Research On Generation Of Terahertz Wave By Stimulated Polariton Scattering With Mid-infrared Crystal

With the wide application of terahertz(THz)waves in the fields of safety inspection,biomedicine,wireless communication and so on,requirements of high power and energy THz radiation sources are also greatly improved.THz waves generation based on the stimulated polariton scattering have the advantages of high power,wide tuning,narrow line width,simple structure and room temperature operation,which is an effective way to realize high energy THz radiation sources.However,the quantum conversion efficiency of this method is low,which needs to be improved.To achieve greater conversion efficiency,this paper mainly proposes an innovation based on the THz waves generated by stimulated polariton scattering.A new method and a new structure are introduced to generate THz waves by coupled cascaded parametric and difference frequency processes with aperiodic crystal,thus improving the output intensity and conversion efficiency of THz waves.The main contents of this paper are as follows:1.A high-efficiency THz wave amplification approach by coupled cascaded optical parametric processes(CCOPP)with orientationally patterned gallium phosphide(OP-GaP)is introduced.First,based on the analysis of the cascaded optical interaction mechanism,two cascaded optical parametric processes(COPP)which are intensely coupled by THz waves and cascaded optical waves via setting a precise periodic vector of OP-GaP.Coupled models of the first and m-order for CCOPP are employed.The difference frequency coupling wave equations of cascaded vector beam are derived.Second,the effects of different cascading orders,phase mismatches,pump intensities,red-shifted and blue-shifted processes,nonlinear coefficients and absorption coefficients,the initial THz intensities on THz output are analyzed.The influence mechanism of phase mismatch of each order difference frequency on cascaded optical energy flow is summarized.CCOPP increases the conversion efficiency,so the THz peak intensity and quantum efficiency of the 12-order CCOPP are 8.03 MW/cm2 and 780%,which exceeds the Manly-Rowe limit.Compared with the traditional non-coupled cascaded optical parametric amplification process,the quantum efficiency of the CCOPP increases to 3.7 times.2.A high power THz wave generation method based on cascaded difference frequency with new structure crystal is proposed.The characteristics of cascaded difference frequency generating THz waves are investigated.The effects of phase mismatches,coupling coefficients,pump intensities,signal intensities and input THz intensities on cascaded optical energy flow are investigated.Based on the analysis results,we propose a THz wave generation approach by cascaded difference frequency generation with an aperiodically poled lithium niobate(APPLN)crystal at cryogenic temperature.By precisely setting the poled wave vector of aperiodically poled lithium niobate crystal,the phase mismatches of cascaded Stokes processes and cascaded anti-Stokes processes are reduced and increased respectively.The variations of phase mismatches enhance the cascaded Stokes processes,and restrain the cascaded anti-Stokes processes,yielding unprecedented energy conversion efficiencies in excess of 30%from optical wave to THz wave at a temperature of 10 K.The peak THz intensities are 364.7 MW/cm~2 and400.7 MW/cm2 by cascaded difference frequency generation from APPLN with schemes of stepwise changing poling periods and gradually changing poling periods,respectively.Compared with traditional cascaded difference frequency generation using periodically poled lithium niobate(PPLN),the THz peak intensities are enhanced by 2.5 and 2.8 times using APPLN with stepwise changing and gradually changing poling periods,respectively.