Impact of focusing and polarization inhomogeneity on SHG in type-II ppKTP

Abstract

Abstract We study the effects of pump beam focusing and polarization inhomogeneity on second-harmonic generation (SHG) in a 3 cm long type-II ppKTP crystal. Focusing leads to an asymmetry in the SHG output power around the phase-matching temperature (PMT), along with a shift of the PMT to a lower temperature. Theoretical predictions are experimentally verified for different focusing parameters at various pump wavelengths, showing maximum SHG of 0.24 %  W−1 (close to optimal focusing). Further, an inhomogeneous polarization across the pump beam cross-section is theoretically predicted to break up the SHG transverse (intensity) profile into several lobes, resulting in an overall decrease in the total output power. We experimentally show that a radially/azimuthally polarized pump beam (generated via a Q-plate) results in a four-lobed SHG similar to the Hermite–Gaussian HG 11 mode with an optimal focusing parameter that is four times higher than that for the homogeneously polarized pump.