Acousto-optic Diffraction Efficiency Research Articles

Purification and growth of mercurous chloride single crystals

Mercurous chloride ( Hg 2 Cl 2) exhibits an extraordinary combination of properties including transmittance from 0.35 to 20 μm, an anomalously slow sheer wave velocity (347 m/s), large birefringence and a high acousto-optic diffraction efficiency which make it an attractive candidate for a number of optical signal processing devices. For this reason we have developed improved techniques to purity and grow single crystals of this compound. While we observed that Hg 2 Cl 2 is insoluble in water, alcohol, and ether, and therefore purification by simple recrystallization is difficult, the material has sufficiently high vapor pressure that it can be purified by repeated sublimation. Purification is complicated by the oxidation of Hg 2 Cl 2 in presence of traces of moisture. We found that the orthorhombic compound Hg 5 Cl 2 O 4, analog to the mineral pinchite, was the main constituent of the black-brown residue formed under vapor transport conditions which produced colored Hg 2 Cl 2 single crystals. Moisture control coupled with multiple sublimations to minimize metallic contaminants has produced colorless oriented H 2 Cl 2 single crystals with reduced light scattering compared to crystals grown from unpurified feedstock.

Convection and Diffusion Effects During Physical Vapor Transport of Hg2Cl2

AbstractWe have carried out extensive experimentation on the physical vapor transport growth of mercurous chloride, which is an important material for opto-electronic devices. Because of the extraordinary combination of properties found in Hg2Cl2, including transmittance from 0.36 to 20 μm, anomalously slow soung velocity, high birefringence, and large acousto-optic diffraction efficiency, we are exploring the conditions for growth of large crystals of device quality. Our experiments have led so far to measured Hg2C12 growth rates that were orders of magnitude smaller than those expected by theories based on laminar, unidirectional flow between source and sink. Slight disturbances of the solid-vapor interface give rise to instabilities which lead to interfacial convection. This kind of convection should enhance the growth rate due to the acceleration of the materials exchange by hydrodynamic coupling, which is contrary to our findings. Interfacial convection is often connected with buoyancy-induced instabilities which dominate other instabilities under 1-g conditions. Despite attempts to reduce the Rayleigh numbers in our system, buoyancy-induced instabilities might be the cause of discrepancy between the measured and calculated growth rates. Observations of these phenomena under a microgravity environment might permit a better assessment of the transport mechanisms in physical vapor phase crystal growth of Hg2C12.

Performance and optical characterization of efficient wideband gallium phosphide Bragg cells

Characteristics of high performance gallium phosphide Bragg cells are reported, including very high diffraction efficiencies and moderate bandwidths (for example, 45%/W, 530 MHz at 633 nm). Optical characteristics, including polarization dependence of acoustooptic diffraction efficiency, are considered explicitly. Design calculations are discussed and verified. Laser beam depolariation observed along the (1,0,−1) direction was found to affect the efficiency calculations. However, the depolarization effect could be isolated by considering an equivalent substrate configuration in which optical polarization was maintained. Then diffraction efficiency was accurately predicted by operating on the photoelastic tensor. Finally, possible models for optical effects observed in bulk GaP are outlined.