Nonlinear optical phenomena at the interface with photorefractive crystals

Characteristics of nonlinear optical phenomena in reflection of light at the interface with photorfractive crystals having photovoltaic nonlinearity are studied. it is shown that under total reflection, hysteretic behavior of the reflection coefficient is possible as a function of the incident radiation intensity or the angle of incidence. It is important that previously required threshold intensities are rather low (≤1 W/cm²) in comparison with the known case of reflection from media with Kerr nonlinearity. The present results are of doubtless interest in view of the development of possible devices for controlling laser radiation. Interdepartmental Institute of Advanced Training of Personnel in New Trends in Engineering and Technology at the State Polytechnical Academy of Belarus, 77, Partizanskii Ave., Minsk-107, 220600 Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 6, pp. 915–920, November–December, 1998.     

KDP晶体金刚石车削参数对晶体雾化的影响

安装在高功率激光系统中的KDP晶体,其表面质量是一个很重要的问题。单点金刚石车削是唯一获得透射波前误差和激光损伤阈值的方法,尤其是在紫外UV波段。KDP晶体表面雾化与金刚石车削参数有关。研究金刚石车削参数主要是为了提高KDP晶体的频率转化效率。研究结果表明,在金刚石切削中使用一种纯净的切削油和适宜的加工参数能大大地减少晶体表面的雾化。     

Coexistence of two colloidal crystals at the nematic-liquid-crystal-air interface

Glycerol droplets at a nematic-liquid-crystal-air interface form two different lattices--hexagonal and dense quasihexagonal--which are separated by the energy barrier and can coexist. Director distortions around each droplet form an elastic dipole. The first order transition between the two lattices is driven by a reduction of the dipole-dipole repulsion through reorientation of these dipoles. The elastic-capillary attraction is essential for the both lattices. The effect has a many-body origin.     

On the transverse electrical susceptibilities of KDP type crystals

Using statistical Green's function technique exact expression for the transverse electrical susceptibility (x_$\text{a}$) for KDP type crsytals has been derived with the total pseudo-spin lattice coupled mode Hamiltonian. Very good fittings of the experimental data of x_$\text{a}$ for KH₂PO₄, RbH₂PO₄ and KH₂AsO₄ with the derived expression have been obtained with a single set of Blinc-de Geenes (BG) parameters being different for different salts. The temperature dependence of the E-mode character along the transverse $\text{a}$-axis has also been studied theoretically and compared with the recent experimental results available for KH₂PO₄ only.     

Preliminary study of the damage resistance of type I doubler KDP crystals at 532 nm

This Letter is concerned with the influence of polarization on the damage performance of type I doubler potassium dihydrogen phosphate crystals grown by the conventional growth method under 532 nm pulse irradiation.Pinpoint density(ppd) and the size distribution of pinpoints are extracted through light scattering pictures captured by microscope. The results show that the ppd of polarization that parallels the extraordinary axis is around 1.5× less than that of polarization that parallels the ordinary axis under the same fluence,although polarization has no influence on size distribution of pinpoints. We also find that the size distribution is independent of fluence,although the number of pinpoints grows with fluence.     

光在两单轴晶体间界面的反射和透射

根据位相匹配条件和应该满足的边界方程,在光轴取向任意的条件下,得到了光在两单轴晶体界面的反射和折射的理论表达式,给出了更普遍的公式计算光能量损失以及两束折射光的能量比。数值模拟表明所得结果满足能量守恒,光轴的取向和入射角大小对晶体中折射的o光、e光的能量比有重大影响,但是对反射率影响很小。只要知道了晶体的有关参数及入射条件,利用所给表达式可以计算各光束光能量比,为晶体器件特性研究提供了有力的工具。由于对光轴取向和入射角度没有任何限制,公式具有普遍性。     

Third harmonic generation of Nd:glass laser with novel composite deuterated KDP crystals

A novel composite partially deuterated KDP crystal used for frequency tripling is experimentally demonstrated for the first time in this paper. The deuteration level is used as a degree of freedom to alter the phase matching wavelength so as to increase the bandwidth acceptance. The results show that the acceptance bandwidth based on such crystal can be increased by a factor of 3 comparing with the conventional method. The obtained results are very useful to develop frequency tripling components in broadband third harmonic generation.     

Study on the optical properties of rapidly grown KDP crystals

KDP crystals were grown at growth rates from 5.0 to 19.0 mm/d. Transmittance, laser damage threshold and light scatter were detected. It has been shown that laser damage thresholds of KDP crystals decrease with the increase of the absorption coefficients. Large-scale impurity is an important factor that causes light absorption in UV range and reduces the damage threshold.     

Surface electromagnetic waves at the interface of photorefractive crystals with a drift-diffusion nonlinearity mechanism

Within the accuracy of the plane-wave approximation, a solution of the boundary-value problem for the Maxwell equations is obtained for surface electromagnetic waves at the interface between a photorefractive crystal and a transparent isotropic medium. It is shown that in this case, of all the kinds of surface waves, only soliton-type waves can be generated. Characteristic parameters of these waves are determined for a photorefractive SBN (strontium-barium-niobate) crystal.     

Ionic liquids at the air/water interface

We present molecular dynamics experiments of Langmuir monolayers of iodide and chloride salts of 1-octyl-3-methylimidazolium adsorbed at water/air interfaces, covering a concentration range that spans from a dilute regime up to the experimental surface saturation for both systems. For the chloride case we observed a propensity to form monolayers with nearly equal surface concentration of both cations and anions; whereas for the iodide system, the more marked propensity to surface solvation of the anionic species leads to the appearance of quasi-double-layered structures. At the surface, the imidazolium rings remain in contact with the aqueous substrate, with a wide variety of orientations with respect to the surface normal direction. The global tilt of the hydrophobic tail of the cations was found to be θ_tl ~ 40°and 50°, for the chloride and iodide salts, respectively. Polarization fluctuations of the interface are analyzed in terms of those describing charge distributions of the adsorbed species and the electrical response of the solvent as well. The characteristics of the local densities for the ionic species at the interface provide arguments for the microscopic interpretation of the differences observed in scattering experiments on the dependence of the surface tension with the surfactant concentration.     

Analysis of broadband third harmonic generation with non-collinear angular dispersion in KDP crystals

A new scheme with non-collinear sum frequency generation (SFG) process of broadband third harmonic generation (THG) with two KDP crystals (one doubler and one mixer) using angular dispersion was proposed. The principle of broadband harmonic generation with non-collinear angular dispersion was presented. The comparison between the schemes of non-collinear and collinear SFG process was performed. The effects of the angular dispersion on the conversion efficiency, the pulse shape and the spectrum of the third harmonic field were discussed. The results show that, if proper angular dispersion is added to the fundamental and the second harmonic field outside the mixing KDP crystal, respectively, the broadband third harmonic conversion efficiency can be improved significantly. However, the difficulty of this scheme arises due to the requirement of two gratings with different angular dispersion outside the mixer. With the new scheme, it can be simpler that only one grating is needed to realize the broadband phase matching in the non-collinear SFG process. Although the fundamental and the second harmonic field inside the mixing crystal are non-collinear except the center wavelength, a blazing grating with proper angular dispersion for the fundamental field (twice as that for second harmonic field) can yield the well compensation for phase mismatching in the SFG process. Consequently, the conversion efficiency and the characteristics of the third harmonic field for the broadband third harmonic generation can be improved significantly.     

Proof of the first order phase transition in the Slater KDP model

The Slater KDP model defined on d-dimensional tetrahedral lattices is proved to have a phase transition for which the entropy and energy are discontinuous functions at a transition temperaturekT _c =/ln2, independent of dimensionality.     

Structure of monolayers at the air/water interface

A perturbation theory based on the random phase approximation with a hard disc reference fluid is used to generate substantially analytic results for the form of the small angle scattering from model two dimensional monolayer systems. When applied to systems with a conventional 12-6 Lennard-Jones interaction potential the theory yields results in good agreement with more sophisticated theories and computer simulation. The method is also used for the 12-5 Lennard-Jones interaction potential that characterizes the in-plane interactions of lipid molecules at the air/water interface and gives qualitative agreement with experimental studies. The inclusion of weak dipolar interactions between the monolayer particles is seen to supress any liquid-vapour phase transition.     

Investigation of etch pits on KDP crystals with porous sol–gel coatings

We have investigated etch-pit formation on potassium dihydrogen phosphate (KDP) crystals with porous anti-reflective coatings. Etch pits develop beneath the sol–gel coatings after exposure to ambient humidity. The etch pits are homogeneously distributed with a density and an average size governed by the relative humidity and the coating thickness. Furthermore, the etch pits are self-similar in shape and possess facets corresponding to low-energy planes of KDP. Results from optical microscopy, light scattering, and atomic force microscopy support the following model for etch-pit formation in this system. Water adsorbed from the environment into the porous sol–gel coating contacts the crystal surface, causing etch-pit nucleation at high undersaturation. The plume of KDP rising from an etch pit slowly diffuses laterally, reducing the undersaturation and shutting off nucleation in the surrounding area. Because surface kinetics are rapid compared to mass transport through the sol–gel, etch pits continually undergo equilibration to maintain a low-energy geometry and generate an average spacing. Growth continues until the reservoir of water in the sol–gel is saturated with KDP. Coarsening is only observed in high relative humidity environments. Received: 13 Juni 2001 / Accepted: 30 July 2001 / Published online: 30 October 2001     

Glassy dynamics of water at interface with biomolecules: A Mode Coupling Theory test

We study the slow dynamics of hydration water upon cooling in two different biological aqueous solutions, one containing a molecule of lysozyme and another with trehalose molecules. In particular we test if the glassy behaviour of these solutions fulfils the predictions of the popular Mode Coupling Theory of glassy dynamics. In particular we test the Time Temperature Superposition Principle and the matching of the exponents of the theory. Our results confirm that this theory is able to describe the dynamical behaviour of supercooled water also in non ideal cases as the ones under investigation in the region of mild supercooling.     

Two-dimensional density of states for electrons bound to impurities inside inversion layers at the semiconductor-insulator interface

The density of states for electrons bound to Na⁺ impurities inside the inversion layer at the semiconductor-insulator interface of an MOS structure is calculated as a function of impurity concentration. The impurity potential is considered unscreened and the electrical quantum limit is assumed. A simple one-electron Hamiltonian is used and the disorder is treated through a cluster calculation. It is shown that the impurity band has a considerable bandwidth for impurity concentrations in a range of the experimental regime (this result agrees with the experimental findings of Hartstein and Fowler), and that the upper Hubbard band stands well above the lower band at very low concentration (in rough agreement with recent calculation done by Phelps and Bajaj on D^? state).     

Liquid/solid interface profile of melt grown oxide crystals II. Crystal quality

Temperature gradients and rotation rates influence the quality of Czochralski grown oxide crystals. Decreasing the heat transfer from the melt level and increasing the rotation rate increases the optical homogeneity and structural perfection of the crystal, due to convex interface becoming flattened. Growth striations due to temperature fluctuations in the melt during growth are affected by the heat losses from the melt level but are practically unaffected by the rotation rate as long as a narrow ring of centrifugally streaming melt in the neighbourhood of the growing crystal is not formed. The parts of the crystals grown in the presence of this ring contain no striations.     

Liquid/solid interface profile of melt grown oxide crystals I. Czochralski growth

The relation between temperature gradient and the liquid/solid interface profile of Czochralski grown Al₂O₃ and YAG crystals is described. Lowering the radial component as well as increasing the axial component of temperature gradient during crystal growth causes the convex or pointed interface to be flattened. In the range of magnitudes both of the components of temperature gradient used, four types of interface profiles were found. An alteration of the interface profile which causes local increase of the growth rate can be accompanied by cell formation. The volume of the portion of the growing crystal submerged in the melt is directly proportional to the optical transmissivity of the crystal and inversely proportional to the optical transmissivity of the melt.