Two-dimensional steady-state photorefractive screening solitons.

We study experimentally steady-state photorefractive screening solitons trapped in both transverse dimensionsand measure their beam profiles as they propagate throughout the crystal. The solitons are observed to be axially symmetric, and they self-bend. We characterize the soliton dependence on the optical intensity, appliedelectric-field strength, and beam diameter.

Steady-state dark photorefractive screening solitons.

We present an experimental study of steady-state dark photorefractive screening solitons trapped in a bulk strontium barium niobate crystal. We compare experimental measurements with theoretical calculations of the soliton properties and find good agreement between theory and experiments. We confirm the shapepreserving behavior of the dark soliton by measuring its beam profile as it propagates throughout a specially cut crystal and by guiding a beam of a different wavelength.

Y junctions arising from dark-soliton propagation in photovoltaic media.

We report the observation of planar Y-junction waveguide splitters that are due to the bulk photovoltaic effect. The junctions are generated by multiple dark-soliton propagation in LiNbO(3) by use of low power levels (20 mW) and average intensities of 10 W/cm(2) at 488 nm. The junctions persist in the dark and can be used to divide input beams of less photorefractive sensitivity. We describe some possibilities for generating other waveguide structures in bulk and thin-film media.

Conjugation fidelity and reflectivity in photorefractive double phase-conjugate mirrors.

We present an experimental study of the behavior of photorefractive double phase-conjugate mirrors that illustrates recent theoretical predictions. We observe a sharp fidelity threshold that significantly depends on the specific feature size in the input beams. Furthermore, we find that if the two input beams have unequal intensities the conjugation process is asymmetric and the steady-state fidelity is better on the side of the crystal on which the more-intense beam enters.

Temporal evolution of fanning in photorefractive materials.

We present detailed calculations of the temporal and spatial evolution of beam fanning in photorefractive crystals that is initiated by scattering from noise. We show that fanning starts from beam coupling between the incident radiation and part of the incident radiation scattered by noise at or near the input plane. We show that scattering within the volume of the crystal has negligible effect on fanning, that absorption affects the time response but not the spatial pattern of the fanning, and that the difference between calculations including only phase-matched terms and those including non-phase-matched terms is negligible.

Temporal evolution of photorefractive double phase-conjugate mirrors.

We present wave-optics calculations of the temporal and spatial evolution from random noise of a double phase-conjugate mirror in photorefractive media that show its image exchange and phase-reversal properties. The calculations show that for values of coupling coefficient times length greater than two the process exhibits excellent conjugation fidelity, behaves as an oscillator, and continues to operate even when the noise required for starting it is set to zero. For values less than two, the double phase-conjugation process exhibits poor fidelity and disappears when the noise is set to zero.

Bistable operation of a phase-conjugate resonator containing an intracavity saturable absorber.

We have demonstrated bistability in a phase-conjugate resonator containing an intracavity saturable absorber. The saturable absorber was a film of fluorescein-doped boric acid glass. The state of the resonator is determined by an external control beam incident upon the saturable absorber. When the resonator is switched to the on state by means of the control beam, it remains in that state even with the control beam turned off.

Picosecond separation and measurement of coexisting photorefractive, bound-electronic, and free-carrier grating dynamics in GaAs.

A novel transient-grating geometry, which is nondegenerate, copropagating, phase matched, and polarization sensitive, is used to isolate and measure independently the ultrafast dynamics of multiple coexisting gratings in GaAs:EL2 with a temporal resolution of <5 psec. This technique permits the measurement of the evolution of the photorefractive grating in materials with zinc blende symmetry, where the photorefractive grating is usually obscured by the stronger free-carrier and instantaneous bound-electronic gratings.

Photorefractive nonlinearities caused by the Dember space-charge field in undoped CdTe.

The photorefractive nonlinearity associated with the Dember space-charge field between electrons and holes produced by two-photon absorption is unambiguously isolated and studied in undoped CdTe by using a nondegenerate, forward-probing, polarization-sensitive, transient-grating technique with a temporal resolution of <5 ps.

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 .

Picosecond photorefractive response of GaAs:EL2, InP:Fe, and CdTe:V.

Measurements and theoretical calculations are presented for the photorefractive effect in three semi-insulating semiconductors (GaAs:EL2, InP:Fe, and CdTe:V) using 29-psec pulses at a wavelength of 1.06 microm. The photorefractive gain is largest in the CdTe crystal and smallest in our InP sample. The major differences between the materials responsible for this are the electro-optic coefficients, the mobilities, the absorption coefficients, and the amount of electron-hole competition.

Hologram fixing in Bi(12)TiO(20) using heating and an ac electric field.

We demonstrate hologram fixing in Bi(12)TiO(20) using a technique involving thermal cycling and an external ac field.

Coupling pulsed dye oscillators using a phase conjugate resonator.

Four pulsed dye oscillators are coherently combined into a single phase conjugate resonator through the selfpumped conjugation process in a single crystal of BaTiO(3). We have measured the locking range and energy scaling behavior of the system. The locking range is of the order of hundreds of wavelengths. Our best results are close to the theoretical limit and show that one can obtain output energy approximately proportional to N, the number of dye gain cells. Since the N output beams are locked in phase, this implies that the peak far-field intensity would scale as N(2). We also report two properties of the photorefractive phase conjugate mirror that are relevant to such a system: the fluence required for start-up (9 J/cm(2)) and the field of view (0.0 sr).

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.

Research on nonlinear optical materials: an assessment.

The seven papers making up this assessment are based on the Workshop on Nonlinear Optical Materials held in April 1986.

Imaging threshold detector using a phase-conjugate resonator in BaTiO(3).

We have demonstrated an imaging threshold detector using a phase-conjugate resonator to provide high spatial resolution and signal regeneration near threshold. The device is based on spatially resolved grating erasure in photorefractive BaTiO(3) and has the capability of high resolution and low cross talk.

Picosecond photorefractive beam coupling in GaAs.

We report the first observation to our knowledge of the photorefractive effect on picosecond time scales. Photorefractive beam coupling in GaAs with picosecond, 1.06-microm pulses is observed owing to charge separation between electrons and the ionized defect EL2(+) at low fluences and to separation between free electrons and holes created by two-photon interband absorption for high fluences. The accompanying processes of linear absorption, two-photon absorption, and transient energy transfer are also observed.