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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-microm-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(eff)(2) congruent with +/-2 x 10(-14) cm(2)/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 n(eff)(2) scales linearly with length and as d(eff)(2) however, for the maximum n(eff)(2) the nonlinear phase distortion becomes sublinear with irradiance for phase shifts near pi/4.

We present measurements of the two-photon absorption coefficients /2 of 10 different semiconductors having band-gap energies between 1.4 and 3.7 eV. We find that 12 varies as Eg-3 , as predicted by theory. In addition, the absolute values of 02 agree with theory, which includes the effect of nonparabolic bands, the average difference being less than 26%. This agreement permits confident predictions of two-photon absorption coefficients of other materials at other wavelengths.

CONTENTS 1. Introduction 2. Theory 3. Experiment and data 3.1. Experiment 3.2. Data 4. Comparison of ß2 values to theory 4.1. Two-photon absorption theory 4.2. Comparison to theory 5. Self-refraction 6. Optical limiter 7. Conclusion 8. Acknowledgments 9. References Abstract. Two-photon absorption coefficients ß2 of ten direct gap semiconductors with band-gap energy Eg varying between 1.4 and 3.7 eV were measured using 1.06 pm and 0.53 pm picosecond pulses. ß2 was found to scale as E93, as predicted by theory for the samples measured. Extension of the empirical relationship between ß2 and Eg to InSb with Eg = 0.2 eV also provides agreement between previously measured values and the predicted ß2. In addition, the absolute values of ß2 are in excellent agreement (the average difference being <26 %) with recent theory, which includes the effects of nonparabolic bands. The nonlinear refraction induced in these materials was monitored and found to agree well with the assumption that the self-refraction originates from the two-photon-generated free carriers. The observed self-defocusing yields an effective nonlinear index as much as two orders of magnitude larger than CS2 for comparable irradiances. This self-defocusing, in conjunction with twophoton absorption, was used to construct a simple, effective optical limiter that has high transmission at low input irradiance and low transmission at high input irradiance. The device is the optical analog of a Zener diode.

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