X-ray and optical pulse interactions in GaAs.

Absorption of hard x-rays in GaAs creates excitations that can dramatically alter the propagation of optical laser pulses with photon energies near the bandgap. Measurements of optical transmission through a thin crystalline wafer of GaAs after absorption of an intense x-ray synchrotron pulse demonstrate how x-ray induced optical transparency depends on the recombination of excited conduction band electrons and valence band holes via Auger, spontaneous emission, and especially stimulated emission processes. The x-ray induced band fluorescence spectrum also reveals amplified spontaneous emission at the high x-ray fluences used, confirming the importance of stimulated emission. For laser pulses with sufficiently high fluence, the interaction of optically excited electrons with x-ray excited electrons can quench the enhanced laser transmission.