Research On Silicon Integrated YIG-based Magneto-optical Thin Films And Optical Isolators

Optical isolators allow one-way transmission of light,protecting sensitive components such as lasers and amplifiers from harmful reflections,avoiding relative intensity noise?RIN?and phase noise.Integrated optical isolator has been a long-term challenge for photonic integrated circuits?PICs?.An ideal integrated optical isotator for a PIC should be made by a monolithic process,have a small footprint,exhibit broadband and polarization-diverse operation,and be compatible with multiple materials platforms.Despite significant progresses,optical isolators reported so far do not meet all of these requirements.The traditional magneto-optical isolators use the non-reciprocity of magnetic materials such as yttrium iron garnet to achieve light isolation.However,due to lattice mismatch and large difference in thermal expansion coefficient,magneto-optical materials are challenging to integrate onto silicon and?-?semiconductor substrates.Heterogeneous integration through wafer bonding can overcome this obstacle to achieve integrated optical isolators.Although excellent performance of epitaxial film can be obtained,the problem is that monolithic integration and large-scale device fabrication cannot be achieved,and the fabricated device has a large size,high cost and low fabrication yield.Besides,optical isolators based on spatio-temporal modulation mechanisms are limited by their narrow operation bandwidths,long device length and large power consumption.Therefore,there is currently no monolithically integrated silicon-based optical isolator.However,silicon integrated magneto-optical materials and devices can effectively overcome the above-mentioned problems.The main issues in this area including three questionsm.Firstly,how to break through the lattice mismatch and thermal mismatch between magneto-optical materials and silicon substrated to achieve high-value magneto-optical materials on silicon substrates?Secondly,How to solve the phase matching difficulty of waveguide Faraday rotatory structure and design an optical isolator based on the new mechanism of non-reciprocal phase shifting?Thirdly,how to develop a universal waveguide isolator integration method based on semiconductor fabrication process for fully polarized and full-substrate materials?Based on the above research background and issues,this dissertation has addressed the key challengs on silicon-based integrated magneto-optical materials preparation,optical iaolators design and farbrication.For the first time,we realized silicon-based monolithic integrated optical isolator with broad band and high-performace.The details list as follows.1.Pulsed laser deposition?PLD?was used to study the preparation temperature and atmosphere of the films to realize monolithic integration of polycrystalline YIG on silicon substrate.The relationships between the preparation temperature and the materails structure,valence state and magnetic properties were obtained.Non-garnet phases or mixed Fe valence states are observed in films when deposited or PDA at too high temperatures,whereas partially amorphous thin films are obtained when deposited or PDA at too low temperatures.Therefore,YIG thin films with bulk-like structure and saturation magnetization are obtained for films deposited at 400°C and PDA at800°C-850°C,respectively.2.By using YIG thin film as bottom seedlayer,a new magneto-optical thin film with record high mageto-optical effect on silicon substrate was realized,which break through the difficulty of integtating high-quality magneto-optical materials on silicon.Based on the first-principles calculation results,by controlling Ce⁴⁺ions formation enthalpy,the concentrations of Ce in polycrystalline Ce:YIG thin films were increased from 1.0 to1.5 by controlling the oxygen vacancy concentration during film preparation.At the wavelength of 1550 nm,the magneto-optical Faraday rotation angle of Ce_1.5Y_1.5Fe₅O₁₂thin films is as high as-6410 deg/cm,which is twice that of the polycrystalline Ce₁Y₂Fe₅O₁₂ thin films,and 1.4 times that of the epitaxial Ce₁Y₂Fe₅O₁₂ thin films as reported so far.The preparation of high Faraday rotation thin films provides the possibility of monolithic integration of magnto-optical isolators.3.The regulation of the magnetic anisotropy of Ce:YIG thin films was achieved by introducing Dy³⁺ions with negative magneoelastic coefficient into Ce:YIG thin films for the demond of TE mode isolators.With the increase of Dy³⁺ions concentration,the magnetization easy axis of Dy:CeYIG thin films gradually changed from in-plane to out-of-plane.Perpendicular magnetic anisotropy was obtained when Y³⁺ions were completely replaced by Dy³⁺ions.Meanwhile,the coercivity can be controlled by changing the thermal stress during film depositon process.For Ce₁Dy₂Fe₅O₁₂ thin films,the coervivity can be gradually increased from 160 Oe to 1100 Oe.In addition,the Faraday rotation of Ce₁Dy₂Fe₅O₁₂ thin films were tested from visible to near infrared wavelength.Its Faraday rotation angle at 1310 nm and 1550 nm were-4700 deg/cm and-2800 deg/cm,similar to Ce₁Y₂Fe₅O₁₂ thin films.Therefore,Dy substitution is an effective way to induce PMA in Ce:YIG thin films without compromising their magneto-optical figure of merit,making this material promising for self-biased transverse electric mode optical isolator applications.4.In order to accurately measure thin films optical loss,a cut-back method was used.By measuring the magneto-optical waveguide loss,the optical loss of magneto-optical materials can be calculated by simulating the confinement factor of each layers.Firstly,silicon waveguides with thickness of 220 nm and a width of 4 um and 500 nm were fabribated by contact photolithography and electron beam lithography?EBL?and RIE etching process.By using these two waveguide structures,the optical loss of YIG thin films as function of thicknesses and Ce:YIG thin films under different depostion oxygen partial pressure were measured.The optical losses of Ce:YIG thin films and Ce:DyIG thin films with perpendicular magnetic anisotropy were 80 dB/cm and 91 dB/cm,respectivity.In addition,by patterning windows with different lengths on top of silicon waveguide for materials depostion,it is possible to accurately measure the loss of silicon integrated magneto-optical thin films over a wide range.5.Based on Mach-Zehnder Interferometer?MZI?structure,TM-type and TE-type integrated broadband magneto-optical isolators were designed by COMSOL FEM method and Lumerical FDTD simulation and then experimentally demonstrated.By depositing high-quality magneto-optical garnet thin films on both the top and sidewalls of the silicon waveguides,we realized monolithic on-chip optical isolators with high isolation ratios,low insertion losses,broadband operations and small footprints.For TM mode optical isolator,we obtained a maximum isolation of 30 dB with an insertion loss of 5-6 dB under a unidirectional magnetic field of 500 Oe.This device has the best performance among the monolithic integrated isolators.The work introduces optical non-recoprocity for silicon-based optoelectronic systems,and paves the path to experimental demonstration of several theoretically proposed magneto-optical photonic crystal structures,magneto-optical waveguide modulators,magneto-optical metasurface and topological photonic devices.