Ultrahigh performance passive radiative cooling by hybrid polar dielectric metasurface thermal emitters

Real-world passive radiative cooling requires highly emissive,selective,and omnidirectional thermal emitters to maintain the radiative cooler at a certain temperature below the ambient temperature while maximizing the net cooling power.Despite various selective thermal emitters have been demonstrated,it is still challenging to achieve these conditions sim-ultaneously because of the extreme difficulty in controlling thermal emission of photonic structures in multidimension.Here we demonstrated hybrid polar dielectric metasurface thermal emitters with machine learning inverse design,en-abling a high emissivity of~0.92 within the atmospheric transparency window 8-13μm,a large spectral selectivity of~1.8 and a wide emission angle up to 80 degrees,simultaneously.This selective and omnidirectional thermal emitter has led to a new record of temperature reduction as large as~15.4°C under strong solar irradiation of~800 W/m2,signific-antly surpassing the state-of-the-art results.The designed structures also show great potential in tackling the urban heat island effect,with modelling results suggesting a large energy saving and deployment area reduction.This research will make significant impact on passive radiative cooling,thermal energy photonics and tackling global climate change.

High Refractive Index Ti3O5 Films for Dielectric Metasurfaces

Ti33O55 films are deposited with the help of an electron beam evaporator for their applications in metasurfaces. The film of subwavelength （632nm） thickness is deposited on a silicon substrate and annealed at 400℃. The ellipsometry result shows a high refractive index above 2.5 with the minimum absorption coefficient in the visible region, which is necessary for high efficiency of transparent metasurfaces. Atomic force microscopy analysis is employed to measure the roughness of the as-deposited films. It is seen from micrographs that the deposited films are very smooth with the minimum roughness to prevent scattering and absorption losses for metasurface devices. The absence of grains and cracks can be seen by scanning electron microscope analysis, which is favorable for electron beam lithography. Fourier transform infrared spectroscopy reveals the transmission and reflection obtained from the film deposited on glass substrates. The as-deposited film shows high transmission above 60%, which is in good agreement with metasurfaces.

Photonic Band Modulation in a Two-Dimensional Photonic Crystal with a One-Dimensional Periodic Dielectric Background

A two-dimensional photonic crystal with a one-dimensional periodic dielectric background is proposed. The photonic band modulation effects due to the periodic background are investigated based on the plane wave expansion method. We find that periodic modulation of the dielectric background greatly alters photonic band structures, especially for the E-polarization modes. The number, width and position of the photonic band gaps （PBGs） sensitively depend on the structure parameters （the layer thicknesses and dielectric constants） of the one-dimensional periodic background,

Asymmetrical photonic spin Hall effect based on dielectric metasurfaces

The photonic spin Hall effect has attracted considerable research interest due to its potential applications in spincontrolled nanophotonic devices.However,realization of the asymmetrical photonic spin Hall effect with a single optical element is still a challenge due to the conjugation of the Pancharatnam-Berry phase,which reduces the flexibility in various applications.Here,we demonstrate an asymmetrical spin-dependent beam splitter based on a single-layer dielectric metasurface exhibiting strong and controllable optical response.The metasurface consists of an array of dielectric nanofins,where both varying rotation angles and feature sizes of the unit cells are utilized to create high-efficiency dielectric metasurfaces,which enables to break the conjugated characteristic of phase gradient.Thanks to the superiority of the phase modulation ability,when the fabricated metasurface is under normal incidence with a wavelength of 1550 nm,the lefthanded circular polarization(LCP)light exhibits an anomalous refraction angle of 28.9°,while the right-handed circular polarization(RCP)light transmits directly.The method we proposed can be used for the flexible manipulation of spin photons and has potentials in high efficiency metasurfaces with versatile functionalities,especially with metasurfaces in a compact space.

Racemic dielectric metasurfaces for arbitrary terahertz polarization rotation and wavefront manipulation

Dielectric chiral metasurface is a new type of planar and efficient chiral optical device that shows strong circular dichroism or optical activity,which has important application potential in optical sensing and display.However,the two types of chiral optical responses in conventional chiral metasurfaces are often interdependent,as their modulation of the amplitudes and phases of orthogonal circularly polarized components is correlated,which limits the further progress of chiral meta-devices.Here we propose a new scheme for independently designing the circular dichroism and optical activity of chiral metasurfaces to further control the polarization and wavefront of transmitted waves.Inspired by mixtures of chiral molecular isomers,we use the dielectric isomer resonators to form“super-units”instead of single meta-atoms for chiral responses in terahertz band,which is called racemic metasurface.By introducing two levels of Pancharatnam-Berry phases between meta-atoms and“super-units”,the polarization rotation angle and wavefront of the beam can be designed without the far-field circular dichroism.We demonstrate the strong control ability on terahertz waves of this scheme through simulation and experiments.In addition,this new type of device with near-field chirality but no far-field circular dichroism may also have important value in optical sensing and other technologies.

Band gaps structure and semi-Dirac point of two-dimensional function photonic crystals

Two-dimensional function photonic crystals, in which the dielectric constants of medium columns are the functions of space coordinates , are proposed and studied numerically. The band gaps structures of the photonic crystals for TE and TM waves are different from the two-dimensional conventional photonic crystals. Some absolute band gaps and semiDirac points are found. When the medium column radius and the function form of the dielectric constant are modulated, the numbers, width, and position of band gaps are changed, and the semi-Dirac point can either occur or disappear. Therefore,the special band gaps structures and semi-Dirac points can be achieved through the modulation on the two-dimensional function photonic crystals. The results will provide a new design method of optical devices based on the two-dimensional function photonic crystals.

Multifunctional light-field modulation based on hybrid nonlinear metasurfaces

The generation characteristics of nonlinear optical signals and their multi-dimensional modulation at micro-nano scale have become a prominent research area in nanophotonics,and also the key to developing various novel nonlinear photonics devices.In recent years,the demand for higher nonlinear conversion efficiency and device integration has led to the rapid progress of hybrid nonlinear metasurfaces composed of nanostructures and nonlinear materials.As a joint platform of stable wavefront modulation,nonlinear metasurface and efficient frequency conversion,hybrid nonlinear metasurfaces offer a splendid opportunity for developing the next-generation of multipurpose flat-optics devices.This article provides a comprehensive review of recent advances in hybrid nonlinear metasurfaces for light-field modulation.The advantages of hybrid systems are discussed from the perspectives of multifunctional light-field modulation,valleytronic modulation,and quantum technologies.Finally,the remaining challenges of hybrid metasurfaces are summarized and future developments are also prospected.

Towards wafer-scale growth of two-dimensional cerium dioxide single crystal with high dielectric performance

Owing to the atomically thin nature,two-dimensional(2D)oxide materials have been widely reported to exhibit exciting transport and dielectric properties,such as fine gate controllability and ultrahigh carrier mobility,that outperform their bulk counterpart.However,unlike the successful synthesis of bulk oxide single crystals,reliable methods for synthesizing large-area single crystal of 2D oxide,that would suppress the negative influence from defective grain boundaries,remain unavailable,especially for nonlayered oxide.Herein,we report that the lattice symmetry between the substrate and cerium dioxide(CeO_(2))would allow for the aligned nucleation and epitaxial growth of CeO_(2)on sapphire substrates,enabling the wafer-sized growth of CeO_(2)single crystal.The careful tuning of the growth temperature and oxygen flow rate contributed to the harvesting of CeO_(2)wafer with reduced thickness and enhanced growth rates.The removal of grain boundaries improved the dielectric performance in terms of high dielectric strength(E_(bd)≈8.8 MV·cm^(-1)),suppressed leakage current,along with high dielectric constants(ε_(r)≈24).Our work demonstrates that with fine dielectric performance and ease of synthesizing wafer-sized single crystals,CeO_(2)can function as potential candidate as gate insulator for 2D-materials based nanoelectronics,and we believe the reported protocol of aligned nucleation can be extended to other 2D oxides.

(Benzimidazolium)_(2)GeI_(4):A layered two-dimensional perovskite with dielectric switching and broadband near-infrared photoluminescence

Two-dimensional(2D)lead halide perovskites have attracted tremendous attention due to their outstanding physical properties.However,the presence of toxic lead is a major problem for large-scale applications.Here,we report the synthesis,phase transition,dielectric and photoluminescence(PL)properties of a lead-free 2D Ge-based perovskite(BIM)_(2)GeI_(4)(BIM=benzimidazolium)which experiences a phase transition at 405 K accompanied by an order-disorder change of organic spacer.Moreover,the step-like dielectric transition near the phase transition temperature makes it suitable for dielectric switching.Meanwhile,(BIM)_(2)GeI_(4)has a direct bandgap of 2.23 eV and broadband emission from 550 to 1000 nm,implying its unique optical property for potential near-infrared lighting and displaying.This work provides a fresh example for the development of lead-free 2D Ge-based perovskite with potential applications in the fields of optoelectronics and high temperature dielectric switching.

Versatile on-chip light coupling and(de)multiplexing from arbitrary polarizations to controlled waveguide modes using an integrated dielectric metasurface

Metasurfaces have found broad applicability in free-space optics,while its potential to tailor guided waves remains barely explored.By synergizing the Jones matrix model with generalized Snell’s law under the phase-matching condition,we propose a universal design strategy for versatile on-chip mode-selective coupling with polarization sensitivity,multiple working wavelengths,and high efficiency concurrently.The coupling direction,operation frequency,and excited mode type can be designed at will for arbitrary incident polarizations,outperforming previous technology that only works for specific polarizations and lacks versatile mode controllability.Here,using silicon-nanoantenna-patterned silicon-nitride photonic waveguides,we numerically demonstrate a set of chip-scale optical couplers around 1.55μm,including mode-selective directional couplers with high coupling efficiency over 57%and directivity about 23 d B.Polarization and wavelength demultiplexer scenarios are also proposed with 67%maximum efficiency and an extinction ratio of 20 d B.Moreover,a chip-integrated twisted light generator,coupling free-space linear polarization into an optical vortex carrying 1 h orbital angular momentum(OAM),is also reported to validate the mode-control flexibility.This comprehensive method may motivate compact wavelength/polarization(de)multiplexers,multifunctional mode converters,on-chip OAM generators for photonic integrated circuits,and high-speed optical telecommunications.

Nonlinear optics in all-dielectric nanoantennas and metasurfaces:a review

Free from phase-matching constraints,plasmonic metasurfaces have contributed significantly to the control of optical nonlinearity and enhancement of nonlinear generation efficiency by engineering subwavelength meta-atoms.However,high dissipative losses and inevitable thermal heating limit their applicability in nonlinear nanophotonics.All-dielectric metasurfaces,supporting both electric and magnetic Mie-type resonances in their nanostructures,have appeared as a promising alternative to nonlinear plasmonics.High-index dielectric nanostructures,allowing additional magnetic resonances,can induce magnetic nonlinear effects,which,along with electric nonlinearities,increase the nonlinear conversion efficiency.In addition,low dissipative losses and high damage thresholds provide an extra degree of freedom for operating at high pump intensities,resulting in a considerable enhancement of the nonlinear processes.We discuss the current state of the art in the intensely developing area of all-dielectric nonlinear nanostructures and metasurfaces,including the role of Mie modes,Fano resonances,and anapole moments for harmonic generation,wave mixing,and ultrafast optical switching.Furthermore,we review the recent progress in the nonlinear phase and wavefront control using all-dielectric metasurfaces.We discuss techniques to realize alldielectric metasurfaces for multifunctional applications and generation of second-order nonlinear processes from complementary metal–oxide–semiconductor-compatible materials.

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 flinctions 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.

High-efficiency all-dielectric transmission metasurface for linearly polarized light in the visible region

We propose and numerically investigate an efficient transmission-mode metasurface that consists of quasi- continuous trapezoid-shaped crystalline silicon nanoantennas on a quartz substrate. This metasurface provides a linear phase gradient and realizes both full 2Jr phase shift and high transmission efficiency in the operating wavelength range from 740 to 780 nm. At the central wavelength around 751 nm, the total transmission efficiency is up to 88.0% and the section of the desired anomalous refraction is 80.4%. The anomalous refraction angle is 29.62°, and larger refraction angle can be achieved by changing the period of the super cell. We demonstrate a refraction angle as large as 38.59°, and the anomalous transmission efficiency reaches 76.6% at wavelength of 741 nm. It is worth mentioning that the structure is much simpler than conventional metasurfaces based on arrays of discrete nanoantennas. Our research may pave the way for designing efficient all-dielectric phase-gradient metasurfaces and applying them in integrated optical devices for wavefront control.

Cascaded metasurfaces enabling adaptive aberration corrections for focus scanning

Scanning focused light with corrected aberrations holds great importance in high-precision optical systems.However,conventional optical systems,relying on additional dynamical correctors to eliminate scanning aberrations,inevitably result in undesired bulkiness and complexity.In this paper,we propose achieving adaptive aberration corrections coordinated with focus scanning by rotating only two cascaded transmissive metasurfaces.Each metasurface is carefully designed by searching for optimal phase-profile parameters of three coherently worked phase functions,allowing flexible control of both the longitudinal and lateral focal position to scan on any custom-designed curved surfaces.As proof-ofconcept,we engineer and fabricate two all-silicon terahertz meta-devices capable of scanning the focal spot with adaptively corrected aberrations.Experimental results demonstrate that the first one dynamically scans the focal spot on a planar surface,achieving an average scanning aberration of 1.18%within the scanning range of±30°.Meanwhile,the second meta-device scans two focal points on a planar surface and a conical surface with 2.5%and 4.6%scanning aberrations,respectively.Our work pioneers a breakthrough pathway enabling the development of high-precision yet compact optical devices across various practical domains.

超高圆二色性近红外全介质二维手性超表面

随着微纳加工技术的发展,手性超表面被证明在光学中有着巨大的应用潜力。传统的金属手性结构,由于其自身存在的欧姆损耗,难以取得理想的圆二色性,而具有良好圆二色性但结构复杂的三维手性结构加工制备困难。采用周期性双Si砖错位拼接的二维全介质手性结构,通过激发介质内部中非正交的面内磁偶极矩和电偶极矩,可以产生明显的光学手性响应,从而得到理想的圆二色性。通过仿真优化,该结构在1550 nm波长附近可以实现超过0.94的超高圆二色性,并且可以实现对入射的左旋圆偏振光到右旋圆偏振光的转化。该全介质二维手性超表面的圆二色性高、结构简单、制备难度低,在近红外偏振成像等领域有着广泛的应用前景。

基于介质超表面的圆偏振复用全息

针对单一相位调控方式无法实现圆偏振复用问题,使用矩形硅砖结构,通过复合传输相位和几何相位,设计了一种工作在1.55μm下可以实现圆偏振复用全息的超表面。仿真结果表明,分别在左旋圆偏振光和右旋圆偏振光照射下,在远场可以观测到两幅清晰的全息图像,透过效率分别为60.8%和61.5%,偏振转化效率分别为79.1%和78.7%。设计的圆偏振复用超表面具有微弱串扰、两通道独立设计以及设计简单等优点,并且超表面元件体积小、重量轻、易于集成,在信息复用、信息存储或编码、防伪等方面具有较大的应用潜力。

Gradient-structure-enhanced dielectric energy storage performance of flexible nanocomposites containing controlled preparation of defective TiO_(2) and ferroelectric KNbO_(3) nanosheets

Next generation power system needs dielectrics with increased dielectric energy density.However,the low energy density of dielectrics limits their development.Here,an asymmetric trilayered nanocomposite,with a transition layer(TL),an insulation layer(IL),and a polarization layer(PL),is designed based on poly(vinylidene fluoride)-polymethyl methacrylate(PVDF-PMMA)matrix using KNbO_(3)(KN)and TiO_(2)(TO)as the nanofillers.The morphology and defect control of the two-dimensional nano KN and nano TO fillers are realized via a hydrothermal method to increase the composite breakdown strength(E_(b))and the composite energy density(U_(e)).The asymmetric trilayered structure leads to a gradient electric field distribution,and the KN and TO nanosheets block charges transfer along z direction.As a result,the development path of the electrical trees is greatly curved,and E_(b) is effectively improved.And the Ue value of the nanocomposites reaches 17.79 J·cm^(-3) at 523 MV·m^(-1).On the basis,the composite Ue is further improved by defect control in TO nanosheets.The nanocomposite KN/TO/PVDF-PMMA containing TO with less oxygen vacancy concentration(calcined at oxygen atmosphere)acquires a high Ue of 21.61 J·cm^(-3) at 548 MV·m^(-1).This study provides an idea for improving the energy storage performance by combining the design of the composite dielectric structure and the control of nanofillers’defect and morphology.

Advanced manufacturing of dielectric metadevices

Metasurfaces,composed of two-dimensional nanostructures,exhibit remarkable capabilities in shaping wavefronts,encompassing phase,amplitude,and polarization.This unique proficiency heralds a transformative paradigm shift in the domain of next-generation optics and photonics,culminating in the development of flat and ultrathin optical devices.Particularly noteworthy is the all-dielectric-based metasurface,leveraging materials such as titanium dioxide,silicon,gallium arsenide,and silicon nitride,which finds extensive application in the design and implementation of high-performance optical devices,owing to its notable advantages,including a high refractive index,low ohmic loss,and cost-effectiveness.Furthermore,the remarkable growth in nanofabrication technologies allows for the exploration of new methods in metasurface fabrication,especially through wafer-scale nanofabrication technologies,thereby facilitating the realization of commercial applications for metasurfaces.This review provides a comprehensive overview of the latest advancements in state-of-the-art fabrication technologies in dielectric metasurface areas.These technologies,including standard nanolithography[e.g.,electron beam lithography(EBL)and focused ion beam(FIB)lithography],advanced nanolithography(e.g.,grayscale and scanning probe lithography),and large-scale nanolithography[e.g.,nanoimprint and deep ultraviolet(DUV)lithography],are utilized to fabricate highresolution,high-aspect-ratio,flexible,multilayer,slanted,and wafer-scale all-dielectric metasurfaces with intricate nanostructures.Ultimately,we conclude with a perspective on current cutting-edge nanofabrication technologies.

基于电介质超表面的光场复振幅调制及应用

超表面作为一种人工设计的二维阵列纳米结构,能够在亚波长尺度上实现光场波前振幅、相位和偏振态的灵活调控,为现代光学器件的小型化、集成化提供了全新的实现途径。随着光学成像、显示等应用的发展,在可见光波段具有高工作效率的微型光学器件的需求日益凸显。近年来,由高折射率、低损耗电介质材料制备的光学超表面得到了极大地发展,在消色差光学超透镜、偏振相关全息显示等方面展现出广泛的应用前景。文中围绕电介质超表面的相关研究,首先介绍广义斯涅耳定律及电介质超表面结构调控光场振幅、相位和偏振态的基本原理,在此基础上,重点回顾近年来关于光场波前单一参量调控和多参量联合调控在全息显示、结构光场产生等方面的研究进展,最后讨论电介质超表面发展的可能挑战与前景。

超构表面红外分光阵列设计

超构表面阵列分光具有高透射、低损耗、高集成度的特点,是当前微纳结构领域的研究热点。本文提出了一种由Si和SiO2组成的超构表面红外分光阵列。以不同波长的红外光入射单元,仅改变圆柱半径,分别计算不同波长的透射率和出射相位随半径的变化。根据广义斯涅尔定律,选择具有不同圆柱半径的若干单元组成超构表面分光周期。该周期对不同波长的入射光具有对应的透射相位梯度,验证了超构表面阵列具有分光功能。对超构表面红外分光阵列可能的应用提出了展望。结果表明:以半径为150,250,300 nm的3个Si纳米柱组成的超构表面周期可以将波长2.7μm和2.9μm的混合入射红外光分离,并且透射率达到70%以上。