Ganesan Ravi

Growth and characterization of 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) single crystals

Abstract Experimental determination of metastable zonewidth and induction period for 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) is reported. The interfacial tension for different supersaturation conditions has been estimated for the DAST solution at 30°C. The solubility of DAST in methanol has been determined and bulk crystals of dimensions 6×5×2xa0mm 3 have been grown by spontaneous nucleation technique with the optimized growth parameters. XRD and FTIR studies confirm the structure and presence of functional groups in the grown crystals. Vickers microhardness studies carried out on {0xa00xa01} face of DAST crystals reveal that these crystals possess higher hardness values than other organic crystals.

Nonvolatile two-color holography in Mn-doped near-stoichiometric lithium niobate

Nonvolatile holograms are recorded in near-stoichiometric LiNbO3 doped with 8 ppm of Mn by use of a two-color holographic recording method. An IR laser with wavelength of 778 nm is used for writing and a UV laser with wavelength of 350 nm is used for gating. Dependences of two-color sensitivity and M/♯ on gating and writing intensities are presented. Significant improvements of sensitivity, M/♯, and dark decay are obtained compared with reduced near-stoichiometric LiNbO3. The obtained high two-color sensitivity of 0.21 cm/J with a gating intensity of 1.5 W/cm2 is attributed to large light-induced absorption of small polarons.

Growth aspects of semi-organic nonlinear optical l-arginine tetrafluoroborate single crystals

Abstract Single crystals of l -arginine tetrafluoroborate ( l -AFB), a semi-organic NLO material, have been grown from aqueous solution. Since l -AFB is known to form highly viscous solution in water, growth by slow cooling has been difficult for optical quality crystals. In this study, highly transparent l -AFB crystals have been successfully grown by slow evaporation by keeping the saturated solution under special condition. The grown crystals were subjected to structural, optical and mechanical property studies. XRD studies reveal that l -AFB crystals possess orthorhombic structure. Fourier transform infrared absorption studies confirm the chemical constituents and presence of functional groups in the grown crystals. l -AFB crystals possess a higher hardness values compared to other semi-organic crystals. The linear and nonlinear optical properties of the grown crystals have also been studied.

Growth and two-color holographic storage properties of Mn-doped lithium niobate crystals with varying Li∕Nb ratio

Near-stoichiometric LiNbO3 crystals slightly doped with Mn from melts with different compositions are grown by the top seeded solution growth method. The growth parameters such as temperature field, pulling, and rotation rates are optimized. Two-color holograms are recorded in these crystals by the use of a near infrared laser of 778 nm for writing and an ultraviolet laser of 350 nm for gating. The light-induced absorption changes and two-color holographic characteristic performances are presented and compared. The results show that crystals with composition closer to stoichiometry exhibit better two-color holographic storage performance.

Light-induced absorption and its relaxation under illumination of continuous wave ultraviolet light in Mn-doped near-stoichiometric LiNbO3

The steady-state light-induced absorption and the temporal relaxation behavior under illumination of cw ultraviolet light in Mn-doped near-stoichiometric LiNbO3 with different crystal compositions are investigated. The ultraviolet-light-induced absorption has been assigned to small polarons NbLi4+ by measuring the absorption spectra at room temperature. The dependences of relaxation behaviors (time constant and stretching factor) of light-induced absorption on various illumination conditions (intensity, polarization) and temperature are presented, which are very different from those observed in Fe-doped LiNbO3 illuminated with highly intense light pulse, though the temporal relaxation follows the same stretched-exponential decay behavior in both cases. The results are explained reasonably by using the model of distance-dependent electron transition probabilities between localized deep traps and small polarons without any additional assumptions, and discussed to tailor doped near-stoichiometric LiNbO3 crys...

Effect of niobium substitution in stoichiometric lithium tantalate (SLT) single crystals

Abstract Optical-quality near-stoichiometric lithium tantalate (SLT) crystals doped with Nb were grown by top seeded solution growth technique using induction-heating system. The crystalline wafers were subjected to optical transmittance studies and coercive field measurements. The optical transmittance spectra of doped crystals show a shift in the absorption edge towards longer wavelength side. A change in Curie temperatures with respect to doping concentrations of Nb has been observed. Hysteresis measurements reveal that the coercive field value increases for increasing niobium concentration. It has also been observed that there is a significant increase in the internal field value for Nb-doped SLT crystals.

Investigation on dark decay of two-color holograms in near-stoichiometric LiNbO 3 and LiTaO 3

We have investigated the dark decay mechanism by measuring dark decay time constants of two-color holograms recorded in undoped and slightly doped near-stoichiometric LiNbO3 and LiTaO3 crystals in the temperature range from 50 to 200 °C. All the samples in this work obey an Arrhenius-type dependence of time constant on absolute temperature, and yield nearly same activation energies of 1.08 eV. The results show that proton compensation mechanism dominated the dark decay process. Lifetimes of holograms at room temperature in LiTaO3 are one order of magnitude longer than that in LiNbO3 if having the same proton concentration.

Photochromic Effect in Stoichiometric LiNbO3:Fe,Mn

Stoichiometric LiNbO3 (LN) crystals doubly doped with Fe and Mn exhibited a strong reversible photochromic effect at room temperature. Irradiation with UV light induced a new absorption band extending from approximately λ=650 nm to the absorption edge, which could also be bleached by subsequent visible illumination. The dark decay times of induced absorption at room temperature were estimated to be approximately 2 months and more than a year for as-grown and oxidized crystals, respectively. Electron paramagnetic resonance analysis confirmed that the photochromic process in LN:Fe,Mn occurs by an intervalence charge transfer between deep Mn2+/3+ and shallow Fe2+/3+ energy levels. It was also found that only a small fraction of the doping ions are involved in the photochromic process.

Growth, Optical and Switching Characteristics of Ferroelectric Pure and Mn Doped Near SLN Crystals

Pure and Mn doped single domain near stoichiometric lithium niobate crystals have been grown by top seeded solution growth from 55 mol% Li solution. A uniform segregation of Mn throughout the crystal was obtained. Doping concentration decreases the Curie temperature while the absorption edge shift towards the longer wavelength region. Two color holographic recording was examined in Mn doped samples. Improved sensitivity, dynamic range and lifetime of the hologram for the doped crystal have been determined. The effect of Mn doping on the switching properties has been analyzed.

Comparison of two-color hologram lifetimes of near-stoichiometric lithium niobate and of tantalate crystals

Lifetimes of two-color nonvolatile holograms recorded in undoped or in slightly doped near-stoichiometric lithium niobate and tantalate crystals were measured and compared by extrapolation of the high-temperature data. A proton-compensation mechanism dominated the dark decay and yielded similar activation energies, of 1.05 and 1.10 eV, for near-stoichiometric lithium niobate and tantalate crystals, respectively. The lifetime of holograms in lithium tantalate was 1 order of magnitude longer than that in lithium niobate with the same proton concentration, which was consistent with our theoretical estimation. The projected lifetime of two-color holograms in lithium tantalate without observable OH-absorption is longer than 50 years.