We monitor the induced phase change produced by a cascaded χ⁽²⁾:χ⁽²⁾ process in KTP near the phase-matching angle on a picosecond 1.06-µm-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 n^eff₂ ≅ ±2 × 10^-14 cm²/W (±1 × 10^-11 esu) for an angle detuning of ±5° 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 n^eff₂ scales linearly with length and as d_eff² however, for the maximum n^eff₂ the nonlinear phase distortion becomes sublinear with irradiance for phase shifts near π/4.

© 1992 Optical Society of America

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