The Studies Of Hybrid Silicon And Lithium Niobate Thin Films And Photonic Devices

In recent years,lithium niobate on insulator(LNOI)is considered to be a promising integrated photonic platform.LNOI has the following advantages:it has a large refractive index contrast,which results in a waveguide cross-section of sub-microns and a tight bending radius of tens of microns,allowing high density photonic integration;it retains the electro-optical(E-O),acousto-optic,and nonlinear-optical effects of lithium niobate(LN)bulk materials and can achieve high performance integrated photonic devices.With the industrialization of large-size and high-quality LNOI wafers,as well as breakthroughs in various micro-nano fabrication technologies,numerous integrated photonic devices have been implemented on LNOI,e.g.,E-O modulators,acousto-optical modulators,optical frequency combs,and nonlinear conversion devices.However,LN is an insulator with insignificant electrical properties.Some light sources,detectors,and driving electronics are difficult to realize on LNOI.Si is a widely used semiconductor material.Silicon on insulator(SOI)is also an attractive material platform in integrated optics that has the following advantages:a large refractive-index contrast between Si thin film and SiO2 layer,low light absorption of Si at wavelengths above 1200 nm,and the fact that Si-based integrated photonic devices can be mass-produced in complementary metal oxide semiconductor(CMOS)fabs.A variety of high-performance integrated photonic devices(such as couplers,polarization control devices,detectors,lasers,and waveguide array gratings)have been implemented on SOI.However,Si is a center-symmetric crystal and lacks E-O,acousto-optical,and second-order nonlinear-optical effects.In recent years,many attempts have been made to combine Si thin film and LN thin film so as to use the excellent physical properties of both materials.By flip-chip integration of LN thin film on SOI,many high-performance E-O modulators have been realized.However,these devices are fabricated by directly bonding LN thin films on top of patterned SOIs.But the direct bonding process requires that both bonding surfaces be flat,which makes the direct bonding process challenging.In addition,the fabrication and connection of electrodes is another challenge.Hybrid mono-crystalline silicon and lithium niobate thin films(Si-LNOI)can be fabricated by ion implantation and wafer bonding techniques.The material from top to bottom is:Si thin film,LN thin film,SiO2 layer and Si substrate.The Si-LNOI combines mature photonic devices and mature fabrication technologies of Si-based photonics,as well as excellent E-O,acousto-optic,and nonlinear-optical effects of LN crystal.Compared with the flip-chip integrated LN thin film on the top of SOI,the Si thin film of Si-LNOI is located on the top of LNOI and can be directly processed,which greatly facilitates the design and fabrication of integrated photonic devices.Si-LNOI will provide a new materials platform for integrated optics.In addition,in the future,Si-LNOI can integrate nanoelectronic functions(such as digital,analog,memory,and storage)with integrated photonic devices(such as E-O modulators,acousto-optic modulators,and nonlinear-optical devices)on the same substrate,realizing the monolithic integration of photonic and electronic integrated chips.At present,Si-LNOI wafers have been successfully fabricated by ion implantation and wafer bonding techniques,and researchers have designed high-performance photonic devices on Si-LNOI,such as M-Z modulators and nonlinear conversion devices.However,since this is a new research field,there are few studies on the physical properties of Si-LNOI.Meanwhile,there are few reports of integrated photonic devices implemented on Si-LNOI.In this paper,we study the physical properties of Si-LNOI,which provides an important reference for the high-performance photonic devices.For integrated photonic devices,efficient coupling between optical fiber and optical waveguide is essential.We have designed and fabricated a spot-size converter for efficient end-face coupling between a Si-LNOI waveguide and optical fiber.The E-O effect is one of the most attractive properties of LN materials.Moreover,we designed,fabricated and characterized various E-O devices based on Si-LNOI.The main results of this thesis are the following:1.Fabrication and characterizations of Si-LNOI wafersFirst,LNOI wafers were fabricated.The lattice properties,domain inversion voltage and E-O coefficient of LNOI were studied.Second,Si-LNOI wafers were fabricated by bonding Si thin film on LNOI wafer,and their lattice and structure properties were studied.2.End-face coupler on Si-LNOIFor photonic devices,the efficient coupling between optical fibers and optical waveguides is essential.We fabricated a spot-size converter on Si-LNOI to match the modes of the waveguide and lensed fiber,so as to achieve efficient end-face coupling.At 1550 nm,TE and TM modes achieve coupling losses of 1.9 and 2.2 dB/facet,respectively.The coupling efficiency is stable in the wavelength range of 1550-1600 nm.3.Phase modulator on Si-LNOIPhase modulators can be used for wavelength tuning of lasers,active mode-locking of lasers,and frequency comb generation.Half-wave voltage(Vπ)is an important parameter for phase modulators.A phase modulator was designed on Si-LNOI with a VπL of only 3.3 V·cm.A phase modulator was fabricated on Si-LNOI with a VπL of 4.74 V·cm.This study provides useful information for the E-O devices on the Si-LNOI4.Electro-optically tunable interleaver on Si-LNOIThe interleaver is widely used in dense wavelength division multiplexing technology.We designed and fabricated an interleaver with an asymmetric Mach-Zehnder interferometer structure on Si-LNOI.In the wavelength range of 1530-1620 nm,the interleaver achieved a channel interval of 55 GHz and extinction ratio of 12-28 dB.It also has an E-O tuning function,and the measured E-O tuning efficiency is 26 pm/V.5.Electro-optically tunable Bragg grating filter on Si-LNOIA Bragg grating filter is an important component in integrated optics.First,a uniform Bragg grating filter was designed on Si-LNOI.The simulation results show that the Bragg grating filter on Si-LNOI could have a good filtering effect(the extinction ratio can reach 55 dB).The phase-shifted Bragg grating filters with a narrow 3 dB bandwidth were designed,and can be used as a narrow passband filter.The E-O modulation efficiency of the designed Bragg grating filters is higher than 16 pm/V.Second,the uniform Bragg grating filter was fabricated with an extinction ratio of 20 dB.