memories, local electrical characterization of marks recorded on as-deposited amorphous Ge(2)Sb(2)Te(5) films provides useful information for furthering research and development efforts in this important area of modern technology.

Fabrication of phase-change Ge2Sb2Te5 nano-rings.

Phase-change material Ge2Sb2Te5 rings with nanometer-scale thickness have been fabricated using the photo-thermal effect of a focused laser beam followed by differential chemical etching. Laser irradiation conditions and etching process parameters are varied to control the geometric characteristics of the rings. We demonstrate the possibility of arranging the rings in specific geometric patterns, and also their release from the original substrate.

Fabrication of phase-change chalcogenide Ge2Sb2Te5 patterns by laser-induced forward transfer.

Femtosecond laser pulses are focused on a thin film of Ge2Sb2Te5 phase-change material, and the transfer of the illuminated material to a nearby substrate is investigated. The size, shape, and phase-state of the fabricated pattern can be effectively controlled by the laser fluence and by the thickness of the Ge2Sb2Te5 film. Results show multi-level electrical and optical reflection states of the fabricated patterns, which may provide a simple and efficient foundation for patterning future phase-change devices.

Cubic-Phase Metasurface for Three-Dimensional Optical Manipulation.

The optical tweezer is one of the important techniques for contactless manipulation in biological research to control the motion of tiny objects. For three-dimensional (3D) optical manipulation, shaped light beams have been widely used. Typically, spatial light modulators are used for shaping light fields. However, they suffer from bulky size, narrow operational bandwidth, and limitations of incident polarization states. Here, a cubic-phase dielectric metasurface, composed of GaN circular nanopillars, is designed and fabricated to generate a polarization-independent vertically accelerated two-dimensional (2D) Airy beam in the visible region. The distinctive propagation characteristics of a vertically accelerated 2D Airy beam, including non-diffraction, self-acceleration, and self-healing, are experimentally demonstrated. An optical manipulation system equipped with a cubic-phase metasurface is designed to perform 3D manipulation of microscale particles. Due to the high-intensity gradients and the reciprocal propagation trajectory of Airy beams, particles can be laterally shifted and guided along the axial direction. In addition, the performance of optical trapping is quantitatively evaluated by experimentally measured trapping stiffness. Our metasurface has great potential to shape light for compact systems in the field of physics and biological applications.

Laser-induced phase transitions of Ge2Sb2Te5 thin films used in optical and electronic data storage and in thermal lithography.

Amorphous thin films of Ge(2)Sb(2)Te(5), sputter-deposited on a ZnS-SiO(2) dielectric layer, are investigated for the purpose of understanding the structural phase-transitions that occur under the influence of tightly-focused laser beams. Selective chemical etching of recorded marks in conjunction with optical, atomic force, and electron microscopy as well as local electron diffraction analysis are used to discern the complex structural features created under a broad range of laser powers and pulse durations. Clarifying the nature of phase transitions associated with laser-recorded marks in chalcogenide Ge(2)Sb(2)Te(5) thin films provides useful information for reversible optical and electronic data storage, as well as for phase-change (thermal) lithography.

Metalens-array-based high-dimensional and multiphoton quantum source.

The development of two-dimensional metasurfaces has shown great potential in quantum-optical technologies because of the excellent flexibility in light-field manipulation. By integrating a metalens array with a nonlinear crystal, we demonstrate a 100-path spontaneous parametric down-conversion photon-pair source in a 10 × 10 array, which shows promise for high-dimensional entanglement and multiphoton-state generation. We demonstrate two-, three- and four-dimensional two-photon path entanglement with different phases encoded by metalenses with fidelities of 98.4, 96.6, and 95.0%, respectively. Furthermore, four-photon and six-photon generation is observed with high indistinguishability of photons generated from different metalenses. Our metalens-array-based quantum photon source is compact, stable, and controllable, indicating a new platform for integrated quantum devices.

Achromatic metalens array for full-colour light-field imaging.

A light-field camera captures both the intensity and the direction of incoming light1-5. This enables a user to refocus pictures and afterwards reconstruct information on the depth of field. Research on light-field imaging can be divided into two components: acquisition and rendering. Microlens arrays have been used for acquisition, but obtaining broadband achromatic images with no spherical aberration remains challenging. Here, we describe a metalens array made of gallium nitride (GaN) nanoantennas6 that can be used to capture light-field information and demonstrate a full-colour light-field camera devoid of chromatic aberration. The metalens array contains an array of 60 × 60 metalenses with diameters of 21.65 µm. The camera has a diffraction-limited resolution of 1.95 µm under white light illumination. The depth of every object in the scene can be reconstructed slice by slice from a series of rendered images with different depths of focus. Full-colour, achromatic light-field cameras could find applications in a variety of fields such as robotic vision, self-driving vehicles and virtual and augmented reality.