Free Carrier Gratings Research Articles

Picosecond photorefractive and free-carrier transient energy transfer in GaAs at 1 mu m

The strength, formation, and decay of photorefractive and free-carrier gratings written in GaAs by 43-ps pulses at a wavelength of 1 mu m are investigated using picosecond-time-resolved two-beam coupling, transient grating, and degenerate-four-wave-mixing techniques. Photorefractive weak-beam gains of a few percent are measured at fluences of a few pJ/ mu m/sup 2/ (0.1 mJ/cm/sup 2/), and gain from transient energy transfer is observed at fluences larger than approximately 10 mJ/cm/sup 2/ in the beam-coupling experiments. The roles of saturation and two-photon absorption in determining the final electron, hole, and ionized-donor populations and the roles of drift and diffusion in determining the quasi-steady-state photorefractive and free-carrier index modulations are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Picosecond photorefractive effect in BaTiO_3

We report the observation of photorefractive index gratings written and read out in BaTiO(3) using single pulses of 30-psec duration and fluences of 1 to 15 mJ/cm(2) at a wavelength of 0.532 microm. While the photorefractive gratings are masked during formation by large free-carrier grating, they are clearly seen from 50 psec to many minutes following the peak in the writing beams. The rapid and complete formation of the photorefractive grating and the complete decay of the free-carrier grating on a scale of 50 to 100 psec indicate either that the carrier-recombination time in our sample of BaTiO(3) is less than 100 psec or that the mobility of the carriers is >23 cm(2)/V sec.

Picosecond pulse amplification by coherent wave mixing in silicon.

Pulses from Nd:YAG lasers (where YAG represents yttrium aluminum garnet) were amplified up to 20 times by noncollinear mixing with a coherent pump wave in silicon. Amplification of long pulses (about 16 ns) was obtained with intersection angles up to 2\ifmmode^\circ\else\textdegree\fi{} whereas angles larger than 10\ifmmode^\circ\else\textdegree\fi{} are possible with short pulses (50 ps). The effect is theoretically described by self-diffraction at a laser-induced dynamic free-carrier grating.

Mott-transition-induced period doubling in transient-grating experiments in CdS

The occurrence of higher-order Fourier components in an originally sinusoidal free-carrier index grating produced in CdS by interference of two picosecond light pulses is demonstrated by simultaneously monitoring the decay of the first- and second-order diffraction intensity of a transparent probe pulse. A pronounced contribution with doubled period can be attributed to an exciton phase grating formed by binding of electrons and holes of the free-carrier grating.

Investigation of non-equilibrium processes in semiconductors by the method of transient holograms

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Diffraction of light by laser generated free carriers in Si: Dispersion or absorption?

We found that the diffraction of light by laser generated free carriers in Si is mainly due to free carrier dispersion and not to free carrier absorption. This was established by studying the self-image of a free carrier grating.