Strong self- and cross-phase modulation effects in chromium-doped KTiOPO4 crystals

Abstract

The presence of chromium impurities in the crystal matrix of chromium-doped KTiOPO4 is shown to be at the origin of a strong cubic optical nonlinearity of this material. As a result, a pronounced self-phase modulation of a Gaussian laser beam at 532 nm is observed in the far field as a diffraction pattern consisting of alternating dark and bright rings. The induced nonparabolic and polarization sensitive refractive index change is also used to modulate a weak probe laser beam at 633 nm. A simple theoretical model incorporating a laser heating process and a thermo-optical effect accounts for the main features observed in our experiments and elucidates the mechanisms leading to the diffraction pattern formation.