Highly efficient acousto-optic modulation using nonsuspended thin-fil1m lithium niobate-chalcogenide hybrid waveguides

A highly efficient on-chip acousto-optic modulator is as a key component and occupies an exceptional position in microwave-to-optical conversion.Homogeneous thin-film lithium niobate is preferentially employed to build the suspended configuration for the acoustic resonant cavity,with the aim of improving the modulation efficiency of the device.However,the limited cavity length and complex fabrication recipe of the suspended prototype restrain further breakthroughs in modulation efficiency and impose challenges for waveguide fabrication.In this work,based on a nonsuspended thin-film lithium niobate-chalcogenide glass hybrid Mach-Zehnder interferometer waveguide platform,we propose and demonstrate a built-in push-pull acousto-optic modulator with a half-wave-voltage-length product VnL as low as 0.03 V cm that presents a modulation efficiency comparable to that of a state-of-the-art suspended counterpart.A microwave modulation link is demonstrated using our developed built-in push-pull acousto-optic modulator,which has the advantage of low power consumption.The nontrivial acousto-optic modulation performance benefits from the superior photoelastic property of the chalcogenide membrane and the completely bidirectional participation of the antisymmetric Rayleigh surface acoustic wave mode excited by the impedance-matched interdigital transducer,overcoming the issue of low modulation efficiency induced by the incoordinate energy attenuation of acoustic waves applied to the Mach-Zehnder interferometer with two arms in traditional push-pull acousto-optic modulators.

Laplace metasurfaces for optical analog computing based on quasi-bound states in the continuum

Laplace operation,the isotropic second-order differentiation,on spatial functions is an essential mathematical calculation in most physical equations and signal processing.Realizing the Laplace operation in a manner of optical analog computing has recently attracted attention,but a compact device with a high spatial resolution is still elusive.Here,we introduce a Laplace metasurface that can perform the Laplace operation for incident lightfield patterns.By exciting the quasi-bound state in the continuum,an optical transfer function for nearly perfect isotropic second-order differentiation has been obtained with a spatial resolution of wavelength scale.Such a Laplace metasurface has been numerically validated with both 1D and 2D spatial functions,and the results agree well with that of the ideal Laplace operation.In addition,the edge detection of a concerned object in an image has been demonstrated with the Laplace metasurface.Our results pave the way to the applications of metasurfaces in optical analog computing and image processing.

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.