Thomas F. Boggess

Formation, decay, and erasure of photorefractive gratings written in barium titanate by picosecond pulses

Photorefractive gratings are written, read out, and erased with single 30-psec pulses at fluences of 1–15 mJ/cm2 and a wavelength of 0.532 μm in a BaTiO3 crystal. The gratings consist of one component that is fully formed 50 psec after the peak in the writing beams and a second component that grows for approximately 30 sec until it starts to decay by dark erasure. The photorefractive index change per absorbed photon for picosecond pulses is approximately the same as that obtained when a cw beam at 0.515 μm is used with the same crystal. The index change is approximately a factor of 30 smaller than that observed at the same fluence in GaAs at 1.06 μm when the same picosecond laser system is used.

Picosecond pump-probe technique to measure deep-level, free-carrier, and two photon cross sections in GaAs

Optical transmission of a weak probe pulse in the presence of a strong cross‐polarized pump pulse is measured as a function of pump‐probe delay. We use pulses of duration 35 ps at a wavelength of 1.06 μm and two samples of undoped, semi‐insulating GaAs cut from two dissimilar boules. Numerical solutions to the rate equations coupled with the nonlinear wave equation for propagation in GaAs are fit to the experimental results in order to determine rate coefficients. We obtain a two‐photon absorption coefficient of 27±5 cm/GW, a free‐carrier cross section (5.5±2)×10−18 cm2, and the ratio of the hole cross section to the electron cross section for the EL2 defect 0.76±0.1.

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.

High-accuracy, high-reflectivity phase conjugation at 1.06 μm by four-wave mixing in photorefractive gallium arsenide

We have used a 20-kHz ac square-wave electric field to enhance the beam-coupling gain and the degenerate four-wave mixing reflectivity of photorefractive GaAs at 1.06 μm. The largest measured four-wave mixing reflectivity in the steady state was 15% at a grating period of 9 μm and an applied voltage of 3.2 kV across 0.4 cm. Theoretical results predict substantially higher reflectivity values for a field of 8 kV/cm; the origin of this discrepancy is thought to be space-charge effects that prevent us from obtaining the full field inside the crystal. We have also measured a transient reflectivity of 510% during the switch on of the backward pump beam. Finally, we have determined the accuracy of wave-front reversal through measurements of piston-error correction and conjugation fidelity.

Two-photon absorption and anisotropic transient energy transfer in BaTiO 3 with 1-psec excitation

We use a cross-polarized pump–probe technique and 1-psec pulses at 596 nm to measure a two-photon absorption coefficient of 0.1 cm/GW in BaTiO3. A long-lived absorption is also observed and is shown to be consistent with free-carrier absorption and charge redistribution between two separate impurity levels. We also observe an anisotropic transient energy-transfer process in which the p-polarized pump is scattered into the s-polarized probe. The energy transfer occurs when the c axis of the crystal is oriented along a grating vector equal to the difference between the pump- and probe-wave vectors.

Cavity-length detuning effects and stabilization of a synchronously pumped femtosecond linear dye laser

We report on the cavity-length detuning characteristics of a synchronously pumped femtosecond linear-resonator cw dye laser. The severe cavity-matching requirement (to within 0.2 μm) for optimum short-pulse generation is addressed by an active cavity-length stabilization scheme based on second-harmonic generation. This allows pulses ~100 fsec in duration to be maintained for periods of several hours.

Polarization-rotation switch using picosecond pulses in GaAs

Photorefractive and free-carrier nonlinearities in GaAs transfer energy from a strong picosecond pump, which is linearly polarized at an arbitrary angle to a weak s-polarized probe, into a p-polarized beam propagating in the probe direction. By placing the GaAs between a crossed pair of high-quality polarizers that are set to extinguish the probe in the absence of the pump, an efficient, high-contrast, high-speed optical switch is realized. By varying the pumpprobe ratio, delay, fluence, and polarization, we can maximize the intensity of the p-polarized component. This intensity with the pump present divided by that without the pump, the on-off ratio, is approximately 2 at fluences as low as 0.03 mJ/cm(2) and approaches 10,000 at fluences of 15 mJ/cm(2) and pump polarizations of 45 degrees .

Piezoreflectance as a supplement to photoreflectance for nondestructive characterization of GaAs/AlxGa1−xAs multiple quantum wells

Piezoreflectance is used to identify allowed and forbidden transitions in the spectra of as‐grown 100 and 50 A GaAs/AlxGa1−xAs multiple quantum well samples from which no photoreflectance spectra suitable for characterization could be obtained. Features in the piezoreflectance spectra are shown to result from excitonic transition by direct comparison with the absorbance measurements made on semitransparent regions of these samples. Transition energies are extracted from the piezoreflectance spectra using a form of the Aspnes equation appropriate for the first derivative of a Lorentzian.

Stimulated emission in indirect gap AlxGa1−xAs

Stimulated emissions from both direct and indirect valleys are observed simultaneously in the picosecond time‐resolved photoluminescence spectra of the indirect gap semiconductor AlxGa1−xAs, for samples with Al concentrations near (but slightly above) the direct‐to‐indirect band‐gap crossover value. Indirect stimulated emission is possible in this material because the long‐wave‐vector scattering is enhanced by the near degeneracy of the direct and indirect conduction‐band valleys and by alloy scattering. Two distinct bands are observed in the luminescence spectra. One of these we associate with the stimulated indirect recombination of electrons from states in the X valley that are below the Γ band gap and are, therefore, not resonantly coupled to states in the direct valley. The other we associate with the recombination of electrons from the central Γ valley or of electrons from the indirect X valley that are energetically resonant with states in the direct valley. The dramatically different temporal dyna...

Beam Amplification By Transient Energy Transfer In GaAs And Si

Two beam coupling experiments with 29-ps (FWHM) pulses at a wavelength of 1.06 μm have been performed in GaAs and Si. By optimization of pump-probe energy ratio, time delay and fluence we obtained optical gain up to 25 in GaAs at a fluence of 25 mJ/cm compared to roughly 100 mJ/cm2 required to obtain similar gain in Si. Optical amplifiers with high gain for cw radiation have been demonstrated over the past ten years using the photorefractive effect in BaTiO3, KNb03, Bi12Si020 and GaAs. Amplification in these materials, which occurs even when the probe (or signal) and pump are of equal intensity, is produced by the shifted grating found in photorefractive materials when diffusion is the dominant transport mechanism or obtained by a frequency offset between the beams when drift is the dominant transport.