SELF-FOCUSING AND SELF-DEFOCUSING BY CASCADED 2ND-ORDER EFFECTS IN KTP

We monitor the induced phase change produced by a cascaded chi(2):chi(2) process in KTP near the phase-matching angle on a picosecond 1.06-mu-m-wavelength beam using the Z-scan technique. This nonlinear refraction is observed to change sign as the crystal is rotated through the phase-match angle in accordance with theory. This theory predicts the maximum small-signal effective nonlinear refractive index of n2eff is-approximately-equal-to +/- 2 X 10(-14) cm2/W (+/- 1 X 10(-11) esu) for an angle detuning of +/- 5-degrees from phase match for this 1-mm-thick crystal with a measured d(eff) of 3.1 pm/V. For a fixed phase mismatch, this n2eff scales linearly with length and as d(eff)2; however, for the maximum n2eff the nonlinear phase distortion becomes sublinear with irradiance for phase shifts near pi/4.