Nonlinear Optical Properties And Ultrafast Carrier Dynamics Of Gallium Oxide （β-Ga₂O₃）

β-Ga2O3 is a novel ultra wide band gap semiconductor material.Because of its advantages such as large band gap(～4.8 eV),high breakdown field strength,low preparation cost and stable physical and chemical properties,β-Ga2O3 has attracted extensive attention in the fields of power electronic devices,UV solar blind detectors and high temperature gas sensors.It has become a domestic and international research hotspot in the direction of ultra wide band gap semiconductors.Due to its transparency in UV-IR caused by large band gap width and integration with Ⅲ-N semiconductors,β-Ga2O3 has potential application value in the field of photonic integrated circuits.However,the research on the nonlinear optical properties of β-Ga2O3 under laser irradiation is still imperfect compared with the electrical properties.Therefore,exploring and revealing the evolution law and photophysical mechanism of the nonlinear optical response of intrinsic and doped β-Ga2O3 at different wavelengths and pulse widths are of great significance for the research of β-Ga2O3 materials and the development of optoelectronic devices.In this dissertation,undoped,Sn-doped and Fe-doped β-Ga2O3 crystals are the main research objects,and experimental methods such as Z-scan,time-resolved pump-probe technique based on phase object and transient absorption spectra are used comprehensively,combined with energy level model and carrier dispersion effect.Theoretically,the bound electron-related nonlinear optical properties,carrier-related nonlinear optical properties and ultrafast carrier dynamics of β-Ga2O3 crystals in UV-Vis are studied.The main research contents and results are as follows:(1)The ultrafast nonlinear optical properties of undoped,Sn-doped and Fe-doped βGa2O3 crystals in the range of 355 nm-750 nm were systematically studied by femtosecond Z-scan.Two-photon absorption coefficient in the range of Eg/2 to Eg,the dispersion relationship between three-photon absorption coefficient and the Kerr refractive index in the range of Eg/3 to Eg/2 were measured.The effects of doping on two-photon absorption coefficient,three-photon absorption coefficient and Kerr refractive index were mainly investigated.It is found that Fe doping can significantly improve three-photon absorption coefficient in the range of 540-600 nm(close to Eg/2).The calculated threshold light intensity of the FOM3PA reached 100 GW/cm2,indicating that β-Ga2O3has great potential in the field of all-optical switching.(2)The carrier dispersion effect of undoped and Sn-doped β-Ga2O3crystals was investigated by degenerate time-resolved pump-probe technique base on phase object excited with picosecond pulses at 355 nm.The nonlinear refraction mechanisms of β-Ga2O3 in different time domains were distinguished.Based on the two-photon induced free carrier model,the free carrier absorption cross section,free carrier refraction volume and free carrier recombination lifetime of β-Ga2O3crystals were determined.It is found that the free carrier absorption and free carrier refraction effects of β-Ga2O3 are significantly smaller than the wide band gap semiconductors GaN and ZnO.The results can provide a reference for the development of β-Ga2O3-based ultra-low loss waveguides and integrated light modulators in the UV spectral range.(3)The effect of doping and intrinsic defects in β-Ga2O3 crystal on nonlinear absorption/refraction dynamics was studied by nondegenerate pump-probe technique(355 nm pump,515 nm probe)based on phase object with femtosecond pulses.Combined with the transient absorption spectrum,the existence of intrinsic defect states in β-Ga2O3 was verified,And the modulation effect of specific probe wavelength on the carrier nonlinearity of β-Ga2O3was explored.It is found that the carrier absorption/refractive index under 515 nm probe is at least one order of magnitude larger than that under 355 nm probe,which was due to the additional absorption of 515nm probe light by intrinsic defects.The carrier recombination process related to Fe-induced deep trapped states was analyzed using the defect state carrier dynamics model and the recombination lifetime of the trapped states was determined to be about 50 ps.In this dissertation,the nonlinear optical properties and carrier dynamics of β-Ga2O3 are systematically studied.On the one hand,the photophysical parameters such as multiphoton absorption coefficient,Kerr refractive index,carrier refractive volume and carrier lifetime of β-Ga2O3are determined.On the other hand,the correlation between electronic states caused by doping/intrinsic defects and nonlinear optical response/carrier dynamics under different pulse widths is revealed.The results of this dissertation clarify the application value of β-Ga2O3 in the field of UV-Vis photonics devices,which can provide a reference for the design and development of UV-Vis optoelectronic devices based on βGa2O3.