Picosecond Photorefractive and Free-Carrier Nonlinearities in Semiconductors

A variety of picosecond time-resolved two-beam coupling, transient grating and degenerate-four-wave mixing techniques are used to investigate the nonlinear loss and to measure the strength, formation and decay of photorefractive gratings written in GaAs and InP:Fe and of free-carrier gratings written in Si, GaAs, and InP by 43-ps pulses at a wavelength of 1 μm. We present data and numerical calculations as a function of fluence, time delay, pump-to-probe ratio, pump polarization, analyzer angle and crystal orientation. We observe photorefractive gains of a few percent at fluences of a few pJ/μm2 (0.1 mJ/cm2) in GaAs and InP, and we identify two sources for the photorefractive space-charge field. It is principally between mobile free carriers and stationary single-photon ionized donors at low fluences and between mobile electrons and holes produced by two-photon absorption at high fluences. We also observe strong transient energy transfer from the nominally "unshifted" free-carrier index gratings written in GaAs and InP by two-photon absorption and in Si by single-photon indirect absorption. We have demonstrated optical switches based on the pump-induced photorefractive rotation of the probe polarization in GaAs with on/off ratios of >2/1 at fluences as low as 400 fJ/μm2 and optical switches based on free-carrier transient-energy-transfer with on/off ratios >20,000/1 at 200 pJ/μm2. We have also used transient-energy-transfer to construct weak beam amplifiers with gains >25 at 30 mJ/cm2. Finally, these techniques have been used to obtain information about the properties of the deep-level (mid-gap) states in GaAs (EL2/EL2+) and InP (Fe2+/Fe3+), such as the cross sections and number densities.