Tunable phase-mismatched mid-infrared difference-frequency generation between 6 and 17?m in CdTe

In parametric conversion, phase-matching techniques such as birefringence and quasi phase-matching (PM) with the designed crystal angle or periodically poled polarities are employed to fulfill the requirement of momentum conserva-tion. However, directly using phase-mismatched interactions in nonlinear media with large quadratic nonlinear coef-ficient remains unheeded. Here, for the first time to the best of our knowledge, we study the phase-mismatched difference-frequency generation (DFG) in an isotropic cad-mium telluride (CdTe) crystal, with the comparison of other DFG processes based on birefringence-PM, quasi -PM, and random-quasi-PM. Long-wavelength mid-infrared (LWMIR) phase-mismatched DFG with an ultra-broadband spectral tuning range of 6-17 mu m based on CdTe is demon-strated. Thanks to the giant quadratic nonlinear coefficient (similar to 109 pm/V) and good figure of merit in the paramet-ric process, the output power up to 100 mu W is obtained, which is comparable to or even better than the DFG out-put from a polycrystalline ZnSe with the same thickness facilitated by random-quasi-PM. A proof-of-concept demon-stration in gas sensing of CH4 and SF6 is conducted based on the phase-mismatched DFG as a typical application. Our results demonstrate the feasibility of phase-mismatched parametric conversion in producing useful LWMIR power and ultra-broadband tunability in a simple and convenient way without the necessity of controlling the polarization, phase-matching angle, or pole periods, which could find applications in the fields of spectroscopy and metrology.(c) 2023 Optica Publishing Group