R. Mazelsky

Nonlinear optical characteristics of binary organic system

Single crystals of 3-nitroaniline-2-chloro-4-nitroaniline (m.NA-CNA) alloy were grown from the melt. The crystals do not show any damage when exposed to 1.06 μm radiation at 1 mJ per pulse (10 Hz repetition rate, 0.35 mm spot size, and 10 ns pulse) corresponding to a 100 MW/cm 2 power density. The second harmonic efficiency was measured by using a beam of 1.5 mm diameter spot 10 ns pulse and 1.06 μm wavelength. The input energy was 0.3 mJ, peak power density was 1.7 MW/cm 2 and the test crystal was 2 mm thick. The observed efficiency was 7.7% indicating that a crystal of 1 cm length will produce very high efficiency. This measured efficiency for the m.NA-CNA alloy is substantially higher than the reported value for the m.NA crystal

Purification and growth of mercurous chloride single crystals

Abstract 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.

Solution growth of vanillin single crystals

Single crystals of vanillin were grown by the solution growth method. Vanillin was observed to be very anisotropic, and crystal morphology was strongly dependent on solvent. Crystals grew in needle morphology in pure methanol and ethanol, and in plate morphology in pure chloroform. A mixture of methanol and chloroform was found to be a suitable solvent for crystal growth and crystals up to the size of 7 × 5 × 4 mm3 were grown for optical characterization. Optical quality of crystal was observed to be good. Preliminary studies on frequency conversion showed that vanillin has a higher figure of merit when compared to other crystals suitable for non-linear optical applications in the visible to near-infrared region.

Lead (II) halides: A new class of acousto-optic and electro-optic materials

Abstract A novel and potentially useful class of crystals, the lead (II) halides were recognized by A.V. Zamkov et al. [Soviet Phys.-Cryst. 24 (1979) 87] for the acousto-optic and electro-optic and electro-optic devices. Lead bromide, PbBr 2 , is the most highly studied of these and was tested for its acousto-optic properties. The crystal possesses certain properties which are highly desirable for signal-processing applications. For this reason, we have developed a method to purify and grow single crystals of this compound. PbBr 2 crystals up to 26 mm in diameter and several cm in length were grown by the Bridgman method. Purification is complicated by oxidation of PbBr 2 in the presence of traces of moisture. Formation of lead chlorite [ Pb ( ClO 2 ) 2 ], mendipite (Pb 3 O 2 Cl 2 ), plattnerite (PbO 2 ) and mixed oxy halides (Pb 3 O 2 BrCl) were observed due to residual chlorine and oxygen impurities. Mendipite was the major constituent of dark bluish residue formed during the purification by directional freezing.

The ZnSb structure; A further enquiry☆

Abstract A redetermination of the crystal structure of ZnSb was undertaken to resolve the question raised by a short Zn-Zn distance found by a previous worker.(1) Our result, Zn-Zn distance = 2.81 A, is in agreement with that found by Toman ,(2) who employed a diffractometer and a powder sample. From the bond distances a calculation of the bond orders and valences by the method of Pauling for intermetallic compounds leads to the interpretation of ZnSb as an electron transfer compound with Zn−1 having a valence of three and Sb+1 having a valence of four. The transference of approximately one electron from Sb to Zn permits better binding through the increased valence of both atoms. In the case of the Zn-Zn interaction it appears likely that a repulsive force exists between the atoms, however, it is not certain whether the force is that due to the close approach of nonbonded atoms or that of a bond in compression. With this uncertainty in mind the effects of normal twinning and polytype formation are discussed as a possible explanation of reported(3) phase transformations. Using a single crystal and a modified powder camera the cell edges were determined at 28.4 ± 2°C to be a0 = 6.202 A, b0 = 7.742 A, co = 8.100 A.

Growth and characterization of lead bromide crystals

Lead(II) bromide was purified by a combination of directional freezing and zone-refining methods. Differential thermal analysis of the lead bromide showed that a destructive phase transformation occurs below the melting temperature. This transformation causes extensive cracking, making it very difficult to grow a large single crystal. Energy of phase transformation for pure lead bromide was determined to be 24.67 cal/g. To circumvent this limitation, crystals were doped by silver bromide which decreased the energy of phase transformation. The addition of silver helped in achieving the size, but enhanced the inhomogeneity in the crystal. The acoustic attenuation constant was almost identical for the pure and doped (below 3000 ppm) crystals.

Growth of large mercurous chloride single crystals by sublimation

Abstract Large, high-quality single crystals of Hg 2 Cl 2 were obtained by directional sublimation of mercurous chloride with temperatures between 300 and 450°C in closed, evacuated quartz ampoules using a vertical pulling technique.

Growth rates and characteristics of mercurous chloride crystals

Abstract The present study demonstrates that high optical-quality crystals of mercurous chloride can be grown by the physical vapor transport (PVT) method when growth conditions are carefully controlled. Growth rates were measured at a fixed temperature gradient in a two-zone transparent furnace, where growth was abruptly disturbed and then re-establishment of equilibrium was allowed. During this process the crystal length was measured as a function of time. Data were taken for crystal orientations between [001] and [110], growth along [001] being faster than along the [110] orientation. Experimental data follow with the equation l t = l e − A exp(- t /τ), where l t is the length of crystal at any time t , l e is the equilibrium length, and A and τ are constants. Results indicate that growth is controlled by surface kinetics. Growth defects such as inclusions, nonstoichiometry, cracking, cleavage, growth rings, and striations were observed in crystals of medium purity. Low defect densities were achieved by control of crystal purity, growth rate, and orientation.

Annealing experiments on silver gallium selenide crystals

Abstract A detailed annealing experiment was carried out to identify the mechanisms of composition changes in AgGaSe 2 crystals. Silver-deficient phases were observed in the as-grown crystal which dissolved by a diffusional process as the result of thermal annealing. The annealing enhanced the optical quality of the as-grown crystal by reducing light scattering.

Growth and characterization of thallium arsenic selenide crystals for nonlinear optical applications

Abstract The synthesis, growth, and characterization of large single crystals of the efficient nonlinear optical sulfosalt material thallium arsenic selenide (TAS) are reviewed. Recent significant improvements in crystal growth and purification technology have enabled us to grow high optical quality crystals as measured by etchpit, x-ray topographic, and birefringent interferometric characterization methods. The excellent crystal quality is reflected in the high CO 2 frequency conversion efficiencies obtained with TAS. In addition to quality improvement in 2.5 cm diameter crystals, larger crystals, up to 5 cm in diameter, have now been grown by a modified Bridgman technique. Etching of crystals grown under a high axial thermal gradient indicated a higher defect density near the crystal surface than at the corresponding central region. In addition, high defect densities were found at the seed-crystal joint. This suggests that the etchpits are related to thermal stresses induced by the temperature distribution in a growing crystal. X-ray topographic analyses confirmed the etching studies, and also revealed a subgrain structure in some crystals which is related to the crystal stress distribution and to seeding defects. Birefringent interference patterns, a measure of optical uniformity, show a direct correlation between subgrain structure and optical homogeneity of the crystals. Improved growth control has eliminated these defects. The nonlinear optical susceptibilities of TAS are large, nearly three times those of proustite. Crystals grown in our laboratory have shown high values of second, third, and fourth harmonic conversion efficiency of the output from a CO 2 laser. Fifth harmonic generation has also been demonstrated. A 4.8 cm long crystal, 25 mm in diameter, was used to double 4.8 μm to 2.4 μm with an efficiency of 6.4% for the relatively low fluence of 8 mJ/cm 2 in an 80 nsec pulse.