Measurement Of Nonlinear Optical Properties And Study On Carrier Dynamics Of Silicon Carbide Based On Femtosecond Laser

Silicon carbide(SiC),as the leading representative of the third-generation semiconductors,has the advantages of wide band gap,high saturated electron mobility,high thermal conductivity,and so on,showing great potential in the field of electronics,micro-electro-mechanical system(MEMS),and optoelectronics.However,high hardness,chemical inertness,and high brittleness of SiC require special processing technologies for SiC device fabrication such as femtosecond laser machining.In addition,SiC has good nonlinear optical properties,which can be utilized to realize optical limiting and optical parametric amplification.Both of these two aspects require systematic study on the nonlinear optical properties and the carrier dynamics of SiC.In this work,the nonlinear optical properties and the carrier dynamics of 6H-SiC and 4H-SiC,two important SiC polytypes,are studied by femtosecond laser with the following content.1.Measurement of nonlinear optical properties.An optical parametric amplifier is used for extending the spectrum of femtosecond laser to conduct Z-scan experiment in the wavelength range of 400-1100 nm.For the open-aperture Z-scan data,the corresponding multi-photon absorption model is used in different spectral ranges to obtain the trend of multi-photon absorption coefficient of samples varied with wavelength in the range of 400-1100 nm.Samples used include semi-insulating(SI)6H-SiC,SI 4H-SiC and N-type doped(N)6H-SiC.The relationship between the results and the electronic band structure of the samples is analyzed.The results of closed-aperture Z-scan experiment show that the nonlinear refraction of the three samples are all of self-focusing type,and the values of nonlinear refractive index in the wavelength range of 500-1000 nm are measured.It is found that the N-type doping leads to the enhancement of nonlinear refraction.2.Time-domain characterization of carrier dynamics.Pump-probe technique is used to characterize the excitation and the following relaxation of carriers in SiC launched by femtosecond laser.First,the process with picosecond timescale in SI 6H-SiC and SI 4H-SiC are detected by the pump-probe experiment with multiple wavelength combinations.The corresponding physical processes are analyzed by considering the electronic band structure and the characteristic times of typical ultrafast processes.Then,the order of nonlinear absorption in the excitation process is determined by studying the relationship between the pump fluence and the amplitude of the transient transmittance change signal of SI 6H-SiC.In addition,it is found that when the pump pulse overlaps with the probe pulse in time domain,the transmittance of SiC to the probe has considerable change.Ultrafast all-optical modulation based on SiC can be realized by using this phenomenon.Through pump-probe experiment,this work studies the modulation effect of 820 nm femtosecond laser to light within the range 420-720 nm.The relationship between the optical modulation effect and the angle between the polarization directions of the pump and the probe is also investigated.In sum,femtosecond laser in a wide spectrum is used to perform Z-scan experiment and pump-probe experiment on SiC.The values of nonlinear optical properties of SiC in a wide spectral range are measured for the first time.The relaxation process of excited carriers in SiC is characterized,and the ultrafast all-optical modulation of transmittance of SiC by femtosecond laser is explored.