Research On Key Technologies Of Chip-Based Wide-Angle Scanning Optical Phased Array Antenna

The scanning and manipulation of free-space light are of great significance in imaging,light detection and ranging（Li DAR）,optical communication,holographic projection,and other systems.Traditional mechanical-based beam-steering devices suffer from low response speed and large size,which can no longer meet the requirements of scientific and technological applications.Optical phased arrays can achieve fast and precise free-space beam scanning through purely electronic control,without mechanical actuators.With the development of optical micro-nano processing technology,antenna arrays composed of hundreds or thousands of radiation elements can be integrated on a coin-sized optical chip by taking advantage of photonic integration technology.Therefore,the size and weight of beam-scanning devices can be significantly reduced.On-chip integrated optical phased arrays are expected to be a potential solution in applications such as autonomous driving and virtual reality,and have attracted more and more attention.Optical antenna is the key component of optical phased arrays.Currently reported optical antennas are simple in topology and single in function,making it difficult to precisely control the amplitude and phase of the antenna aperture field to realize complex free-space optical function.Optical phased arrays are generally composed of optical antennas periodically arranged on a one-dimensional（1-D）or two-dimensional（2-D）plane.By actively adjusting the phase of each antenna element,the array radiation wavefront can be manipulated to achieve the control of free-space beams,including beam scanning.Particularly,optical phased arrays composed of 1-D millimeter-scale waveguide grating antennas have been extensively investigated.Ideally,aliasing-free beam scanning can be ensured by a half-wavelength antenna element pitch.However,strong couplings between waveguides will occur even when the element pitch in the array is below one wavelength,causing the independent phase control of each antenna element unachievable.Therefore,the coupling between the waveguides and the element pitch poses a limitation to the wide-angle scanning of optical phased array antennas.Periodic structures have been widely employed in photonic integrated devices for their special and fantastic responses to electromagnetic waves.This dissertation aims to make full use of the fantastic characteristics of periodic structures to conduct in-depth research on wide-angle beam scanning of optical phased array antennas.At the same time,on-chip plasmonic nanoantenna and arrays have been intensively investigated for their potential to manipulate free-space light fields.The main contents of this dissertation are summarized as follows:1.A silicon-nitride（Si N）based optical phased array antenna loaded with a near-wavelength high-contrast grating（High-contrast Grating,HCG）matching layer is proposed.Taking advantage of the peculiar optical transmission spectrum caused by Rayleigh anomaly,an HCG periodic structure that selectively transmits plane waves at different incident angles is designed and loaded as the matching layer of a Si N-based optical phased array antenna.The proposed design achieves a total scanning range of 28°under the antenna element pitch of 1.55λ₀,with the sidelobes lower than-10 d B,with the far-field grating lobe suppression caused by large antenna element pitch demonstrated（Typically,the element pitch in currently reported Si N-based phased array is larger than2λ₀,which leads to many grating lobes in the real space and limits the field of view below30°）.On the other hand,optical phase shifters are generally based on the thermo-optic effect of silicon.However,the thermo-optic coefficient of Si N is only one tenth of that of silicon,resulting in high power consumption and low efficiency of phase shifters on Si N platforms.To solve the problem,an electro-optic phase shifter based on electro-optic polymer cladding modulation is also designed.2.Based on the commercial silicon-on-insulator（SOI）platform,a subwavelength-pitch low-crosstalk optical phased array is proposed for wide-angle beam steering.In the proposed design,reflective boundaries composed of 1-D periodic dielectric blocks are inset between the transmission waveguides to suppress the coupling between waveguides.Meanwhile,the application of the one-dimensional periodic structure makes the transmission waveguide an optical antenna,enabling the energy of the evanescent field to radiate into free space at an extremely weak rate,and an extremely narrow far-field 3-d B beamwidth can be obtained.By loading the 1-D periodic structure,the proposed optical phased array can be arrayed with subwavelength even nearly half-wavelength element pitch with low crosstalk between waveguides maintained.Therefore,far-field wide-angle beam scanning can be enabled.3.A wide-angle metasurface-based focal plane switch array for far-field beam scanning is proposed.Based on quadratic phase distributions,an optical metasurface composed of six discrete high-performance units is designed,and its wide-angle focal plane focusing properties are first verified.Then,an ultra-compact plasmonic optical nanoantenna is also proposed as the metasurface feeding source.A beam scanning range of±50°is demonstrated by switching the excitation of the 11 feed antennas on the focal plane of the metasurface,with the maximum directivity fluctuation among the beams less than 2.83 d B.The proposed design addresses the defocusing problem of conventional metasurfaces based on parabolic phase distributions at large incident angles.Furthermore,using the proposed ultra-compact optical nanoantenna as the metasurface feed source will facilitate the large-scale integration of antenna arrays on the focal plane and reduce the scanning blind zone.4.A high-efficiency aperture-coupled nanoantenna array based on the SOI platform is proposed.By analyzing nano-blocks with high refractive index under periodic conditions,a 2-D HCG periodic structure with broadband reflection properties is designed and employed as the ground of an optical antenna to suppress the waves leaking down to the substrate.In addition,the antenna radiation aperture is formed by etching periodic H-shaped slots on the top mental film.Finally,the proposed aperture-coupled nanoantenna array achieves the desired broadband unidirectional radiation property and a radiation efficiency exceeding 75%.It shows a significant gain enhancement of 3.59 d B over the structure without employing the HCG periodic structure.5.A sinusoidally modulated optical leaky-wave antenna based on a plasmonic gap waveguide is proposed.To show the powerful wavefront shaping abilities of the proposed antenna,the amplitude and phase on the antenna radiation aperture are implemented in the following two different methods:（a）Based on the amplitude holography method,the modulation amplitude along the guided wave in the waveguide for generating specific far field can be holistically designed.For proof of concepts,a series of sophisticated free-space optical functions,such as high directional beams,far-field dual beams with arbitrarily designed relative amplitude and radiation angle,focused beams,and Airy beam,have been designed and demonstrated through theoretical analysis,simulation and experiment results.By using the proposed method,various complex one-dimensional wavefront shaping can be achieved.（b）Each modulation period in the proposed antenna can be individually designed to control the field amplitude along the antenna radiation aperture.As an example,a Chebyshev amplitude distribution along the antenna aperture is elaborately designed,achieving a far-field low sidelobe level of below-15 d B,according to the experimental results.