Nanoparticle Deep-Subwavelength Dynamics Empowered by Optical Meron-Antimeron Topology.

Optical meron is a type of nonplanar topological texture mainly observed in surface plasmon polaritons and highly symmetric points of photonic crystals in the reciprocal space. Here, we report Poynting-vector merons formed at the real space of a photonic crystal for a Γ-point illumination. Optical merons can be utilized for subwavelength-resolution manipulation of nanoparticles, resembling a topological Hall effect on electrons via magnetic merons. In particular, staggered merons and antimerons impose strong radiation pressure on large gold nanoparticles (AuNPs), while focused hot spots in antimerons generate dominant optical gradient forces on small AuNPs. Synergistically, differently sized AuNPs in a still environment can be trapped or orbit in opposite directions, mimicking a coupled galaxy system. They can also be separated with a 10 nm precision when applying a flow velocity of >1 mm/s. Our study unravels a novel way to exploit topological textures for optical manipulation with deep-subwavelength precision and switchable topology in a lossless environment.

Artificial Intelligence in Meta-optics.

Recent years have witnessed promising artificial intelligence (AI) applications in many disciplines, including optics, engineering, medicine, economics, and education. In particular, the synergy of AI and meta-optics has greatly benefited both fields. Meta-optics are advanced flat optics with novel functions and light-manipulation abilities. The optical properties can be engineered with a unique design to meet various optical demands. This review offers comprehensive coverage of meta-optics and artificial intelligence in synergy. After providing an overview of AI and meta-optics, we categorize and discuss the recent developments integrated by these two topics, namely AI for meta-optics and meta-optics for AI. The former describes how to apply AI to the research of meta-optics for design, simulation, optical information analysis, and application. The latter reports the development of the optical Al system and computation via meta-optics. This review will also provide an in-depth discussion of the challenges of this interdisciplinary field and indicate future directions. We expect that this review will inspire researchers in these fields and benefit the next generation of intelligent optical device design.

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.

Superhybrid Mode-Enhanced Optical Torques on Mie-Resonant Particles.

Circularly polarized light carries spin angular momentum, so it can exert an optical torque on the polarization-anisotropic particle by the spin momentum transfer. Here, we show that giant positive and negative optical torques on Mie-resonant (gain) particles arise from the emergence of superhybrid modes with magnetic multipoles and electric toroidal moments, excited by linearly polarized beams. Anomalous positive and negative torques on particles (doped with judicious amount of dye molecules) are over 800 and 200 times larger than the ordinary lossy counterparts, respectively. Meanwhile, a rotational motor can be configured by switching the s- and p-polarized beams, exhibiting opposite optical torques. These giant and reversed optical torques are unveiled for the first time in the scattering spectrum, paving another avenue toward exploring unprecedented physics of hybrid and superhybrid multipoles in metaoptics and optical manipulations.

Inverse Optical Torques on Dielectric Nanoparticles in Elliptically Polarized Light Waves.

Elliptically polarized light waves carry the spin angular momentum (SAM), so they can exert optical torques on nanoparticles. Usually, the rotation follows the same direction as the SAM due to momentum conservation. It is counterintuitive to observe the reversal of optical torque acting on an ordinary dielectric nanoparticle illuminated by an elliptically or circularly polarized light wave. Here, we demonstrate that negative optical torques, which are opposite to the direction of SAM, can ubiquitously emerge when elliptically polarized light waves are impinged on dielectric nanoparticles obliquely. Intriguingly, the rotation can be switched between clockwise and counterclockwise directions by controlling the incident angle of light. Our study suggests a new playground to harness polarization-dependent optical force and torque for advancing optical manipulations.

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.

Dual-layer achromatic metalens design with an effective Abbe number.

Planar achromatic metalenses with a thickness of the order of wavelength have attracted much attention for their potential applications in ultra-compact optical devices. However, realizing single-layer achromatic metalenses across a wide bandwidth requires that the corresponding meta-atoms have complex cross-sections for correct phase profile and dispersion compensation. Herein, we introduce an effective Abbe number and use lens maker equations to design a dual-layer achromatic metalens in which we compensate the dispersion by using a plano-convex liked metalens combined with a plano-concave liked metalens. The stacked metalens are designed based on simple high refractive index dielectric cylindrical meta-atoms with different radii, which simplify the design and fabrication processes. We demonstrate that a dual-layer achromatic metalens has a small focal length difference across the visible wavelength range and an average focusing efficiency above 50%, which proves that the design method is promising for many potential applications in multi-functional flat optical devices.

Extraordinary Multipole Modes and Ultra-Enhanced Optical Lateral Force by Chirality.

Strong mode coupling and Fano resonances arisen from exceptional interaction between resonant modes in single nanostructures have raised much attention for their advantages in nonlinear optics, sensing, etc. Individual electromagnetic multipole modes such as quadrupoles, octupoles, and their counterparts from mode coupling (toroidal dipole and nonradiating anapole mode) have been well investigated in isolated or coupled nanostructures with access to high Q factors in bound states in the continuum. Albeit the extensive study on ordinary dielectric particles, intriguing aspects of light-matter interactions in single chiral nanostructures is lacking. Here, we unveil that extraordinary multipoles can be simultaneously superpositioned in a chiral nanocylinder, such as two toroidal dipoles with opposite moments, and electric and magnetic sextupoles. The induced optical lateral forces and their scattering cross sections can thus be either significantly enhanced in the presence of those multipoles with high-Q factors, or suppressed by the bound states in the continuum. This work for the first time reveals the complex correlation between multipolar effects, chiral coupling, and optical lateral force, providing a distinct way for advanced optical manipulation.

Generating Third Harmonic Vacuum Ultraviolet Light with a TiO2 Metasurface.

Dielectric metasurfaces have recently been shown to provide an excellent platform for the harmonic generation of light due to their low optical absorption and to the strong electromagnetic field enhancement that can be designed into their constituent meta-atoms. Here, we demonstrate vacuum ultraviolet (VUV) third harmonic generation from a specially designed dielectric metasurface consisting of a titanium dioxide (TiO2) nanostructure array. The metasurface was designed to enhance the generation of VUV light at a wavelength of 185 nm by tailoring its geometric design parameters to achieve an optical resonance at the fundamental laser wavelength of 555 nm. The metasurface exhibits an enhancement factor of nominally 180 compared to an unpatterned TiO2 thin film of the same thickness, evidence of strong field enhancement at the fundamental wavelength. Mode analysis reveals that the origin of the enhancement is an anapole resonance near the pump wavelength. This work demonstrates an effective strategy for the compact generation of VUV light that could enable expanded access to this useful region of the electromagnetic spectrum.

Vacuum Ultraviolet Light-Generating Metasurface.

Vacuum ultraviolet (VUV) light has important applications in many fields, ranging from device fabrication to photochemistry, from environmental remediation to microscopy and spectroscopy. Methods to produce coherent VUV light frequently utilize high harmonic generation in media such as rare gases or atomic vapors; nonlinear optical crystals that support second harmonic generation into the VUV are quite rare. Here, we demonstrate an all-dielectric metasurface designed for the nonlinear optical generation of VUV light. Consisting of an array of zinc oxide nanoresonators, the device exhibits a magnetic dipole resonance at a wavelength of 394 nm. When excited with ultrafast laser pulses at this wavelength, the second harmonic at 197 nm is readily generated. Manipulation of the metasurface design enables control over the radiation pattern. This work has the potential to open the door toward simple and compact VUV sources for new applications.

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.

Red/green/blue LD mixed white-light communication at 6500K with divergent diffuser optimization.

Enabling laser white-lighting at a correlated color temperature (CCT) of 6500K with the use of only red/green/blue (RGB) tri-color laser diodes (LDs) is demonstrated, which can further perform wavelength division multiplexing (WDM) communication with a high-spectral-usage 16 QAM-OFDM data stream at 11.2 Gbps over 0.5 m. The sampling rate of encoded data is optimized to avoid the aliasing effect and to effectively amplify the signal with high on/off extinction and modulation depth. Proper oversampling can decrease the peak-to-average power ratio (PAPR) of the OFDM data and filter out unwanted noise. There are also six different diffusers used to diverge the white-light mixed by the RGB LD beam. By analyzing the color-casting transmittance, surface roughness, CCT uniformity, divergent angle of the diffuser, and the data transmission capacity, the frosted glass (FG2.8) diffuser with high transmittance diverges the white light with the divergent angle of ± 20° and supports the highest data rate of 14 Gbps over 0.5 m. To fit the day-light CCT, the blue LD power at an optimized bias current is further attenuated with a 0.6-optical density filter for reducing CCT from 100000K to 6500K; however, such an adjustment also degrades the SNR ratio to sacrifice the achievable data rate of the blue LD. The polycarbonate (PC1.5) diffuser with proper surface roughness diverged white-light exhibits the best CCT uniformity and a divergent angle of ± 30° but supports a data rate of only 6.4 Gbps over 0.5 m. The poly (methyl methacrylate) PMMA1.5 diffuser scatters the white light with the largest angle of ± 40°; however, the data rate also decreases to 4.8 Gbps over 0.5 m.

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.

Gate-Tunable Conducting Oxide Metasurfaces.

Metasurfaces composed of planar arrays of subwavelength artificial structures show promise for extraordinary light manipulation. They have yielded novel ultrathin optical components such as flat lenses, wave plates, holographic surfaces, and orbital angular momentum manipulation and detection over a broad range of the electromagnetic spectrum. However, the optical properties of metasurfaces developed to date do not allow for versatile tunability of reflected or transmitted wave amplitude and phase after their fabrication, thus limiting their use in a wide range of applications. Here, we experimentally demonstrate a gate-tunable metasurface that enables dynamic electrical control of the phase and amplitude of the plane wave reflected from the metasurface. Tunability arises from field-effect modulation of the complex refractive index of conducting oxide layers incorporated into metasurface antenna elements which are configured in reflectarray geometry. We measure a phase shift of 180° and ∼30% change in the reflectance by applying 2.5 V gate bias. Additionally, we demonstrate modulation at frequencies exceeding 10 MHz and electrical switching of ±1 order diffracted beams by electrical control over subgroups of metasurface elements, a basic requirement for electrically tunable beam-steering phased array metasurfaces. In principle, electrically gated phase and amplitude control allows for electrical addressability of individual metasurface elements and opens the path to applications in ultrathin optical components for imaging and sensing technologies, such as reconfigurable beam steering devices, dynamic holograms, tunable ultrathin lenses, nanoprojectors, and nanoscale spatial light modulators.

Lambertian thermal emitter based on plasmonic enhanced absorption.

In this paper, a narrow band thermal emission at 10 µm is demonstrated using a one dimensional metasurface. The proposed metasurface structure provides magnetic resonance mode that enhances the phonon absorption of SiO2. The proposed metasurface thermal emitter shows a Lambertian distribution. Additionally, 5.8-folds enhancement of emissivity is achieved by optimizing the cavity thickness of the metasurfaces. This type of thermal emitter will be useful for IR sensing applications.

Plasmonic Archimedean spiral modes on concentric metal ring gratings.

Plasmonic Archimedean spiral modes on concentric silver (Ag) ring gratings are investigated by FDTD simulations and theoretical analyses. These modes are generated by placing the ring grating under an Ag nanorod to extract the orbital angular momentum (OAM) of spiral surface plasmon (SSP) modes on the nanorod and transform it into the orbital motion of SP on the grating. The formation of Archimedean spiral patterns is ascribed to two factors: both the r- and θ-directional wavevectors are conserved for SSP on nanorod coupling into SP on ring grating and both the r- and θ-directional velocities of SP keep unchanged when it propagates on the ring grating. The number of strands of Archimedean spiral pattern is determined by the topological charge of SSP mode. The plasmonic Archimedean spiral modes have potential applications in the fields of data storage, dielectric microparticle manipulation, biosensing and directional switching.

Introduction to the Feature Issue on nanophotonics.

This feature issue is a partial collection of contributions from authors who presented their research at the 9th International Conference on Nanophotonics (ICNP 2016) held during March 21-25, 2016 at Academia Sinica, Taipei, Taiwan. ICNP is an independent conference series dedicated to nanophotonics research and applications. This feature issue collects 28 papers related to research presented at ICNP 2016.

Integrated plasmonic metasurfaces for spectropolarimetry.

Plasmonic metasurfaces enable simultaneous control of the phase, momentum, amplitude and polarization of light and hence promise great utility in realization of compact photonic devices. In this paper, we demonstrate a novel chip-scale device suitable for simultaneous polarization and spectral measurements through use of six integrated plasmonic metasurfaces (IPMs), which diffract light with a given polarization state and spectral component into well-defined spatial domains. Full calibration and characterization of our device is presented, whereby good spectral resolution and polarization accuracy over a wavelength range of 500-700 nm is shown. Functionality of our device in a Müller matrix modality is demonstrated through determination of the polarization properties of a commercially available variable waveplate. Our proposed IPM is robust, compact and can be fabricated with a single photolithography step, promising many applications in polarization imaging, quantum communication and quantitative sensing.