Energy Conversion In Second Order Cascaded Nonlinear Optical Frequency Conversion

Usually third-order nonlinear effect is less than second-order nonlinear effect in nonlinear optical frequency conversion efficiency. Thus, a cascaded second-order nonlinear effect can be used instead of a third-order effect to improve the efficiency of conversion.For example, following the double frequency conversion of the fundamental frequency light wave ωâ†'2ω, the frequency-summed wave ω+2ω=3ω obtained by coupling the second-harmonic (SH) and fundamental waves can be substituted for the third-harmonic generation (THG) process obtained using direct third-order nonlinear effects. The efficiency of this multi-parametric process has made it the subject of cutting-edge research in nonlinear optics in recent years.The physics of the generation of the TH with a cascaded second-order nonlinear optical frequency conversion is that the SH is first generated owing to the fundamental frequency light field (with an associative coupling parameter a) and the frequency-summed wave is then obtained by coupling the SH and fundamental waves (with an associative coupling parameter β).Clearly, the physics is associated with two coupling parameters. In this case, the relationship between the two coupling parameters for maximum energy conversion during TH generation in the crystal is the key scientific issue.Based on the energy conservation condition, we obtained the functions of the coupling parameters ratio under the complete energy conversion. We described the form and features of these functions. These functions can achieve maximum energy conversion during TH generation by numerically calculated.The results of this study will be of great significance to the future design and study of cascade second-order nonlinear frequency conversion devices.This paper is mainly composed of four parts,as follow:1.The developments of a cascaded second-order process.2.This part focus on the coupling wave equations, quasi-phase matching (QPM), the process of using two second-order nonlinear effects to achieve TH, a three-level atomic system, the stimulated Raman adiabatic passage (STIRAP) method, large-signal theory et al. After establishing these basic concepts,we illustrates the physical mechanism of the cascaded second-order nonlinear light field energy conversion effect.3.The physical process of the THG produced by cascaded second-order nonlinear optical effects is investigated in order to obtain the maximum energy conversion in nonlinear optical crystals. Based on the energy conservation condition, the functional form and characteristics of two coupling parametric ratios are obtained under the maximum THG conversion efficiency. A set of functions that can get the maximum energy output of third-harmonic are verified using numerical calculation.4.Based on the STIRAP principle and stability characteristics of the dark state, we studied the energy transfer and nonlinear coupling process of the third harmonic produced by cascaded second-order nonlinear optical effects.We obtained the adiabatic condition in the nonlinear system, analyzed the physical mechanism of energy conversion with high efficiency,and put forward the fundamental condition that the coupling coefficients should satisfy in order to realize the complete energy transfer. The result had great significance in optical device designing that produced third harmonic.