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Photorefractive Effects, Materials, and Devices

Kinetics of induced nanosecond absorption in photorefractive InP:Fe

P. U. Halter, Jean Claude Fabre, Gérald Roosen

Published Jan 1, 1990• 0 citations

Abstract

For understanding and optimization of the photo refractive gain it is important to know the basic parameters of photo-ionization and recombination processes. In the nanosecond regime the pulse energies required for maximum coupling gain in semiconductors easily reach the limit for saturation of the deep centers. especially if one of the recombination times is larger than the pulse duration. In that case, induced absorption (or bleaching) will appear due to a redistribution of the population of the different levels of the deep center and counterbalance photorefractive amplification. The present simultaneous study of the kinetics of the induced absorption and the photocurrent in an InP:Fe sample reveals a number of interesting parameters like electron and hole recombination times, ionization cross sections and Fe2+/Fe3+ concentrations.

DOI: 10.1364/pmed.1990.a11

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Induced nanosecond absorption in InP:Fe

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Cafer Özkul, S. Jamet, Philippe Gravey +2 more

Iron-ion concentrations have been determined from measurements of the photoinduced-grating erasure times. We checked the accuracy of this method by performing two-wave mixing experiments at λ = 1.32 μm without an applied field. After verifying the fit of the extended single-defect model with indirect transitions and the two-wave mixing gain data at λ = 1.06 μm, we conclude that the indirect transitions involving the excited state of Fe2+ can induce a reduction of the photorefractive effect in InP:Fe when the dominant photocarriers are holes at room temperature.

Journal of the Optical Society of America B•Sep 1, 1994•4 citations

For understanding and optimization of the photo refractive gain it is important to know the basic parameters of photo-ionization and recombination processes. In the nanosecond regime the pulse energies required for maximum coupling gain in semiconductors easily reach the limit for saturation of the deep centers. especially if one of the recombination times is larger than the pulse duration. In that case, induced absorption (or bleaching) will appear due to a redistribution of the population of the different levels of the deep center and counterbalance photorefractive amplification. The present simultaneous study of the kinetics of the induced absorption and the photocurrent in an InP:Fe sample reveals a number of interesting parameters like electron and hole recombination times, ionization cross sections and Fe2+/Fe3+ concentrations.

Induced absorption was observed in an InP:Fe crystal under nanosecond pulse illumination at 1.06- mu m wavelength. The maximum induced absorption was 0.05 cm/sup -1/ when probed at a 1.06- mu m wavelength and 0.30 cm/sup -1/ when probed at a 1.30- mu m wavelength. The kinetics are studied together with the photocurrent giving the recombination times of holes (100 ns) at room temperature and an estimate for that of electrons (0.5 ns). The induced absorption is attributed to a population redistribution of the different iron impurity states. Computer simulations are able to explain all the features found in the experiments and give values for the photoionization cross sections, in particular for the unknown cross section Fe/sup 2+*/ to Fe/sup 3+/+e/sup -/.>

We show that even if there is no direct hole–electron competition in InP:Fe the gain is lower than expected. We attribute this reduction of the gain to a thermally induced hole–electron competition that involves the excited state of Fe2+ (5T2). We present a new model for the photorefractive effect in InP:Fe, which we use to interpret our beam-coupling results. Some material parameters of the InP:Fe sample and particularly of the excited state are deduced. The same energy-transfer measurements conducted in GaAs give us the ratio of hole and electron photoionization cross sections.

We have observed two-beam coupling gain (0.273 cm−1) that exceeds absorption (0.145 cm−1) in InP:Fe without applying an external field at λ=1.32 μm. Measurements of the gain coefficient as a function of grating spacing fit the photorefractive theory for simultaneous electron-hole transport, with the effective density of traps NE = (3.86 ± 1.1)NE=(3.86±1.1)× 1015 cm−3 and ξ, the factor that accounts for competition between electrons and holes, equaling −0.729±0.026. Absorption measurements, combined with the photoionization cross sections of electrons and holes, confirm these results. The agreement between theory and experiment suggests how to choose impurity concentrations that optimize net gain.

Iron-ion concentrations have been determined from measurements of the photoinduced-grating erasure times. We checked the accuracy of this method by performing two-wave mixing experiments at λ = 1.32 μm without an applied field. After verifying the fit of the extended single-defect model with indirect transitions and the two-wave mixing gain data at λ = 1.06 μm, we conclude that the indirect transitions involving the excited state of Fe2+ can induce a reduction of the photorefractive effect in InP:Fe when the dominant photocarriers are holes at room temperature.