Photonics Devices For Spatial Light Manipulation Based On Subwavelength Structures

The physical dimension resources of the light field include amplitude,phase,time,frequency,polarization,spatial structure.The capacity of optical fiber communication systems can be effectively improved by the modulation or multiplexing of these physical dimensions.In addition to waveguide mode multiplexing,free-space vortex beam multiplexing has been widely studied in large-capacity communication systems.In order to utilize the space dimension of light waves,it is necessary to effectively manipulate the spatial beam,including generation,detection,and conversion.Traditional optical components used to manipulate the spatial light field have disadvantages such as large volume,single function and low integration,which are not suitable for the efficient,flexible and compact spatial manipulation of optical field.In contrast,subwavelength photonic devices have the advantages of small size,easy integration,short diffraction distance and flexible manipulation,achieving great application value in three device platforms including silicon photonics,III-V In P photonics and metasurface.In this thesis,a series of researches focusing on subwavelength photonic devices and spatial field manipulation are carried out based on the above three platforms.The details are as follows:（1）Two kinds of on-chip silicon vortex light generators based on Phc-like subwavelength structures are proposed and investigated.The silicon Phc-like sub-wavelength structure with high manufacturing tolerance is used to couple the on-chip guided mode into the vertical spatial beam,combined with the inverse design optimization algorithm:1)according to the difference between mode field distributions of high and low-order transverse modes（TE₀ and TE₁ mode）,a high-purity,broadband silicon orbital angular momentum（OAM）emitter is designed and fabricated,which can generate 8 kinds of OAM light fields including 2 polarizations and 4 topological charges.The measured average purity of the light field is>0.8 and the maximum value is 0.813 in the C-band（1530～1565 nm）;2)according to difference of the mode structure and refractive index of the polarization mode（TE₀ and TM₀ modes）,an OAM emitter is designed and fabricated,which can generate 4kinds of OAM light with measured average purity>0.83 and maximum value as 0.86 in the C-band.These two devices establish the connection between guide mode multiplexing and OAM mode multiplexing,which broaden the design of silicon vortex light source.（2）Two In P surface emitting spatial beam distributed feedback lasers based on subwavelength structure are proposed and studied.According to physical mechanism of the coupling guided mode into the vertical beam by grating:1)using the three-segment subwavelength grating structure"subwavelength first-order grating+second-order grating+first-order grating",combined with high temperature resistance of Al Ga In As quantum well materials and mature DFB laser fabrication process,a 1.3μm high-speed modulated surface-emitting Gaussian beam laser is designed and fabricated,which successfully achieves stable single-longitudinal mode in a wide temperature range（20～70°C）,high output power>4 m W,3 d B modulation bandwidth up to 21 GHz,and supports high-speed 25 Gb/s signal transmission;2)using three-segment subwavelength grating structure“first-order grating+fork grating+first-order grating”,a 1.3μm surface emitting vortex beam laser is simulated,which can achieve high quality OAM light with purity as 0.83,with simple structure,insensitivity to wavelength and easy mode expansibility.This device offers new methods for the design of highly compact on-chip vortex light source.（3）The devices of spatial beam detection and conversion based on subwavelength metasurface are proposed and studied.According to the advantages of accurate optical field regulation and multi-functional integration of multilayer metasurfaces:1)using the near-field directional excitation of surface wave and far-field light demodulation and focusing,a novel multi-functional spatial light field detector is simulated.It can detect 4 kinds of spin angular momentum state（X,Y,LCP,RCP polarization）and 5 kinds of orbital angular momentum state（topological charge l=-2,-1,0,1,2）,enriching the near and far-fields functionalities of the metasurface and have easy expansibility of more polarizations and topological charge channels;2)integrating the abilities of the independent phase manipulations of X and Y polarization,polarization conversion,strong reflection for Y polarization and strong transmission for X polarization by three-layer metasurface,a novel asymmetric multifunctional spatial beam converter is simulated,which can realize high-efficiency optical conversion between four kinds of Gaussian beam and vortex beams,opening up a new application scenario for the spatial conversion of subwavelength devices.