Optical parametric generation of a midinfrared continuum in orientation-patterned GaAs

We have generated an ultrabroad mid-infrared continuum by using single-pass optical parametric generation (OPG) in orientation-patterned GaAs (OP-GaAs). The spectrum spans more than an octave, from 4.5 to 10.7 m, measured 20 dB down from the peak. The 17.5 mm long, 0.5 mm thick, all-epitaxially-grown OPGaAs sample with a 166.6-m quasi-phase-matching period was pumped with 3.1-3.3 m wavelength, 1 ps pulses up to 2 J in energy. The OPG threshold was observed at 55 nJ pump energy with the pump polarized along the [111] crystal direction. The slope efficiency near threshold was 51%, and the external conversion efficiency was as high as 15%. © 2006 Optical Society of America OCIS codes: 190.2620 Gallium arsenide is a promising material for parametric frequency conversion because of its large nonlinear susceptibility (d 14 = 94 pm/ V for frequency doubling of f =4 m), 1 broad infrared transparency range ͑ = 0.9-17 m͒, and high thermal conductivity. Also of interest are its highly symmetric nonlinear susceptibility tensor and linear optical isotropy, which facilitate nonlinear optical interactions that use a rich variety of polarization configurations. However, the optical isotropy of GaAs prevents birefringent phase matching; efficient interactions can instead be obtained by quasi-phase-matching. Orientation-patterned GaAs (OP-GaAs) is a type of quasi-phase-matched (QPM) GaAs in which periodic inversions of the crystallographic orientation are grown into the material. The advantage of OP-GaAs compared with other QPM GaAs methods [such as diffusion-bonded GaAs (Ref. 2) and Fresnel QPM GaAs (Ref. 3)] is that OP-GaAs has lithographically defined periods, which permit excellent periodicity control and access to small QPM periods. To fabricate OP-GaAs, a process based on photolithography and molecular beam epitaxy is used to grow a thin-film template with periodic crystal inversions. 4,5 A thick film (0.5-1 mm) is then grown upon this template by hydride vapor phase epitaxy to produce bulk OP-GaAs. 6,7 With OP-GaAs, efficient second-harmonic generation, 1 difference-frequency generation, Anomalously wide tuning bandwidths for parametric processes are associated with wavelength 0 , where the group-velocity dispersion, d 2 k /d 2 , of a material goes to zero. 11 If one pumps a nonlinear crystal at 0 / 2, broadband gain around the degenerate signal-idler wavelength can be obtained. Use of OPG is a convenient way to investigate the broadband gain that occurs for GaAs when pumping at ϳ 0 / 2 = 3.31 m. Slightly detuning the pump from 0 / 2 and choosing an appropriate QPM period to compensate for the phase-mismatch results in even broader gain bandwidths (at the expense of some gain variation), as plotted i

Planform selection in two-layer Benard-Marangoni convection

Benard-Marangoni convection in a system of two superimposed liquids is investigated theoretically. Extending previous studies the complete hydrodynamics of both layers is treated and buoyancy is consistently taken into account. The planform selection problem between rolls, squares and hexagons is investigated by explicitly calculating the coefficients of an appropriate amplitude equation from the parameters of the fluids. The results are compared with recent experiments on two-layer systems in which squares at onset have been reported.Comment: 17 pages, 7 figures, oscillatory instability included, typos corrected, references adde

Absolute measurements of the soft X-ray emission from vacuum spark discharges

Low-inductance vacuum spark discharges display the occurence of hot tiny micropinches, which consist of anode material and which are intensive emitters of x-ray radiation. This report presents absolute measurements of the brightness and the energy emitted in the wavelength interval 7-14 Aaand in the water window between 24-27 Aa. Elements were Mg, Al, Ti, and Cu. The energy in the resonance line of He-like Mg can reach 100 mJ. (orig.)SIGLEAvailable from TIB Hannover: D.Dt.F.QN1(12,7)+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Forschung und Technologie (BMFT), Bonn (Germany)DEGerman

Onset of Absolute Instability Induced by Viscous Dissipation in the Poiseuille-Darcy-Benard Convection of a Newtonian Fluid

The present paper investigates the transition from convective to absolute instability induced by viscous dissipation. As far as the authors are aware, this is the first time such a study is reported in the literature. Its framework is provided by the Poiseuille-Darcy-B ́enard convection of a Newtonian fluid. We found the same behaviour observed in the absence of viscous dissipation whenever the Gebhart number is smaller than Ge 4.31, the cross flow always destabilises this onset. The latter two conditions create a scenario where viscous dissipation is capable of inducing a transition to absolute instability in the absence of wall heating, i.e. with a zero Rayleigh number