Marvin B. Klein

Optical limiting with C60 in polymethyl methacrylate

We demonstrate optical limiting for the C(60) fullerene in polymethyl methacrylate (PMMA) as a solid polymer host. It is shown that the optical-limiting behavior is consistent with excited-state absorption (reverse saturable absorption) as a mechanism. We suggest that a higher threshold for optical limiting compared with that of C(60) in toluene is due to nonlinear scattering for the liquid. The performance of C(60) in PMMA is compared with that in chloroaluminum phthalocyanine, N-methylthioacridone, Kings complex, and ruthenium Kings complex in PMMA. Optical damage thresholds are reported.

Beam coupling in undoped GaAs at 1.06 μm using the photorefractive effect

We have observed beam coupling and degenerate four-wave mixing in high-resistivity, undoped GaAs at 1.06 microm that is due to the photorefractive effect. The photorefractive species is thought to be the deep donor EL2. The measured values of two-wave gain are comparable with those measured in Bi(12)SiO(20). The response time is measured to be 20 microsec at an intensity of 4 W/cm(2). This exceptionally fast photorefractive response time (compared with that of oxide electro-optic materials) is due primarily to the large mobility of GaAs.

Beam coupling in BaTiO3 at 442 nm

Steady‐state beam coupling is used to measure the concentration of empty traps, the sign of the dominant photocarrier, and the effective electro‐optic coefficient in seven crystals of BaTiO3. The measured value of the effective electro‐optic coefficient gives the product of the fractional poling of the crystal, the relative conductivity of the crystal, and the electro‐optic coefficient. Anomalously low beam coupling gain measured in an eighth crystal suggests that this crystal is nearly perfectly compensated; i.e., the photoconductivities due to electrons and holes are nearly equal.

Photorefractivity at 1.5 μm in CdTe:V

We have for the first time demonstrated two‐beam coupling energy transfer at a wavelength of 1.5 μm. Beam coupling gain coefficients of 0.6 cm−1 have been obtained in vanadium ‐doped CdTe with only 5 mW/cm2 incident intensity. These gain coefficients exceed typical gain coefficients in GaAs at 1.06 μm wavelength by 50%. In preliminary measurements using the moving grating technique, we have measured a gain coefficient of 2.4 cm−1. Through adjustment of the doping level, CdTe:V can be used as a sensitive photorefractive material through the 0.9–1.5 μm spectral range.

SPECTROSCOPIC AND PHOTOREFRACTIVE PROPERTIES OF INFRARED-SENSITIVE RHODIUM-DOPED BARIUM TITANATE

We have grown and characterized crystals of BaTiO(3) doped with Rh. These crystals have a blue-green color that is probably due to charge-transfer optical transitions involving Rh(4+) and/or Rh(3+). Electron paramagnetic resonance spectroscopy confirmed the presence of Rh(4+) centers. Self-pumped phase-conjugation measurements were performed at wavelengths between 776 and 990 nm. Reflectivities near 80% were obtained throughout this wavelength range. In addition, self-pumped phase-conjugation buildup times were measured as a function of input power at 851 nm and are significantly faster than in undoped or Co-doped crystals studied previously. BaTiO(3):Rh is therefore a promising material for near-infrared photorefractive applications.

Degenerate four‐wave mixing near the band gap of semiconductors

We present a theoretical calculation and experimental data on the effective third‐order susceptibilities χ(3) for degenerate four‐wave mixing near the band gap of semiconductors. The closeness of the calculated and experimental values for the effective χ(3) in silicon at 1.06 μm indicates the utility of our simple calculation. The large values of χ(3) indicate the possibility of high‐efficiency degenerate four‐wave mixing in semiconductors, especially at longer wavelengths.

Photorefractive effect in BaTiO 3 : microscopic origins

We have used a number of experimental techniques to identify the photorefractive species in commercial samples of BaTiO3. We find that Fe impurities (in the Fe2+ and Fe3+ states) are the predominant photorefractive species. Techniques for optimizing the photorefractive properties of BaTiO3 are discussed.

DEPTH-RESOLVED HOLOGRAPHIC IMAGING THROUGH SCATTERING MEDIA BY PHOTOREFRACTION

A depth-resolved near-infrared imaging system has been demonstrated for recording three-dimensional images of objects embedded in diffuse media. Time-gated holographic imaging employing rhodium-doped barium titanate as the recording medium is used to acquire whole depth-resolved two-dimensional images in 1 s. Millimeter depth resolution has been achieved with a transverse resolution of ~ 30 microm.

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.

Laser-based ultrasound detection using photorefractive quantum wells

We demonstrate a laser-based adaptive ultrasonic homodyne receiver using dynamic holography in AlGaAs/GaAs photorefractive multiple quantum wells. The dynamic hologram acts as an adaptive beamsplitter that compensates wavefront distortions in the presence of speckle and requires no path-length stabilization. The photorefractive quantum wells have the unique ability to achieve maximum linear homodyne detection regardless of the value of the photorefractive phase shift by tuning the excitonic spectral phase. We achieve a root mean square noise-equivalent surface displacement of 6.7×10−7 A(W/Hz)1/2.