Research On Cascaded Difference Frequency Generation Of Terahertz Wave

With the wide application of terahertz?THz?wave in the fields of wireless communication,biomedicine,security inspection,spectral imaging and so on,the demand of terahertz radiation source is greatly increased.The method of terahertz wave by nonlinear optics difference frequency generation?DFG?is an effective technique,which has the advantages of high output power,wide tuning range,narrow linewidth,simple structure and room temperature operation.However,the quantum conversion efficiency of optical method is low now,which needs to be improved urgently.In this paper,we will study and analyze how to improve conversion efficiency of terahertz wave via cascaded DFG.The main contents of this paper are as follows:?1?THz waves generated based on periodically polarized KTiOPO₄?KTP?crystal via cascaded difference frequency is theoretically analyzed.The gain and quantum conversion efficiency of THz waves are calculated from the coupling wave equation.Compared with the non-cascade DFG process,the optics intensity of THz wave in the 10-order cascading process increases to 5.53 times,corresponding the quantum conversion efficiency is 479.4%,which exceeds the Manly-Rowe limit.?2?Two pairs of double wavelength output characteristics of Adhesive-Free-Bonded periodically polarized KTP and LiNbO₃ crystal optical parametric oscillator are theoretically analyzed.The output characteristics of THz waves via DFG in the same KTP crystal of optical parametric oscillator are calculated and analyzed.Compared with the non-cascade DFG process,the optics intensity of THz wave in 4-order cascading process increases to 5.56times,corresponding the quantum conversion efficiency reaches 259%.In the same way,the output characteristics of THz waves via DFG in the same LiNbO₃ crystal of optical parametric oscillator are calculated and analyzed.Compared with the non-cascade DFG process,the optics intensity of THz wave in 3-order cascading processing increases to 2.57times,the quantum conversion efficiency is 106%.Both of them exceed the Manly-Rowe limit and provide an effective method to generate highly power THz waves.?3?Theoretical analysis of THz waves based on GaP ridge waveguide by cascaded DFG shows the optics intensity of 10-order cascading DFG process is 5.48 times higher than that of the non-cascade DFG process,moreover the quantum conversion efficiency is 177.9%,which breaks the Manly-Rowe limit.?4?The relationship between the nonlinear coefficient and the effective frequency-doubling polarization coefficient when the wave vector propagates in the coordinate plane of the biaxial crystal spindle coordinate system is analyzed by using the nonlinear polarizability theory.The special case of wave vector propagating along the main axis plane of biaxial crystal spindle is investigated from normal mode equation,and the absorption characteristics of the biaxial crystal are explained according to the Fresnel equation.?5?Simultaneous generation of two orthogonally polarized THz waves by optical parametric process with A₁ and E Symmetric mode of LiNbO₃ crystal are researched.The periodic polarization structure is used to provide perturbation for the phase matching conditions of dual orthogonal THz waves mentioned above,and four THz waves are generated via the coupling optical parametric process simultaneously.