Defect characterization of KTP single crystals

Potassium titanyl phosphate (KTP) is a relatively new nonlinear optical material with excellent combination of physical properties. This paper presents the combined etching and X-ray topographic studies carried out on KTP crystals with a view to characterizing their defects. KTP crystals employed in this investigation were grown from flux. Optical microscopic study of habit faces revealed growth layers and growth hillocks on (100) and (011) faces respectively. Etching of (011) habit faces proved that growth hillocks corresponded to the emergence point of dislocation out crops on these faces. The suitability of the new etchant to reveal dislocation was confirmed by etching the matched pairs obtained by cleaving. The defects present in the crystal were also studied by X-ray topography. The defect configuration in these crystals is characteristic of crystals grown from solution. The dislocations arc predominantly linear with their origin either at the nucleation centre or inclusions. In general, grown crystals were found to have low dislocation density and often large volumes of crystals free from dislocation could be obtained.

Geometry and quadratic nonlinearity of charge transfer complexes in solution: A theoretical study

In this paper, we have computed the quadratic nonlinear optical (NLO) properties of a class of weak charge transfer (CT) complexes. These weak complexes are formed when the methyl substituted benzenes (donors) are added to strong acceptors like chloranil (CHL) or di-chloro-di-cyano benzoquinone (DDQ) in chloroform or in dichloromethane. The formation of such complexes is manifested by the presence of a broad absorption maximum in the visible range of the spectrum where neither the donor nor the acceptor absorbs. The appearance of this visible band is due to CT interactions, which result in strong NLO responses. We have employed the semiempirical intermediate neglect of differential overlap (INDO/S) Hamiltonian to calculate the energy levels of these CT complexes using single and double configuration interaction (SDCI). The solvent effects are taken into account by using the self-consistent reaction field (SCRF) scheme. The geometry of the complex is obtained by exploring different relative molecular geometries by rotating the acceptor with respect to the fixed donor about three different axes. The theoretical geometry that best fits the experimental energy gaps, beta(HRS) and macroscopic depolarization ratios is taken to be the most probable geometry of the complex. Our studies show that the most probable geometry of these complexes in solution is the parallel displaced structure with a significant twist in some cases. (C) 2011 American Institute of Physics. doi:10.1063/1.3526748]

Dielectric properties of Li2O–3B2O3 glasses

The frequency and temperature dependences of the dielectric constant and the electrical conductivity of the transparent glasses in the composition Li2O–3B2O3 were investigated in the 100 Hz–10 MHz frequency range. The dielectric constant and the loss in the low frequency regime were electrode material dependent. Dielectric and electrical relaxations were, respectively, analyzed using the Cole–Cole and electric modulus formalisms. The dielectric relaxation mechanism was discussed in the framework of electrode and charge carrier (hopping of the ions) related polarization using generalized Cole–Cole expression. The frequency dependent electrical conductivity was rationalized using Jonscher’s power law. The activation energy associated with the dc conductivity was 0.80±0.02 eV, which was ascribed to the motion of Li+ ions in the glass matrix. The activation energy associated with dielectric relaxation was almost equal to that of the dc conductivity, indicating that the same species took part in both the processes. Temperature dependent behavior of the frequency exponent (n) suggested that the correlated barrier hopping model was the most apposite to rationalize the electrical transport phenomenon in Li2O–3B2O3 glasses. These glasses on heating at 933 K/10 h resulted in the known nonlinear optical phase LiB3O5.

Near-resonant two-photon absorption in luminescent CdTe quantum dots

We report the nonlinear optical absorption studies in two differently sized water-soluble cadmium telluride quantum dot (QD) samples, exhibiting first excitonic absorption peaks at 493 nm and 551 nm, respectively. An optical limiting behavior is observed for near-resonant excitation at 532 nm using nanosecond laser pulses, originating from the effective two-photon absorption (TPA) mechanism. The effective TPA coefficient (beta(eff)) is measured to be in the range of 10(-12) m/W. This is one order of magnitude higher than the TPA coefficient (beta) reported for off-resonant excitation. At this excitation wavelength, the smaller QD shows a relatively weaker photoluminescence and stronger nonlinear absorption. (C) 2012 American Institute of Physics. [doi:10.1063/1.3687695]

Polymorphism and tautomeric preference in fenobam and the utility of NLO response to detect polymorphic impurities

Crystal structures of polymorphs and solvatomorphs of the potential anxiolytic drug fenobam exhibit an exclusive preference for one of the two possible tautomeric structures. A novel methodology based on nonlinear optical response has been successfully employed to detect the presence of a polymorphic impurity in a mixture of polymorphs.

Simple theoretical analysis of the effective electron mass in semiconductor nanowires

In this paper we study the effective electron mass (EEM) in Nano wires (NWs) of nonlinear optical materials on the basis of newly formulated electron dispersion relation by considering all types of anisotropies of the energy band constants within the framework of k . p formalism. The results for NWs of III-V, ternary and quaternary semiconductors form special cases of our generalized analysis. We have also investigated the EEM in NWs of Bi, IV-VI, stressed Kane type materials, Ge, GaSb and Bi2Te3 by formulating the appropriate 1D dispersion law in each case by considering the influence of energy band constants in the respective cases. It has been found that the 1D EEM in nonlinear optical materials depend on the size quantum numbers and Fermi energy due to the anisotropic spin orbit splitting constant and the crystal field splitting respectively. The 1D EEM is Bi, IV-VI, stressed Kane type semiconductors and Ge also depends on both the Fermi energy and the size quantum numbers which are the characteristic features of such NWs. The EEM increases with increase in concentration and decreasing film thickness and for ternary and quaternary compounds the EEM increases with increase in alloy composition. Under certain special conditions all the results for all the materials get simplified into the well known parabolic energy bands and thus confirming the compatibility test.

Growth and characterization of L-histidinium 2-nitrobenzoate single crystals: A new NLO material

Crystals of a new nonlinear optical (NLO) material, viz., L-histidinium 2-nitrobenzoate (LHNB) (1) were grown by slow evaporation of an aqueous solution containing equimolar concentrations of L-histidine and 2-nitrobenzoic acid. The structure of the title compound which crystallizes in the non-centrosymmetric monoclinic space group P2(1) was elucidated using single crystal X-ray intensity data. The UV-Vis-NIR spectrum of 1 reveals its transparent nature while the vibrational spectra confirm the presence of the functional groups in 1. The thermal stability and second harmonic generation (SHG) conversion efficiency of 1 were also investigated. (C) 2012 Elsevier GmbH. All rights reserved.

Effect of ultrasonic treatment on surface crystallization of calcium bismuth borate glasses

Transparent glasses in CaO-Bi2O3-B2O3 system were fabricated via the conventional melt-quenching technique. X-ray powder diffraction (XRD) and differential thermal analysis (DTA) carried out on the as-quenched samples confirmed their amorphous and glassy nature respectively. The surface crystallization behaviour of these glasses with and without ultrasonic surface treatment (UST) was monitored using XRD, optical microscopy and scanning electron microscopy (SEM). The volume fraction, depth of crystallization and the (001) orientation factor for the heat treated samples with and without UST were compared. The ultrasonically-treated samples on subsequent heat treatment were found to crystallize at lower temperatures associated with the highest degree of orientation factor (0.95) in contrast with those of non-UST samples. These surface crystallized glasses were found to exhibit nonlinear optical behaviour emitting green light (532 nm) when they were exposed to the infrared radiation (1064 nm) using Nd:YAG laser.

Dopant-configuration controlled carrier scattering in graphene

Controlling optical and electronic properties of graphene via substitutional doping is central to many fascinating applications. Doping graphene with boron (B) or nitrogen (N) has led to p- or n-type graphene; however, the electron mobility in doped-graphene is severely compromised due to increased electron-defect scattering. Here, we demonstrate through Raman spectroscopy, nonlinear optical and ultrafast spectroscopy, and density functional theory that the graphitic dopant configuration is stable in graphene and does not significantly alter electron-electron or electron-phonon scattering, that is otherwise present in doped graphene, by preserving the crystal coherence length (L-a).

New transparent crystallized glasses with optical nonlinear LiBGeO4 crystals

Crystallization behaviors of the glass with a composition of 25Li(2)O.25B(2)O(3).50GeO(2) corresponding to lithium borogermanate LiBGeO4 have been examined. It has been confirmed that the LiBGeO4 crystalline phase is formed at the surface of heat-treated glasses. The second harmonic (SH) generation is found from transparent surface crystallized glasses, demonstrating for the first time that the LiBGeO4 phase shows optical nonlinearity. The SH intensity of LiBGeO4 crystallites (powdered state) prepared through crystallization is about ten times as large as that of pulverized alpha-quartz. The SH intensity of transparent crystallized glasses (bulk state) with crystalline layers of 3-4.5 mum thickness increases with increasing heat treatment temperature (540-560degreesC) and time (1-6 h), and the maximum SH intensity among the samples studied is in the order of 1/10 in comparison with that of alpha-quartz single crystal. The transparent crystallized glass obtained by heat treatment at 550alphaC for 3 h exhibits a clear and fine Maker fringe pattern, indicating a highly orientation of LiBGeO4 crystals at the surface.

Influence of non-stoichiometric defects on nonlinear absorption and refraction in Nd:Zn co-doped lithium niobate

Nonlinear absorption and refraction phenomena in stoichiometric lithium niobate (SLN) pure and co-doped with Zn and Nd, and congruent lithium niobate (CLN) were investigated using Z-scan technique. Femtosecond laser pulses from Ti:Sapphire laser (800 nm, 110 fs pulse width and 1 kHz repetition rate) were utilized for the experiment. The process responsible for nonlinear behavior of the samples was identified to be three photon absorption (3PA). This is in agreement with the band gap energies of the samples obtained from the linear absorption cut off and the slope of the plot of Ln(1 − TOA) vs. Ln(I0) using Sutherland’s theory (s = 2.1, for 3PA). The nonlinear refractive index (n2) of Zn doped samples was found to be lower than that of pure samples. Our experiments show that there exists a correlation between the nonlinear properties and the stoichiometry of the samples. The values of n2 fall into the same range as those obtained for the materials of similar band gap.

Double walled carbon nanotubes as ultrafast optical switches

Pristine and molybdenum filled double walled carbon nanotubes (DWNTs) suspended in D2O show excellent ultrafast optical switching properties investigated through femtosecond Z-scan and degenerate pump-probe method using 50 fs pulses with central photon energy of 1.57 eV. For pristine-DWNT, the two photon absorption coefficient, beta and nonlinear refraction coefficient, n2 are 4.9×10−8 cm/W, and 9.5×10−11 cm2/W, respectively, which yield one photon figure of merit, W=133 and two photon figure of merit, T=0.4. The degenerate pump-probe measurements show strong photoinduced bleaching with biexponential decay with time constants ~150 and 600 fs. ©2009 American Institute of Physics

On the stability of a nonlinear system with periodic coefficients

A quasi-geometric stability criterion for feedback systems with a linear time invariant forward block and a periodically time varying nonlinear gain in the feedback loop is developed.

Suppression of the zero-order term in off-axis digital holography through nonlinear filtering

We present experimental validation of a new reconstruction method for off-axis digital holographic microscopy (DHM). This method effectively suppresses the object autocorrelation,namely, the zero-order term,from holographic data,thereby improving the reconstruction bandwidth of complex wavefronts. The algorithm is based on nonlinear filtering and can be applied to standard DHM setups with realistic recording conditions.We study the robustness of the technique under different experimental configurations,and quantitatively demonstrate its enhancement capabilities on phase signals.

Singular value decomposition based computationally efficient algorithm for rapid dynamic near-infrared diffuse optical tomography

Purpose: A computationally efficient algorithm (linear iterative type) based on singular value decomposition (SVD) of the Jacobian has been developed that can be used in rapid dynamic near-infrared (NIR) diffuse optical tomography. Methods: Numerical and experimental studies have been conducted to prove the computational efficacy of this SVD-based algorithm over conventional optical image reconstruction algorithms. Results: These studies indicate that the performance of linear iterative algorithms in terms of contrast recovery (quantitation of optical images) is better compared to nonlinear iterative (conventional) algorithms, provided the initial guess is close to the actual solution. The nonlinear algorithms can provide better quality images compared to the linear iterative type algorithms. Moreover, the analytical and numerical equivalence of the SVD-based algorithm to linear iterative algorithms was also established as a part of this work. It is also demonstrated that the SVD-based image reconstruction typically requires O(NN2) operations per iteration, as contrasted with linear and nonlinear iterative methods that, respectively, requir O(NN3) and O(NN6) operations, with ``NN'' being the number of unknown parameters in the optical image reconstruction procedure. Conclusions: This SVD-based computationally efficient algorithm can make the integration of image reconstruction procedure with the data acquisition feasible, in turn making the rapid dynamic NIR tomography viable in the clinic to continuously monitor hemodynamic changes in the tissue pathophysiology.