Enhancements of Raman oscillation and amplification and frequency upconversion by phonon-polariton resonances

Phonon-polariton resonances are manifestation of the strong interaction between an electromagnetic wave interacts and transverse-optical phonons in a crystalline solid. Such resonances result in great enhancements of ionic second-order nonlinearities [1]. For LiNbO3, dominant element of the ionic second-order nonlinear susceptibility tensor coefficients d33 at 7.5 THz is a factor of about 50 larger than its electronic nonlinearity. Semiconductor materials such as GaAs, GaP, and GaN all exhibit these huge enhancements of the ionic second-order nonlinear coefficients by their polariton resonances. Obviously, one can exploit these huge nonlinear coefficients for greatly enhancing efficiencies of nonlinear parametric processes. The drawback, though, is extremely large absorption being companied with the huge nonlinearities within these strong resonances. Although sum-frequency generation [2], second-harmonic generation [3,4], and generation of THz [5,6], far-IR [7] and mid-IR [8] were investigated near the polariton resonances, the extremely low conversion efficiencies due to extremely large absorption and phase mismatch [2,7] made these nonlinear effects have no practical value.