S. W. McCahon

Photorefractive measurement of photoionization and recombination cross sections in InP:Fe

The photorefractive effect combined with standard Hall measurements is used to determine photoionization and recombination cross sections and deep‐level number densities in a crystal of iron‐doped InP. The results are well fit by iron densities [Fe2+]=6×1016 cm−3 and [Fe3+]=2.5×1015 cm−3, photoionization cross sections at 1.06 μm, se= 4×10−18 cm2, and sh= 3.1×10−16 cm2, and recombination cross sections σe= 10−14 cm2 and σh= 3×10−16 cm2. The sign of the photorefractive beam coupling gain implies that holes dominate this process even though the dark conductivity and photoconductivity are dominated by electrons.

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.

Measurement of electro‐optic and electrogyratory effects in Bi12TiO20

We have conducted measurements of the linear electro‐optic coefficient (r41) and electrogyratory coefficient (η41) in Bi12TiO20 at 633 nm. A precise knowledge of these coefficients is important for evaluating the holographic recording properties of this photorefractive material. We have employed both transverse and longitudinal electric field geometries. For each geometry, we present the results of dc as well as ac techniques. Based on these results we find r41=5.75±0.10 pm/V and ‖η41‖≤0.30±0.05 pm/V. We critically examine our measurements and discuss their relative accuracy. Our findings are compared to those previously reported by other investigators.

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.

Generation of 3-4- mu m femtosecond pulses from a synchronously pumped, critically phase-matched KTiOPO4 optical parametric oscillator.

The operation of a Ti:sapphire-pumped, femtosecond optical parametric oscillator (OPO) based on the nonlinear material KTiOPO4 is described. By empirically optimizing the parametric interaction geometry, we have extended the maximum idler wavelength beyond that reported for similar systems. The OPO typically produces 175-fs idler and signal pulses tunable in the ranges of 2.9–3.96 and 1.05–1.16 μm, respectively.

Materials inspection and process control using compensated laser ultrasound evaluation (CLUE): demonstration of a low-cost laser ultrasonic sensor

We demonstrate the use of a nonsteady-state photo-induced-emf adaptive photodetector as a robust, low-cost laser ultrasonic sensor. This class of sensor enables high-fractional bandwidth ultrasound detection and, in addition, all-optical compensation of adverse in-factory noise, including vibration, speckle, relative platform motion, and optical fiber modal dispersion. Reference-beam and fiber-based time-delay interferometric configurations were demonstrated, as well as the use of a diode laser as a compact optical probe.

Picosecond investigations of optical limiting mechanisms in King's complex

We have investigated the nonlinear optical mechanisms responsible for optical limiting of both picosecond and nanosecond 532-nm optical pulses in the organometallic compound cyclopentadienyliron carbonyl tetramer (Kings complex). For fluences below ~200 mJ/cm 2 , picosecond pump-probe measurements in solutions of the Kings complex reveal a prompt reverse saturable absorption (RSA) that recovers with a time constant of 120 ps. We attribute this RSA to excited-state absorption within the singlet system of the Kings complex, and we demonstrate that the RSA is completely characterized by a simple three-level model. We find, however, that the material parameters extracted from these picosecond measurements cannot account for the strong optical limiting previously observed in identical solutions of this compound using nanosecond excitation at higher fluences. Picosecond measurements at fluences greater than 200 mJ/cm 2 reveal the onset of an additional loss mechanism that appears ~1 ns after excitation. The magnitude of this loss depends on both the laser repetition rate and the solvent, indicating that the loss is not directly related to the intrinsic properties of the Kings complex but is most likely thermal in origin. Using nanosecond excitation pulses, we have performed angularly resolved transmission and reflection measurements, which reveal strong forward- and backward-induced scattering at these fluences. Furthermore, when the Kings complex is incorporated in a solid host, we observe negligible induced scatter and the response is completely described by the singlet parameters extracted from the picosecond measurements. These observations indicate that the nanosecond optical limiter response of solutions of Kings complex is dominated by thermally induced scattering.

Compensated High-Bandwidth Laser Ultrasonic Detector Based on Photo-Induced Emf in GaAs

The trend toward intelligent manufacturing has produced an increase in the need for sensors which can nondestructively evaluate components and processes in real-time. One commonly used nondestructive approach is ultrasonic inspection. The most common method for generating and sensing ultrasound in materials makes use of contacting piezoelectric transducers. A gel or water interface is often used to match the acoustic impedance between the sensor and part. This constraint can impose limitations on their applicability for some types of in-process industrial control or inspection, specifically, inspection of moving parts at elevated temperatures or in vacuum. While noncontact receivers have been made using capacitance or magnetic induction, often their spacing to the workpiece must be maintained within a close tolerance. The lack of a substantial standoff distance for these sensors also reduces their usefulness in some industrial inspection and process control applications. Specifically, contacting schemes and close proximity sensors are not well suited for conditions such as extreme vibrations and fast moving parts with irregular surfaces. Furthermore, it may be more cost effective, from the users perspective, to use a long standoff, remote sensing system which could be applied to a wide range of materials including metals, semiconductors and composites.

Optical limiting with reverse saturable absorbers

We have used a physically realistic model to investigate the spatial and temporal behavior of reverse saturable absorption and optical limiting in organometallic cluster compounds. An algorithm was developed to solve numerically the model and is used for the case of a collimated beam to determine various material parameters by fitting theory to experimental optical limiting data. Using these parameters, the algorithm was then extended to the case of converging and/or diverging beams.

Optimization of optical limiting properties of organometallic cluster compounds

Ground and excited state absorption spectra along with optical limiting measurements have been made on a series of systematically altered organometallic cluster compounds. Measurements using 8 ns pulses at 532 nm on methylene chloride solutions of iron-tricobalt organometallic cluster complexes (70% nominal transmission) show limiting throughputs of 350-700 mJ/cm2 without focusing. The optical limiting properties are shown to be minimally dependent on counter-ion substitution indicating little perturbation in the cluster bonding. Optical limiting is shown to be strongly dependent on ligand substitution. Ground state and excited state absorption spectra reveal broad features indicating the materials can be used as limiters over a large spectral range (>lOOnm) . Temporal pulse narrowing of 8 ns pulses to 5 ns at 532 nm was observed, indicating subnanosecond intersystem crossing. The lifetime of the excited state was determined to be 115 ns, much longer than the pulse length, indicating that efficient limiting occurs for longer pulses. Optimization of optical limiting properties is discussed.