Optical limiting with semiconductors

We present a detailed characterization of the output of passive semiconductor-based optical limiters. These devices utilize two-photon absorption along with photogenerated carrier defocusing within the material to limit the output fluence and irradiance. In addition to protecting downstream optical components, the focusing geometry combined with these nonlinearities makes the devices self-protecting. Such devices have a broad working wavelength range since both the initial two-photon absorption and the subsequent carrier refraction are slowly varying funtions of wavelength. For example, ZnSe should have a useful range of from 0.5 to 0.85 μm. In this material we have observed the onset of limiting at input powers as low as 80 W when using 10-nsec, 0.53 μm input pulses. At the same wavelength, when 30 psec pulses into a monolithic ZnSe limiter are used, limiting begins at ≃300 W or 10 nJ. We also monitored the output spatial energy distribution along with the temporal response at each position, using a 2-psec-resolution streak camera. We found that the output fluence along with the output irradiance is effectively limited below detector damage thresholds over an input range of 4 orders of magnitude. Additionally, since both two-photon absorption and the associated self-defocusing increase with decreasing band-gap energy, similar devices using narrow-gap semiconductors should have considerably lower limiting thresholds.

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