Dielectric Metasurfaces Research Articles

Enhanced Optical Activity with Zero Circular Dichroism in a Dielectric Chiral Metasurface

AbstractEnhanced chiral optical responses of metamaterials have widespread applications in chiral biosensor and polarization optics. Metamaterials with intrinsic chirality are commonly composed of complicated 3D structures, which leads to significant fabrication challenges. Recent works involving planar chiral metasurfaces have demonstrated chiral optical response by symmetry breaking, but the intrinsic optical chirality at normal incidence is usually weak. Here, a dielectric chiral metasurface is designed and fabricated, and its strong intrinsic optical chirality in visible spectrum is experimentally demonstrated. Particularly, giant optical activity with zero circular dichroism can be obtained, implying that linear polarization of the incident light passing through the metasurface can be rotated. A phenomenological model is proposed to explain the chiral optical response of the metasurface based on the superposition of two chiral optical resonances in opposite handedness. The multipole decomposition of the electric fields corroborates how the high‐order multipoles are related to the chiral optical response. These results enable the development of flat compact devices to manipulate the polarization of light flexibly.

A dielectric metasurface for controllable generation of tightly focused vector beams

Tightly focused vector beams (TFVBs) play an important role in optical microscopy, laser processing and optical micromanipulation due to their unique polarization and intensity distribution. Conventional implementation of TFVBs requires a series of bulky optical elements, increasing the complexity and possibility of optical loss. Here, we present a method for directly generating TFVBs under illumination of linearly polarized lights based on a single dielectric metasurface. The metasurface is designed using geometric phase of anisotropic dielectric meta-atoms with identical shape but varying rotation angles. Under illumination of horizontally and vertically polarized lights, it is demonstrated that TFVBs with pure longitudinal and transverse fields can be generated respectively. In addition, more different TFVBs can be generated by simply altering the polarization of incident light. Furthermore, the metasurface is demonstrated to work well within a broad band ranging from 530 to 730 nm. The proposed strategy provides a minimalist design of a metasurface for controllably generating TFVBs, which offers promising opportunities to generate more complex vector beams for advanced applications in particle trapping and data storage.

A Metalens Design for On- and Off-Center Focusing with Amorphous Silicon Hydrogenated (a-Si:H)-Based 1D Array in Visible Spectrum

The use of optical systems in medical imaging, computer electronics, large-scale industries, and space exploration is common. The performance of these devices is closely related to the compactness and fast responses of lenses that are used in these optical systems. Typical lenses suffer from several key issues, including limited efficiency, significant size, and the presence of diffraction-induced distortions that compromise their overall performance. Herein these limitations are addressed by designing and simulating an ultra-thin compact metalens also known as a flat lens using a dielectric metasurface. A 1D array of 31 nano-cylinders is placed on a glass substrate that is utilized for focusing the incident wave both on and off center in the focal plane using simulations. The nano-cylinders are comprised of amorphous silicon hydrogenated (a-Si:H), which has a varying radius in a 1D configuration. Amorphous silicon hydrogenated (a-Si:H) nano-cylinders are utilized for the manipulation of the phase of the incident beam working at a frequency of 474 THz. Three metalenses are introduced with focal lengths of 7.46 μm, 10 μm, and 12.99 μm, each having a numerical aperture (NA) of 0.7, 0.6, and 0.5, respectively. The designed single-array metalens showed a transmission efficiency of 73%. The nano-cylinders obtained a full 0–360 phase control that is beneficial in focusing the beam at the center and beyond the center. Symmetric focusing is obtained in the case of off-center focusing on both sides of the optical axis. The design and simulations of the metalens are performed using finite difference time domain (FDTD) simulation tools.

Spectral responses of resonant dielectric metasurfaces induced by Kerr nonlinearity in pump-probe scheme: Theoretical study

In resonant nonlinear nanostructures, Kerr nonlinear responses and optical bistability have already been extensively studied. In spite of great importance, their spectral responses modified by the pump via nonlinear processes, however, have rarely been investigated. In this work, we analytically and numerically study the spectral responses of resonant dielectric metasurfaces induced by Kerr nonlinearity in pump-probe scheme, taking a gallium phosphide metasurface exhibiting strong resonance driven by quasi-bound states in the continuum, as an example. The intensity enhancement in the nonlinear nanoresonator at pump wavelength plays the central role in their nonlinear spectral responses: in the transmission spectrum obtained by scanning probe wavelength, the spectral position of resonance displays red-shift determined by the spectral profile of the enhancement modified by the pump, while the modulation depth of transmittance becomes shallower due to two-photon absorption. For further higher pump intensity, the metasurface exhibits hysteresis for the change of both intensity and wavelength of pump, resulting in spectral bistability. Comparison with numerical results shows that the analytical model based on nonlinear coupled-mode theory well describes their nonlinear spectral responses. The results can be served as theoretical tool for practical applications in characterizing their nonlinear responses and fabricating nonlinear photonic devices.

Chiral electroluminescence from thin-film perovskite metacavities.

Chiral light sources realized in ultracompact device platforms are highly desirable for various applications. Among active media used for thin-film emission devices, lead-halide perovskites have been extensively studied for photoluminescence due to their exceptional properties. However, up to date, there have been no demonstrations of chiral electroluminescence with a substantial degree of circular polarization (DCP) based on perovskite materials, being critical for the development of practical devices. Here, we propose a concept of chiral light sources based on a thin-film perovskite metacavity and experimentally demonstrate chiral electroluminescence with a peak DCP approaching 0.38. We design a metacavity created by a metal and a dielectric metasurface supporting photonic eigenstates with a close-to-maximum chiral response. Chiral cavity modes facilitate asymmetric electroluminescence of pairs of left and right circularly polarized waves propagating in the opposite oblique directions. The proposed ultracompact light sources are especially advantageous for many applications requiring chiral light beams of both helicities.

A Route toward High-Detectivity and Low-Cost Short-Wave Infrared Photodetection: GeSn/Ge Multiple-Quantum-Well Photodetectors with a Dielectric Nanohole Array Metasurface.

High-detectivity and low-cost short-wave infrared photodetectors with complementary metal-oxide-semiconductor (CMOS) compatibility are attractive for various applications such as next-generation optical communication, LiDAR, and molecular sensing. Here, GeSn/Ge multiple-quantum-well (MQW) photodetectors with a dielectric nanohole array metasurface were proposed to realize high-detectivity and low-cost SWIR photodetection. The Ge nanohole array metasurface was utilized to enhance the light absorption in the GeSn/Ge MQW active layer. Compared with metallic nanostructures, the dielectric nanohole structure has the advantages of low intrinsic loss and CMOS compatibility. The introduction of metasurface architecture facilitates a 10.5 times enhanced responsivity of 0.232 A/W at 2 μm wavelength while slightly sacrificing the dark current density. Besides, the metasurface GeSn/Ge MQW photodetectors benefit 35% improvement in the 3 dB bandwidth compared to control GeSn/Ge MQW photodetectors, which can be attributed to the reduced RC delay. Due to the high responsivity and low dark current density, the room temperature specific detectivity at 2 μm is as high as 5.34 × 109 cm·Hz1/2/W, which is the highest among GeSn photodetectors and is better than commercial InSb and PbSe photodetectors operating at the similar wavelength. This work offers a promising approach for achieving low-cost and effective photodetection at 2 μm, contributing to the development of the 2 μm communication band.

Photosensitizing Metasurface Empowered by Enhanced Magnetic Field of Toroidal Dipole Resonance.

Photochemical reaction exploiting an excited triplet state (T1 ) of a molecule requires two steps for the excitation, i.e., electronic transition from the ground (S0 ) to singlet excited (S1 ) states and intersystem crossing to the T1 state. A dielectric metasurface coupled with photosensitizer that enables energy efficient photochemical reaction via the enhanced S0 →T1 magnetic dipole transition is developed. In the direct S0 →T1 transition, the photon energy of several hundreds of meV is saved compared to the conventional S0 → S1 →T1 transition. To maximize the magnetic field intensity on the surface, a silicon (Si) nanodisk array metasurface with toroidal dipole resonances is designed. The surface of the metasurface is functionalized with ruthenium (Ru(II)) complexes that work as a photosensitizer for singlet oxygen generation. In the coupled system, the rate of the direct S0 →T1 transition of Ru(II) complexes is 41-fold enhanced at the toroidal dipole resonance of a Si nanodisk array. The enhancement of a singlet oxygen generation rate is observed when the toroidal dipole resonance of a Si nanodisk array is matched with the direct S0 →T1 transition wavelength of Ru(II) complexes.

Wide gamut dynamic color manipulation based on dielectric metasurface on a stretchable substrate

Dynamic color adjustment technology has broad prospects in invisibility, anti-counterfeiting, and display, and is an important research direction in structural color. This study investigates the excitation of magnetic dipole resonances in the visible region by titanium dioxide (TiO2) metasurfaces using elastic dielectric polydimethylsiloxane (PDMS) and applied to dynamic color modulation. The near-field interaction between the resonant dielectric elements can be modulated using the mechanical strain of the PDMS substrate, enabling the advection of the resonance peak of the structure throughout the visible range, thereby achieving a wide color gamut. The color structure of TiO2 metasurface is sensitive to the refractive index of the surrounding medium. Moreover, active switching between the “ON” state (highly reflective at a selected wavelength) and the “OFF” state (transparent) can be achieved through the deformation of the flexible PDMS substrate. The proposed method can be applied to fields such as sensor displays and spectroscopy and for studying stretchable and wearable photonics.

Fluorescence enhancement of PbS colloidal quantum dots from silicon metasurfaces sustaining bound states in the continuum

Abstract PbS colloidal quantum dots (CQDs) can be considered a promising lighting material, but their emission performance is mired by defect sites, strong photo-induced activity, and interaction with the environment. Here, we utilize periodic silicon metasurface sustaining a symmetry-protected bound state in the continuum to enhance the near-infrared emission of PbS CQDs at room temperature. In the experimental investigation, it is observed that the fluorescence of the coated PbS CQDs is enhanced by 10 times by the fabricated metasurface, and the emission peak has a quality factor up to 251 at wavelength 1408 nm. Meanwhile, the potential of this work in sensing is demonstrated by showing that the enhanced emission is disturbed by the introduction of sparse gold nanoparticles. In all, this work confirms that dielectric metasurfaces sustaining bound states in the continuum can be adopted to efficiently improve the emission performance of PbS CQDs which may find various practical applications including on-chip silicon-based optical sources and integrated sensors.

Room Temperature Exciton-Polariton Condensation in Silicon Metasurfaces Emerging from Bound States in the Continuum.

We show the first experimental demonstration of room-temperature exciton-polariton (EP) condensation from a bound state in the continuum (BIC). This demonstration is achieved by strongly coupling stable excitons in an organic perylene dye with the extremely long-lived BIC in a dielectric metasurface of silicon nanoparticles. The long lifetime of the BIC, mainly due to the suppression of radiation leakage, allows for EP thermalization to the ground state before decaying. This property results in a condensation threshold of less than 5 μJ cm-2, 1 order of magnitude lower than the lasing threshold reported in similar systems in the weak coupling limit.

High-Efficiency Second-Harmonic Generation Using Quasi-Bound State in LiNbO3 Metasurface

We numerically demonstrated a high-efficiency second-harmonic generation (SHG) using quasi-bound state in the continuum (quasi–BIC) in thin film LiNbO3 (TFLN) metasurface. The TFLN possessed exceptionally high second-order nonlinear coefficients, contributing to the enhanced SHG performance. An eccentric cylinder unit cell was presented to achieve high Q–factor resonances associated with the asymmetric parameter introduced. Simulations showed that the high efficiency of the second-harmonic conversion was obtained by using the high Q–factor of the asymmetric dielectric cylinder metasurface, and it achieved a high SHG efficiency of 6.5% at pump intensities as low as 1 MW/cm2 at a normal incident. Furthermore, the simulation results indicated that breaking the symmetry through oblique incidence was more effective in achieving a higher Q–factor compared to altering the structural parameters. Specifically, under 1° oblique incidences, the conversion efficiency could reach 1.2% at an incident power of 1 kW/cm². We have proposed a method to achieve a high conversion efficiency of second-harmonic generation in low-refractive-index materials. Our work not only offers theoretical support but also provides valuable insights for the advancement of efficient nonlinear frequency doubling technology, optical communication, and sensing applications.

Color Printing and Encryption with Polarization-Switchable Structural Colors on All-Dielectric Metasurfaces.

Metasurface-based structural color with high resolution is promising for color printing and encryption. However, achieving tunable structural colors in practical applications is challenging owing to the immutability after the fabrication of metasurfaces. Herein, we proposed the polarization-switchable dielectric metasurfaces with full colors. The colorful images can be switched on/off by controlling the polarization of incident light. For the nanorods metasurfaces, all colors turned to black in the "off" mode because of the near-zero reflection, and the uniform black was advantageous for designing encryption applications. For the nanocrosses metasurfaces, colors reversed in two different "on" modes and images hidden in the "off" mode. With the polarization-sensitive metasurfaces, a fish-bird image, an overlapped dual-channel image, and a green-red heart image were obtained, respectively. The demonstrations can be applied to dynamic displays, optical cryptography, multichannel imaging, and optical data storage.

Recent Progress on Plasmonic and Dielectric Chiral Metasurfaces: Fundamentals, Design Strategies, and Implementation

AbstractOver the years, researchers have been exploring ways to artificially design chiral structures and materials, namely metamaterials and metasurfaces. They exhibit unique optical properties that can be used for various applications. However, metasurfaces comprise symmetry‐breaking structures that provide a more convenient solution for planar chiral optics regardless of whether they are plasmonic or dielectric. In general, plasmonic chiral metasurfaces are more suitable for applications requiring a high confinement level and substantial optical near‐field enhancement. In contrast, dielectric chiral metasurfaces are ideal for wide operating wavelength ranges and low losses. This review summarizes the recent progress on plasmonic and dielectric chiral metasurfaces. It includes the fundamental concepts, design strategies, and their implementation for applications in holographic displays, imaging and sensing, and detection. Moreover, an overview of chiral metasurfaces to generate the nonlinear effects, hosting bound states in the continuum, and the significant role of machine‐learning‐based design approaches are also discussed. Finally, some future developments are highlighted where chiral metasurfaces are expected to play a vital role.

Nonlinear Optical Wavefront Engineering with Geometric Phase Controlled All‐Dielectric Metadevice

AbstractOptical wavefront engineering with artificial nanostructures has important applications in laser manufacturing, bioimaging, optical communications, etc. In the linear optical regime, metasurfaces are widely utilized to realize wavefront engineering functionalities, such as imaging with a flat lens and generation of vortex beams and holographic images. On the other hand, it is highly desirable to directly control the wavefront of light during nonlinear optical processes. Here, the nonlinear optical wavefront engineering with geometric phase controlled all‐dielectric metadevices is experimentally demonstrated. This is realized by integrating the multifunctional dielectric metasurface with the conventional nonlinear optical crystals. It is demonstrated that the nonlinear metadevice can be used to generate optical vortices and nonreciprocal holographic images at second harmonic frequencies. The proposed all‐dielectric metadevice represents an important strategy to develop compact and multifunctional nonlinear optical sources.

Compact and scalable polarimetric self-coherent receiver using a dielectric metasurface

Polarimetric self-coherent systems using direct-detection-based Stokes-vector receivers (SVRs) are a promising technology to meet both the cost and capacity requirements of short-reach optical interconnects. However, conventional SVRs require a number of optical components to detect the state of polarization at high speed, resulting in substantially more complicated receiver configurations compared with the current intensity-modulation–direct-detection counterparts. Here, we demonstrate a simple and compact polarimetric self-coherent receiver based on a thin dielectric metasurface and a photodetector array (PDA). With a single 1.05-µm-thick metasurface device fabricated on a compact silicon-on-quartz chip, we implement functionalities of all the necessary passive components, including a 1×3 splitter, three polarization beam splitters with different polarization bases, and six focusing lenses. Combined with a high-speed PDA, we demonstrate self-coherent transmission of 20-GBd 16-ary quadrature amplitude modulation and 50-GBd quadrature phase-shift keying signals over a 25-km single-mode fiber. Owing to the surface-normal configuration, it can easily be scaled to receive spatially multiplexed channels from a multicore fiber or a fiber bundle, enabling compact and low-cost receiver modules for future highly parallelized self-coherent systems.

Widely Tunable Femtosecond Sources with Continuously Tailorable Bandwidth Enabled by Self‐Phase Modulation

AbstractGenerating ultrafast pulses with better spectrotemporal control is crucial for optimizing and characterizing nonlinear light–matter responses, yet it is limited by the gain bandwidth of laser media or the phase‐matching geometry of nonlinear processes. This work proposes a simple approach to independently manage a femtosecond source's spectral location and bandwidth. Self‐phase‐modulation‐enabled spectral broadening is first analyzed, which is potentially energy‐scalable using hollow‐core capillaries or multipass cells. It is demonstrated that the outmost lobes in the broadened spectrum show different dependencies on the initial pulse energy and duration. A simple yet effective toy model is introduced that successfully predicts broadband spectral tuning, and the impact of other nonlinear effects, dispersion, and input pulse asymmetry on the experimental scenario is also discussed. Thus a fiber‐based versatile source is demonstrated, which is compressible down to its transform‐limit duration, as short as 12.2 fs centered at 920 nm. In addition, bandwidth‐dependent third‐harmonic generation spectroscopy is performed from a dielectric metasurface with an optimized nonlinear response, and the dependency of laser bandwidth and pulse duration is investigated on the signal‐to‐background ratio of two‐photon images. It is believed that this demonstration will advance the investigation of bandwidth‐dependent nonlinear spectroscopy and microscopy.

Through thick and thin: how optical cavities control spin

Abstract When light interacts with matter by means of scattering and absorption, we observe the resulting color. Light also probes the symmetry of matter and the result is encoded in its polarization. In the special case of circularly-polarized light, which is especially relevant in nonlinear optics, quantum photonics, and physical chemistry, a critical dimension of symmetry is along the longitudinal direction. We examine recent advances in controlling circularly-polarized light and reveal that the commonality in these advances is in judicious control of longitudinal symmetry. In particular, in the use of high quality-factor modes in dielectric metasurfaces, the finite thickness can be used to tune the modal profile. These symmetry considerations can be applied in multiplexed optical communication schemes, deterministic control of quantum emitters, and sensitive detection of the asymmetry of small molecules.

Dielectric dual-dimer metasurface for enhanced mid-infrared chiral sensing under both excitation modes

Abstract Chirality (C) is a fundamental symmetry property of objects. Detecting and distinguishing molecular chirality in the infrared spectrum is important in life sciences, biology, and chemistry. In this paper, we demonstrate an achiral metasurface based on a gaped dual-germanium-dimer array for enhanced mid-infrared chiral sensing under both circularly polarized light (CPL) and linearly polarized light (LPL) excitations. With the metasurface, strong electric and magnetic dipole resonances with large field enhancement can be generated, resulting in an accessible superchiral hotspot in the dimer gaps under both excitation modes. The maximum electric and magnetic field enhancements exceed 220 and 100 for the bare metasurface, and exceed 70 and 60 for the metasurface coated with a 50 nm chiral biolayer under both excitations, respectively. Importantly, a high volume-averaged C enhancement C E_ave of 241 (444) and C E_ave_bio of 161 (102) under CPL (LPL) excitation can be achieved for the bare metasurface and it coated with the chiral biolayer, respectively. These results may open up new possibilities for ultrasensitive vibrational circular dichroism (VCD) and rotational optical dispersion (ORD) spectroscopy in the mid-infrared range.

Bound states in the continuum in asymmetric dual-patch metasurfaces.

We investigate the bound states in the continuum (BICs) in dielectric metasurfaces consisting of asymmetric dual rectangular patches in the unit cell of a square lattice. Various types of BICs are identified in the metasurface at normal incidence, associated with very large quality factors and vanishing spectral linewidths. In particular, symmetry-protected (SP) BICs occur when the four patches are fully symmetric, which exhibit antisymmetric field patterns that are decoupled from the symmetric incident waves. By breaking the symmetry of patch geometry, the SP BICs degrade to quasi-BICs that are characterized by Fano resonance. Accidental BICs and Friedrich-Wintgen (FW) BICs occur when the asymmetry is introduced in the upper two patches, while holding the lower two patches symmetric. The accidental BICs occur on isolated bands when the linewidth of either the quadrupole-like mode or LC-like mode vanishes by tuning the upper vertical gap width. The FW BICs appear when the avoided crossing is formed between the dispersion bands of dipole-like and quadrupole-like modes by tuning the lower vertical gap width. At a special asymmetry ratio, the accidental BICs and FW BICs may appear in the same transmittance or dispersion diagram, accompanied with the concurrence of dipole-like, quadrupole-like, and LC-like modes.

Directional Emission from Electrically Injected Exciton–Polaritons in Perovskite Metasurfaces

We present a new approach to achieving strong coupling between electrically injected excitons and photonic bound states in the continuum of a dielectric metasurface. Here a high-finesse metasurface cavity is monolithically patterned in the channel of a perovskite light-emitting transistor to induce a large Rabi splitting of ∼200 meV and more than 50-fold enhancement of the polaritonic emission compared to the intrinsic excitonic emission of the perovskite film. Moreover, the directionality of polaritonic electroluminescence can be dynamically tuned by varying the source-drain bias, which induces an asymmetric distribution of exciton population within the transistor channel. We argue that this approach provides a new platform to study strong light-matter interactions in dispersion engineered photonic cavities under electrical injection and paves the way to solution-processed electrically pumped polariton lasers.