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

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 nonlinearit y of this material. As a result, a pronounced self-phase modulation of a Ga ussian laser beam at 532 nm is observed in the far field as a diffraction p attern consisting of alternating dark and bright rings. The induced nonpara bolic and polarization sensitive refractive index change is also used to mo dulate a weak probe laser beam at 633 nm. A simple theoretical model incorp orating a laser heating process and a thermo-optical effect accounts for th e main features observed in our experiments and elucidates the mechanisms l eading to the diffraction pattern formation. (C) 2001 American Institute of Physics.