Evolution of growth and enhancement in power factor of InSb bulk crystal

Indium antimonide crystals were synthesized from the respective component elements using the vertical Bridgman technique. The grown crystals were characterized by using X-ray analysis, EDX, electrical conductivity and thermoelectric power measurements. The calculated structural parameters for the prepared crystal have a good agreement with the standard values. Crystallite size (D) of the obtained InSb crystals was calculated to be 62.4 nm. The measurements reveal higher values for Seebeck coefficient, electrical conductivity and power factor than the published results for the same compound.     

Weak surface energy in nematic dispersions: Saturn ring defects and quadrupolar interactions

The role of surface energy in the behavior of colloidal particles in liquid crystalline phases is investigated. When the surface energy dominates, a hedgehog defect is formed and, according to an electrostatic analogy, the distortions around the particles exhibit a dipolar character. By contrast, for weaker anchoring, the configuration becomes quadrupolar as evidenced by the structure of latex clusters in lyotropic systems and the observation of defects reminiscent of Saturn rings in thermotropic systems. Received 21 June 1999     

Electron capture into few-electron heavy ions: Independent particle model

We apply the density matrix theory to re-investigate the radiative electron capture into heavy ions with one valence electron. Attention has been paid particularly to the magnetic sublevel population of the residual ions, as described in terms of alignment parameters. Simple method, based on an independent particle model, which takes into account the Pauli principle, is proposed for evaluating the alignment of the excited ionic states. By making use of this method, detailed calculations are performed for electron capture into (initially) hydrogen-like and lithium-like europium, gold and uranium ions, and are compared with the results of the multiconfiguration Dirac-Fock approach.As seen from the calculations and from the comparison with available experimental results, the independent particle model provides a good estimate for the alignment parameters of few-electron heavy ions. Therefore, our simple model may help to understand the basic properties of the X-ray emission from heavy, few-electron ions without the need for invoking sophisticated MCDF calculations.     

Identification of a TGBA liquid crystal phase via its defects

We have shown that cholesteryl nonanoate, a thermotropic compound which is well known to exhibit pretransitional effects at the smectic A (SmA) cholesteric (N*) transition (W.L. McMillan, Phys. Rev. A 4, 1238 (1971); 6, 936 (1972)), has in fact a TGBA phase in between. Our arguments rely on the observation of new TGBA defects, either in Robinson spherulites cooled from the N* phase or in free-standing films. The same new defects can be obtained in a well-documented TGBA phase of a tolane compound. We analyze qualitatively the TGBA defects in both geometries, in particular their relation to the disclination radius of the N* Robinson spherulites. Received 12 February 2001     

Folded diffractive laser resonators with user-defined fundamental mode

The adjustment effort required for the alignment of laser resonators with arbitrarily shaped end mirrors can be significantly reduced by folding the resonator path half-way between the two end mirrors and by slightly tilting the resonator axis. Thus the resulting resonator consists only of one diffractive mirror and a plane mirror, and the separate alignment of both diffractive end mirrors with respect to each other can be avoided. This principle is demonstrated by a Nd:YAG laser with super-Gaussian output beam. Furthermore, it is shown theoretically that the influence of phase quantization effects on the beam quality can be strongly reduced as a consequence of the tilted resonator axis.     

Generation of a radially polarized doughnut mode of high quality

We present an experimental set-up to generate laser beams with locally varying polarization distribution. In a linear set-up, a radially polarized beam of high quality regarding intensity distribution, polarization and phase-front distortion is generated. This beam can be used for tight focusing. Further applications are discussed.     

Second-harmonic generation of 405-nm light using periodically poled KTiOPO4 pumped by external-cavity laser diode with double grating feedback

A frequency-doubled laser diode system for generation of blue–UV light is described. The system is based on an external-cavity high-power laser diode with double feedback from the zeroth and the first orders of a diffraction grating. Light at 405 nm is generated in a single-pass configuration using periodically poled KTiOPO₄. We show that the double grating feedback improves the second harmonic conversion efficiency by several orders of magnitude as compared to the freely running laser. The conversion efficiency may be improved further such that higher second-harmonic powers may be generated.     

Generation of subpicosecond infrared pulses tunable between 5.2 μm and 18 μm at a repetition rate of 76 MHz

2 (2 mm thick) and GaSe (1 mm thick) crystals are applied as nonlinear media. GaSe has a larger tuning range and is more efficient in the whole spectral range than AgGaS₂. The average IR power reaches up to 2.0 mW at 8.5 μm when the GaSe crystal is used, and up to 1.3 mW at the same wavelength when AgGaS₂ is applied for the frequency conversion. Received: 17 February 1997/Revised version: 14 July 1997     

Mode profiles in resonators with a near 90° roof-top mirror

The transverse modes of a laser resonator comprising a near 90° roof-top mirror and a feedback mirror are studied theoretically. We relate this configuration to an almost plane roof-top resonator and apply the Fox and Li technique to compute the lowest-order symmetric and antisymmetric modes. The effects of perturbations to the vertex angle of the roof-top mirror, misalignments of the feedback mirror and curved feedback mirrors are examined.     

First-order optical systems with real eigenvalues

It is shown that a lossless first-order optical system whose real symplectic ray transformation matrix can be diagonalized and has only real eigenvalues, is similar to a separable hyperbolic expander in the sense that the respective ray transformation matrices are related by means of a similarity transformation. Moreover, it is shown how eigenfunctions of such a system can be determined, based on the fact that simple powers are eigenfunctions of a separable magnifier. As an example, a set of eigenfunctions of a hyperbolic expander is determined and the resemblance between these functions and the well-known Hermite-Gauss modes is exploited.