Many materials with large second-harmonic (SH) coefficients cannot practially be used in frequency-doubling devices, because the large difference between the refractive indices at fundamental and SH wavelengths cannot be compensated by the natural birefringence of the material. It was first shown1 that this natural obstacle can be circumvented by using nonlinear structures with linear and/or nonlinear properties periodically modulated—so-called quasi-phase-matching (QPM). Since then, numerous experiments have been performed2,3 with periodically modulated structures. Typically, to achieve high SH generation efficiency (SHGE), the sign of the SH coefficient has to be periodically modulated, typically by domain inversion. Unfortunately, domain inversion is difficult to achieve in many materials, such as II–VIs. In this presentation, we show theoretically, and offer experimental proof, that high SHGE can be achieved by periodic modulation of the index mismatch (PMIM).
© 1991 Optical Society of AmericaPDF Article