Study Of Photogalvanic Effect And Fabrication Of Photodetectors Based On Bi₂Se₃

Topological insulators（TIs）are a new material system discovered in recent years.In2009,TIs were predicted theoretically and then realized experimentally by Angle resolved photoemission spectroscope（ARPES）.After 10 years of development,the theory of TIs has been gradually improved,and various devices based on TIs have also been demonstrated,including photodetectors,spin-electronics devices,and transistors.TIs are one of the most promising material systems to realize quantum information processing.TIs,with strong spin-orbit coupling,have metallic surface states and insulating bulk states.The surface states are protected by time reversal symmetry behaving as a spin-polarized Dirac cone,where different spins move in opposite directions,namely,the Spin-momentum locking effect（SML）.Circular photogalvanic effect（CPGE）can happen in TIs.The circularly polarized light selectively excites the surface electrons with corresponding spin direction.Due to SML,a spin-polarized current is induced.Based on Bi₂Se₃,photodetectors were fabricated and the interaction between surface states and polarization of light or spin momentum of photons was studied.The CPGE in waveguide-integrated photodetectors was realized.All the fabrications and measurements were carried at room temperature.The CPGE in TIs in free space was studied.In oblique incidence,the current in Bi₂Se₃ varies with the polarization of light.A quarter-wave plate（QWP）was used to obtain different polarizations and the results were fitted according to the periodicity.Due to the Seebeck effect,a thermal current can be generated by unbalanced illumination on contacts.By changing light polarization and spot position,different contributions were extracted mathematically.Then the amplitude-frequency responses of photo current and thermal current were measured.Photo current and thermal current can be classified in measurenment according to cut-off frequency.The anisotropic photogalvanic effect was studied.In different devices,the bulk contributions are different due to the anisotropy of the crystal structure.In the device with multiple pairs of contacts,anisotropic photocurrents were observed.The photocurrents vary periodically corresponding to the 3-folded symmetry of Bi₂Se₃ crystal structure.Different contributions were extracted by curve fitting.The bulk contribution is related to the crystal structure while the surface contribution is not.Therefore,the bulk contribution can be reduced after orientation calibration.Then a waveguide-integrated photodetector was fabricated to study the interaction between guided photons and surface electrons in TIs.On a SOI substrate,TE and TM grating couplers and directional couplers were fabricated and optimized.Directional couplers were used to couple different modes into one waveguide interacting with one sample.Bi₂Se₃ flakes with thickness less than 50nm were exfoliated and transferred to the determined position on substrates.Ohm contact between the sample surfaces and the metal contacts was achieved by ion milling and electrical shock.The fabrication processes and measurements were carefully studied.The photons guided by an optical waveguide or fiber also carry spin angular momentum（SAM）.The SAM of TM and TE modes in a rectangular waveguide were calculated respectively.TM mode carries significant transverse SAM while TE mode does not.The sign of angular momentum reverses with the direction of light propagation.A Bi₂Se₃ flake was put above the waveguide.Photocurrents excited by TM and TE modes were measured,respectively.A longitudinal spin-polarized photocurrent is generated by photons carrying transverse SAM,which is similar to CPGE in free space.Because of SML,changing light propagation direction reverses photon spin and thus the direction of the photocurrent.The results of TM and TE modes vary with the distribution of SAM.This waveguide-integrated photodetector can work without bias voltage,recognize propagating direction and mode,and generate spin-polarized current.In the waveguide-integrated photodetector based on Bi₂Se₃,the SML effect between photons and surface electrons is realized.The path information is directly converted to current information.This device provides a new way to couple photons and electrons and may lead to applications in opto-spintronics,quantum computing and quantum communications.