Study On Terahertz Generation By Optical Rectification Based On Femtosecond Bessel Beam

Terahertz wave has many excellent properties due to its unique location in the spectrum lying between infrared waves and microwaves,and it has a variety of promising applications in many fields.However,the output power of most THz sources in current use is very low,and in order to improve the output power of terahertz sources based on optical rectification,we used a Bessel pump beam to improve the non-linear interaction length between the pump beam and the nonlinear crystal considering the fact that the size of the center spot of the Bessel beam remains unchanged during transmission.Compared with a Gaussian beam,the Bessel beam has a longer effective non-linear interaction length in the same crystal,and we expect that the terahertz generation efficiency based on optical rectification could be improved greatly.The terahertz wave generation efficiency between the Bessel and Gaussians beam was compared in experiment and the results would be of value for higher-power THz sources.In the thesis,the background,applications,generation and detection methods of terahertz wave were introduced firstly so as to highlight the need for high power terahertz wave generation.Then,several factors that influenced the optical rectification efficiency were analyzed in order to help optimize the terahertz generation process theoretically.In order to compare the signal-noise-ratio and the spectral characteristics of the terahertz signals generated by Gaussian and Bessel beams,a THz time-domain spectroscopy system based on the Gaussian pump beam was first built to be prepared for the following work.Next,the Bessel beam was discussed with its characteristics and the main generation methods,and we analyzed the Bessel transmission characteristics produced in our experiment by an axicon,and the non-diffraction range was determined to be ready for the following experiment.In addition we discussed the reasons for the on-axis oscillations of the Bessel beam behind the axicon,and made a detailed analysis of the experimental results.Theoretical analysis of the relationship between the intensity distribution of the center spot of the Bessel beam and the Gaussian function was made,and our experimental results fitted well with the theoretical analysis.Finally,GaP bulk crystals and GaP-waveguides were pumped with Bessel and Gaussian beams to generate terahertz radiation.Depending on the experimental results,we found that the spatial distribution of the terahertz field fitted well by a Gaussian function just as center spot of the incident Bessel beam did.The experiment proved that the the center spot of the Bessel beam could improve the terahertz wave generation than the Gaussian beam when they had the same power in both GaP bulk crystals and GaP waveguides,and the relative efficiency depended on the length of the emitters.