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.

The next generation of infrared (IR) sensor systems will include active imaging capabilities. One example of such a system is a gated active/passive system. The gated active/passive system promises target detection and identification at longer ranges compared to conventional passive-only imaging systems. A detector that is capable of both active and passive modes of operation opens up the possibility of a self-aligned system that uses a single focal plane. The mid-wave infrared (MWIR) HgCdTe electron injection avalanche photodiode (e-APD) provides state-of-the-art 3 lm to 5 lm performance for the passive mode and high, low-noise, gain in the active mode, and high quantum efficiency at 1.5 lm. Gains of greater than 1000 have been measured in MWIR e-APDs with a gain-independent excess noise factor of 1.3. This paper reports the application of the mid-wave HgCdTe e-APD for near-IR gated-active/ passive imaging. Specifically a 128 · 128 focal-plane array (FPA) composed of 40-lm-pitch MWIR cutoff APD detectors and custom readout integrated circuit was designed, fabricated, and tested. Median gains as high as 946 at 11 V bias with noise equivalent photon inputs as low as 0.4 photon were measured at 80 K and 1 ls gate times. This subphoton sensitivity is consistent with the high gains, low excess noise factor, and low effective gain normalized darkcurrent densities, near or below 1 nA/cm 2 , that were achieved in these FPAs. A gated imaging demonstration system was designed and built using commercially available parts. High resolution and precision gating was demonstrated in this system by imagery taken at ranges out to 9 km.