Theoretical Investigation on Collinear Phase Matching Stimulated Polariton Scattering Generating THz Waves with a KTP Crystal

Abstract

We present a theoretical research concerning terahertz (THz) wave generation with $KTiOPO_4$ (KTP) by collinear phase matching (CPM) stimulated polariton scattering (SPS). Both CPM and corresponding nonzero nonlinear coefficients can be simultaneously realized with $s{\rightarrow}f+f$ in yz plane, $s{\rightarrow}f+s$ with ${\theta}$ < ${\Omega}$ in xz plane and $s{\rightarrow}f+f$ with ${\theta}$ < ${\Omega}$ in xz plane. The effective nonlinear coefficients including electronic nonlinearities and ionic nonlinearities are calculated. Based on the parameter values of refractive indices, absorption coefficients and effective nonlinear coefficients, we simulate THz wave intensities generated with CPM SPS by solving coupled wave equations and give the relationship among the maximum THz wave intensity, optimal crystal length and the angle ${\theta}$. The calculation results demonstrate that CPM SPS with KTP can generate THz waves with high intensities and quantum conversion efficiencies.

FIG. 1. Refractive indices and absorption coefficients of a THz wave at 1 THz. (a) and (b) xy plane, (c) and (d) yz plane, (e) and (f) xz plane.

FIG. 2. THz frequencies versus θ with CPM for KTP with a pump wavelength of 1.064 μm. (a) s→f+s with θ < Ω and s→f+f with θ > Ω in the xz plane, (b) s→f+f in the yz plane.

FIG. 3. The effective nonlinear coefficient deff versus THz frequencies by a pump wavelength of 1.064 μm with θ of 5°, 30°, 45°, 60° and 90°.

FIG. 4. THz wave intensities versus crystal length with a pump wavelength of 1.064 μm. Pump intensity Ip = 10 MW/mm², initial Stokes intensity Is = 10 W/mm². (a) s→f+s with θ < Ω in the xz plane, θ = 5°, 10° and 15°, (b) s→f+f with θ > Ω in the xz plane, θ = 21°, 24° and 27°, (c) s→f+f in the yz plane, θ = 4°, 8° and 12°.

FIG. 5. The maximum THz wave intensity and optimal crystal length versus the angle θ with a pump wavelength of 1.064 μm. Pump intensity I_p = 10 MW/mm², initial Stokes intensity I_s = 10 W/mm².

FIG. 6. THz wave intensities versus crystal length for s→f+f in the xz plane with a pump wavelength of 1.064 μm and θ of 5°. Pump intensity I_p = 10, 20, 30, 40 and 50 MW/mm², initial Stokes intensities I_s = 10 W/mm².

FIG. 7. The maximum THz wave intensity and quantum conversion efficiency versus pump intensity I_p for s→f+s in the xz plane with a pump wavelength of 1.064 μm and θ of 5°. Initial Stokes intensity I_s = 10 W/mm².

TABLE 1. CPM conditions and the corresponding nonlinear coefficients for KTP with pump wavelength of 1.064 μm. Y indicates CPM can satisfy, and N indicates CPM cannot satisfy