Femtosecond pulse laser-induced self-organized nanogratings on the surface of a ZnSe crystal

Periodic nanostructures are observed on the surface of ZnSe after irradiation by a focused beam of a femtosecond Ti:sapphire laser, which are aligned perpendicular to the laser polarization direction. The period of self-organized grating structures is about 160 nm. The phenomenon is interpreted in terms of interference between the incident light field and the surface scattered wave of 800-nm laser pulses. With the laser polarization parallel to the moving direction we produce long-range Bragg-like gratings by slowly moving the crystal under a fixed laser focus. The nanograting orientation is adjusted by laser polarization and the accumulation effect.

Coherent enhancement of broadband frequency up-conversion in BBO crystal by shaping femtosecond laser pulses

An optimal feedback control of broadband frequency up-conversion in BBO crystal is experimentally demonstrated by shaping femto-second laser pulses based on genetic algorithm, and the frequency up-conversion efficiency can be enhanced by similar to 16%. SPIDER results show that the optimal laser pulses have shorter pulse-width with the little negative chirp than the original pulse with the little positive chirp. By modulating the fundamental spectral phase with periodic square distribution on SLM-256, the frequency up-conversion can be effectively controlled by the factor of about 17%. The experimental results indicate that the broadband frequency up-conversion efficiency is related to both of second harmonic generation (SHG) and sum frequency generation (SFG), where the former depends on the fundamental pulse intensity, and the latter depends on not only the fundamental pulse intensity but also the fundamental pulse spectral phase. (c) 2006 Elsevier B.V. All rights reserved.

Upconversion luminescence and optical power limiting effect based on two- and three-photon absorption processes of ZnO crystal

Near-infrared to UV and visible upconversion luminescence was observed in single-crystalline ZnO under an 800 nm infrared femtosecond laser irradiation. The optical properties of the crystal reveal that the UV and VIS emission band are due to the exciton transition (D0X) bound to neutral donors and the deep luminescent centers in ZnO, respectively. The relationship between the upconversion luminescence intensity and the pump power of the femtosecond laser reveals that the UV emission belongs to three-photon sequential band-to-band excitation and the VIS emission belongs to two-photon simultaneous defect-absorption induced luminescence. A saturation phenomenon and polarization-dependent effect are also observed in the upconversion process of ZnO. A very good optical power limiting performance at 800 nm has been demonstrated. The two- and three-photon absorption coefficients of ZnO crystal were measured to be 0.2018 cm GW(-1) and 7.102 x 10(-3) cm(3) GW(-2), respectively. The two- and three-photon cross sections were calculated to be 1.189 x 10(-51) cm(4) s and 1.040 x 10(-80) cm(6) s(2), respectively. (c) 2007 Elsevier B.V. All rights reserved.

Microstructures induced in the bulk of SrTiO3 crystal by a femtosecond laser

Microstructures with the total length of hundreds of mu m were induced by fixing the focal point of the femtosecond laser at a certain depth in the bulk of SrTiO3 crystal. By different combination of the focusing conditions with the laser parameters, different morphologies have been observed, such as void array, necklace-shaped structures, continuous/segmental filaments and etc. The possible mechanism of the formation of those diversiform structures is discussed.

Orientation-controllable self-organized microgratings induced in the bulk SrTiO3 crystal by a single femtosecond laser beam

Self-organized microgratings were induced in the bulk SrTiO3 crystal by readily scanning the laser focus in the direction perpendicular to the laser propagation axis. The groove orientations of those gratings could be controlled by changing the irradiation pulse number per unit scanning length, which could be implemented either through adjusting the scanning velocity at a fixed pulse repetition rate or through varying the pulse repetition rate at a fixed scanning velocity. This high-speed method for fabrication of microgratings will have many potential applications in the integration of micro-optical elements. The possible formation mechanism of the self-organized microgratings is also discussed. (C) 2007 Optical Society of America.

I. The crystal structure of trimesic acid. II. Topics in crystallographic calculations

I. Trimesic acid (1, 3, 5-benzenetricarboxylic acid) crystallizes with a monoclinic unit cell of dimensions a = 26.52 A, b = 16.42 A, c = 26.55 A, and β = 91.53° with 48 molecules /unit cell. Extinctions indicated a space group of Cc or C2/c; a satisfactory structure was obtained in the latter with 6 molecules/asymmetric unit - C₅₄O₃₆H₃₆ with a formula weight of 1261 g. Of approximately 12,000 independent reflections within the CuK_α sphere, intensities of 11,563 were recorded visually from equi-inclination Weissenberg photographs.

The structure was solved by packing considerations aided by molecular transforms and two- and three-dimensional Patterson functions. Hydrogen positions were found on difference maps. A total of 978 parameters were refined by least squares; these included hydrogen parameters and anisotropic temperature factors for the C and O atoms. The final R factor was 0.0675; the final "goodness of fit" was 1.49. All calculations were carried out on the Caltech IBM 7040-7094 computer using the CRYRM Crystallographic Computing System.

The six independent molecules fall into two groups of three nearly parallel molecules. All molecules are connected by carboxylto- carboxyl hydrogen bond pairs to form a continuous array of sixmolecule rings with a chicken-wire appearance. These arrays bend to assume two orientations, forming pleated sheets. Arrays in different orientations interpenetrate - three molecules in one orientation passing through the holes of three parallel arrays in the alternate orientation - to produce a completely interlocking network. One third of the carboxyl hydrogen atoms were found to be disordered.

II. Optical transforms as related to x-ray diffraction patterns are discussed with reference to the theory of Fraunhofer diffraction.

The use of a systems approach in crystallographic computing is discussed with special emphasis on the way in which this has been done at the California Institute of Technology.

An efficient manner of calculating Fourier and Patterson maps on a digital computer is presented. Expressions for the calculation of to-scale maps for standard sections and for general-plane sections are developed; space-group-specific expressions in a form suitable for computers are given for all space groups except the hexagonal ones.

Expressions for the calculation of settings for an Eulerian-cradle diffractometer are developed for both the general triclinic case and the orthogonal case.

Photographic materials on pp. 4, 6, 10, and 20 are essential and will not reproduce clearly on Xerox copies. Photographic copies should be ordered.