Characterization of biaxial crystals for tangentially phase-matched frequency conversion

Ultraviolet radiation has been generated by tangentially phase-matched sum-frequency mixing in biaxial L-arginine phosphate (LAP) crystal for the first time using Nd:YAG output at 1064 nm and Rh 6G dye laser output at 560 nm as the two input sources. Characterization has also been made of such a cheap, biaxial crystal for its possible use in devices for tangentially phase-matched short wavelength generation. If the crystal is of proper cut, thickness and quality so that its maximum capability can be exploited it can replace the potassium dihydrogen phosphate (KDP) group of crystals for various applications.

Applications of reverse saturable absorbers in laser science

A variety of applications exist for reverse saturable absorbers (RSAs) in the area of optical pulse processing and computing. An RSA can be used as power limiter/pulse smoother and energy limiter/pulse shortner of laser pulses. A combination of RSA and saturable absorber (SA) can be used for mode locking and pulse shaping between high power laser amplifiers in oscillator amplifier chain. Also, an RSA can be used for the construction of a molecular spatial light modulator (SLM) which acts as an input/output device in optical computers. A detailed review of the theoretical studies of these processes is presented. Current efforts to find RSAs at desired wavelength for testing these theoretical predictions are also discussed.

External-cavity semiconductor laser sensor

The spectral characteristics of a diode laser are significantly affected due to interference caused between the laser diode output and the optical feedback in the external-cavity. This optical feedback effect is of practical use for linewidth reduction, tuning or for sensing applications. A sensor based on this effect is attractive due to its simplicity, low cost and compactness. This optical sensor has been used so far, in different configuration such as for sensing displacement induced by different parameters. In this paper we report a compact optical sensor consisting of a semiconductor laser coupled to an external cavity. Theoretical analysis of the self- mixing interference for optical sensing applications is given for moderate optical feedback case. A comparison is made with our experimental observations. Experimental results are in good agreement with the simulated power modulation based on self-mixing interference theory. Displacements as small as 10-4 nm have been measured using this sensor. The developed sensor showed a fringe sensitivity of one fringe per 400nm displacement for reflector distance of around 10cms. The sensor has also been tested for magnetic field and temperature induced displacement measurements.

Measuring the linewidth of a stabilized diode laser

We demonstrate a straightforward technique to measure the linewidth of a grating-stabilized diode laser system - known as an external cavity diode laser (ECDL) - by beating the output of two independent ECDLs in a Michelson interferometer, and then taking the Fourier transform of the beat signal. The measured linewidth is the sum of the linewidths of the two laser systems. Assuming that the two are equal, we find that the linewidth of each ECDL measured over a time period of 2. s is about 0.3 MHz. This narrow linewidth shows the advantage of using such systems for high-resolution spectroscopy and other experiments in atomic physics.

Finite Element Solution Of Surface-Tension Driven Flows In Laser Surface-Melting

Lasers are very efficient in heating localized regions and hence they find a wide application in surface treatment processes. The surface of a material can be selectively modified to give superior wear and corrosion resistance. In laser surface-melting and welding problems, the high temperature gradient prevailing in the free surface induces a surface-tension gradient which is the dominant driving force for convection (known as thermo-capillary or Marangoni convection). It has been reported that the surface-tension driven convection plays a dominant role in determining the melt pool shape. In most of the earlier works on laser-melting and related problems, the finite difference method (FDM) has been used to solve the Navier Stokes equations [1]. Since the Reynolds number is quite high in these cases, upwinding has been used. Though upwinding gives physically realistic solutions even on a coarse grid, the results are inaccurate. McLay and Carey have solved the thermo-capillary flow in welding problems by an implicit finite element method [2]. They used the conventional Galerkin finite element method (FEM) which requires that the pressure be interpolated by one order lower than velocity (mixed interpolation). This restricts the choice of elements to certain higher order elements which need numerical integration for evaluation of element matrices. The implicit algorithm yields a system of nonlinear, unsymmetric equations which are not positive definite. Computations would be possible only with large mainframe computers.Sluzalec [3] has modeled the pulsed laser-melting problem by an explicit method (FEM). He has used the six-node triangular element with mixed interpolation. Since he has considered the buoyancy induced flow only, the velocity values are small. In the present work, an equal order explicit FEM is used to compute the thermo-capillary flow in the laser surface-melting problem. As this method permits equal order interpolation, there is no restriction in the choice of elements. Even linear elements such as the three-node triangular elements can be used. As the governing equations are solved in a sequential manner, the computer memory requirement is less. The finite element formulation is discussed in this paper along with typical numerical results.

Growth and characterization of $SrBi_{2}Nb_{2}O_{9}$ thin films by pulsed-laser ablation

Polycrystalline films of SrBi2Nb2O9 were grown using pulsed-laser ablation. The ferroelectric properties were achieved by low-temperature deposition followed by a subsequent annealing process. The lower switching voltage was obtained by lowering the thickness, which did not affect the insulating nature of the films. The hysteresis results showed an excellent square-shaped loop with results (P-r=6 mu C/cm(2), E-c=100 kV/cm) in good agreement with earlier reports. The films also exhibited a dielectric constant of 250 and a dissipation factor of 0.02. The transport studies indicated an ohmic behavior, while higher voltages induced a bulk space charge.

Growth and study of antiferroelectric lead zirconate thin films by pulsed laser ablation

Antiferroelectric lead zirconate (PZ) thin films were deposited by pulsed laser ablation on platinum-coated silicon substrates. Films showed a polycrystalline pervoskite structure upon annealing at 650 degrees C for 5-10 min. Dielectric properties were investigated as a function of temperature and frequency. The dielectric constant of PZ films was 220 at 100 kHz with a dissipation factor of 0.03. The electric field induced transformation from the antiferroelectric phase to the ferroelectric phase was observed through the polarization change, using a Sawyer-Tower circuit. The maximum polarization value obtained was 40 mu C/cm(2). The average fields to excite the ferroelectric state, and to reverse to the antiferroelectric state were 71 and 140 kV/cm, respectively. The field induced switching was also observed through double maxima in capacitance-voltage characteristics. Leakage current was studied in terms of current versus time and current versus voltage measurements. A leakage current density of 5x10(-7) A/cm(2) at 3 V, for a film of 0.7 mu m thickness, was noted at room temperature. The trap mechanism was investigated in detail in lead zirconate thin films based upon a space charge limited conduction mechanism. The films showed a backward switching time of less than 90 ns at room temperature.

Efficient Output-Sensitive Construction of Reeb Graphs

The Reeb graph tracks topology changes in level sets of a scalar function and finds applications in scientific visualization and geometric modeling. This paper describes a near-optimal two-step algorithm that constructs the Reeb graph of a Morse function defined over manifolds in any dimension. The algorithm first identifies the critical points of the input manifold, and then connects these critical points in the second step to obtain the Reeb graph. A simplification mechanism based on topological persistence aids in the removal of noise and unimportant features. A radial layout scheme results in a feature-directed drawing of the Reeb graph. Experimental results demonstrate the efficiency of the Reeb graph construction in practice and its applications.

Resolidification behaviour of laser surface-melted metals and alloys

The solidification behaviour is described of two pure metals (Bi and Ni) and two eutectic alloys (A1-Ge and AI-Cu) under nonequilibrium conditions, in particular the microsecond pulsed laser surface melting. The resolidification behaviour of bismuth shows that epitaxial regrowth is the dominant mechanism. For mixed grain size, regrowth of larger grains dominates the microstructure and can result in the development of texture. In the case of nickel, epitaxial growth has been noted. For lower energy pulse-melted pool, grain refinement takes place, indicating nucleation of fresh nickel grains. The A1-Ge eutectic alloy indicates the nucleation and columnar growth of a metastable monoclinic phase from the melt-substrate interface at a high power density laser irradiation. An equiaxed microstructure containing the same monoclinic phase is obtained at a lower power density laser irradiation. It is shown that the requirement of solution partition acts as a barrier to eutectic regrowth from the substrate. The laser-melted pool of A1-Cu eutectic alloy includes columnar growth of c~-A1 and 0-A12Cu phase followed by the dendritic growth of A12Cu phase with ct-Al forming at the interdendritic space. In addition, a banded microstructure was observed in the resolidified laser-melted pool.

X-ray diffraction of permalloy nanoparticles fabricated by laser ablation in water

Permalloy (NiFeMo) nanoparticles were fabricated by laser ablation of bulk material in water with a UV pulsed laser. Transmission electron microscope images showed that approximately spherical particles about 50 nm in diameter were formed in the ablation process. All diffraction peaks corresponding to the bulk material were present in the nanoparticles. In addition to these peaks several new peaks were observed in the nanoparticles, which were attributed to nickel oxide.

Ion beam processing of oriented CuO films deposited on (100) YSZ by laser ablation

A systematic study of Ar ion implantation in cupric oxide films has been reported. Oriented CuO films were deposited by pulsed excimer laser ablation technique on (1 0 0) YSZ substrates. X-ray diffraction (XRD) spectra showed the highly oriented nature of the deposited CuO films. The films were subjected to ion bombardment for studies of damage formation, Implantations were carried out using 100 keV Arf over a dose range between 5 x 10(12) and 5 x 10(15) ions/cm(2). The as-deposited and ion beam processed samples were characterized by XRD technique and resistance versus temperature (R-T) measurements. The activation energies for electrical conduction were found from In [R] versus 1/T curves. Defects play an important role in the conduction mechanism in the implanted samples. The conductivity of the film increases, and the corresponding activation energy decreases with respect to the dose value.

Determination of beta-turn conformation by laser Raman spectroscopy

To correlate the Raman frequencies of the amide I and III bands to beta-turn structures, three peptides shown to contain beta-turn structure by x-ray diffraction and NMR were examined. The compounds examined were tertiary (formula: see text). The amide I band of these compounds is seen at 1,668, 1,665, and 1,677 cm-1, and the amide III band appears at 1,267, 1,265, and 1,286 cm-1, respectively. Thus, it is concluded that the amide I band for type III beta-turn structure appears in the range between 1,665 and 1,677 cm-1 and the amide III band between 1,265 and 1,286 cm-1.

A laser flash photolysis study of pivalothiophenone triplets: steric and electronic effects in thione photoreaction kinetics

Upon laser pulse excitation (Aex = 532 nm) into the lowest-lying '(n,a*) band system, pivalothiophenones in benzene solutions give rise to short-lived triplets (Ama: = 325-335 nm, em: = (1 1-15) X lo3 M-' cm-I) with quantitative intersystem crossing efficiencies. The triplet yields decrease slightly (by 10-30%) upon changing A, to 308 nm (Le., upon excitation into S2). Kinetic data are presented for intrinsic triplet lifetimes, self-quenching, and quenching by oxygen, di-tert-butylnitroxy radical, and various reagents capable of interacting with the triplets via energy, electron, or hydrogen-atom transfer and by biradical formation (possibly leading to cycloaddition). The mechanisms of the quenching processes are discussed. Relative to rigid aromatic thiones, namely, xanthione and thiocoumarin, the interaction of pivalothiophenone triplets with most of the quenchers are kinetically inefficient. This is interpreted primarily as a manifestation of the steric crowding at positions a to the thiocarbonyl group.

Laser surface cladding of MRI 153M magnesium alloy with (Al+Al2O3)

Laser surface cladding was carried out on a creep-resistant MRI 153M magnesium alloy with a mixture of Al and Al2O3 powders using a pulsed Nd:YAG laser at scan speeds of 21, 42, 63 and 84 mm/s. The Al2O3 particles partially or completely melted during laser irradiation and re-solidified with irregular shapes in the size range of 5–60 µm along with a few islands as large as 500 µm, within the grain-refined Mg-rich dendritic matrix. More than an order of magnitude improvement in wear resistance after cladding was attributed to the presence of ultra-hard Al2O3 particles, increased solid solubility of Al and other alloying elements, and a very fine dendritic microstructure as a result of rapid solidification in the cladded layer. However, corrosion resistance of the laser cladded alloy was reduced by almost an order of magnitude compared to that of the as-cast alloy mainly due to the presence of cracks and pores in the cladded layer.

A non-linear analogue-to-digital converter for linearization of transducer input-output relationships

Analogue and digital techniques for linearization of non-linear input-output relationship of transducers are briefly reviewed. The condition required for linearizing a non-linear function y = f(x) using a non-linear analogue-to-digital converter, is explained. A simple technique to construct a non-linear digital-to-analogue converter, based on ' segments of equal digital interval ' is described. The technique was used to build an N-DAC which can be employed in a successive approximation or counter-ramp type ADC to linearize the non-linear transfer function of a thermistor-resistor combination. The possibility of achieving an order of magnitude higher accuracy in the measurement of temperature is shown.