Forming of the Optical Beam with the Rotating Polarization Vector

A method for the optical beam production with the rotating polarization vector based on the interference of two beams with the circular polarizations is proposed. The frequency shift between beams is implemented by means of acousto-optic (AO) diffraction. The method is used for the amplitude light modulation with the frequency nf where f is acoustic frequency and n is integer. AO modulators are fabricated from paratellurite crystal. Modulators allow modulating the optical radiation with wavelength of 0.63 mcm at the quadruple frequency of the acoustic wave. The modulation frequency achieves 180 MHz. Keywords: acousto-optic diffraction, Bragg regime, frequency shift, rotating polarization vector

Interaction of Lamb modes with an inclusion

The interaction of Lamb modes propagating in a steel plate containing a thin inclusion is analyzed for cases where the inclusion material has elastic parameters similar to the ones of the plate, and where the inclusion is in perfect mechanical contact with the surrounding plate material. A modal decomposition method is used to calculate the conversion of an incident Lamb mode to other modes. Hence, the influence of the type of incident mode, of the location and geometry of the inclusion, and of the elastic parameters of the inclusion and plate material on the mode conversion coefficients is analyzed. Besides the expected increase of the conversion efficiency with increasing cross section of the inclusion, it is found that due to reasons of symmetry, the presence of an inclusion leads to an efficient conversion of an incident S0 mode into reflected and transmitted A0 modes, unless the inclusion is located very close to the plate center. On the other hand, the conversion efficiency of an incident A0 mode into a reflected A0 mode is found to be strongly dependent on the depth of the inclusion, this conversion even disappearing for some location depths. For the cases studied, the inclusion location dependence of the mode conversion seems to be correlated with the normal profile of the longitudinal normal stress component. As intuitively expected, the mode conversion efficiency increases with the mismatch of an acoustic impedance like factor between the uniform plate and the inclusion region.status: publishe

X-valley influence on hot free electron absorption and optical nonlinearities at 10.6

A theoretical overview is given about the influence of the presence of the X-valley in highly doped n-GaAs on hot free-electron absorption and optical nonlinearities at 10.6 μm wavelength. The implications of the extension of the quantum-mechanical model from two to three valleys are discussed. For electron temperatures above 600 K the X-valley presence starts to be observed. We reveal that it is difficult to trace the individual contributions of different X-electron related inter- and intravalley absorption and relaxation phenomena and therefore we suggest to introduce an effective X-valley related deformation potential which is a weighted combination of all the X-valley contributions. We discuss how nonlinear optical experiments can be conducted to determine the LL-intervalley and this effective X-valley deformation potential

A multi-valley model for hot free-electron nonlinearities at 10.6

When the frequency of infrared light and the plasma frequency of highly doped n-GaAs are in resonance (e.g. for a doping concentration N = 7 × 1018cm−3 and a wavelength $\lambda=10.6 \mu$m), the free-electron induced optical nonlinearity is soundly pronounced. At such high doping concentrations it is necessary to extend the rigid quantum mechanical description of the free-electron induced nonlinearity to a multi-valley model. The central valley of GaAs was treated as a fully nonparabolic degenerated electron gas, whereas the satellite valley was modeled as an anisotropic electron gas of arbitrary degeneracy. The following intra- and intervalley absorption mechanisms were taken into account: impurity assisted, thermal and hot polar optical phonon assisted intravalley absorption on one hand and intervalley phonon assisted absorption in equivalent and nonequivalent intervalley absorption on the other hand. The dependence of the different absorption and energy relaxation mechanisms on the doping concentration, free electron heating, optical power density and the equivalent LL-intervalley deformation potential are discussed. We demonstrated for the first time that the behavior of the optical intervalley nonlinearity, i.e. the nonlinear absorption and nonlinear intervalley transfer, strongly depend on the equivalent LL-intervalley deformation potential. In the linear regime the model calculations are in good agreement with experimental results

Low threshold for subharmonic generation in fractal piezoelectric composites

We observed subharmonic Lamb modes in composite piezoelectric plates with self-similar structure. The thresholds for their generation are very low as compared to those of homogeneous or periodical plates with comparable acoustical and electrical parameters. In the frame of a first order perturbative theory, this can be explained in terms of the unusually favourable frequency and spatial matching of coupled localized and extended modes in these structures

Determination of thermoelastic material properties by differential heterodyne detection of impulsive stimulated thermal scattering

The underlying working principle of detecting impulsive stimulated scattering signals in a differential configuration of heterodyne diffraction detection is unraveled by involving optical scattering theory. The feasibility of the method for the thermoelastic characterization of coating-substrate systems is demonstrated on the basis of simulated data containing typical levels of noise. Besides the classical analysis of the photoacoustic part of the signals, which involves fitting surface acoustic wave dispersion curves, the photothermal part of the signals is analyzed by introducing thermal wave dispersion curves to represent and interpret their grating wavelength dependence. The intrinsic possibilities and limitations of both inverse problems are quantified by making use of least and most squares analysis.publisher: Elsevier articletitle: Determination of thermoelastic material properties by differential heterodyne detection of impulsive stimulated thermal scattering journaltitle: Photoacoustics articlelink: http://dx.doi.org/10.1016/j.pacs.2015.05.001 content_type: article copyright: Copyright © 2015 The Authors. Published by Elsevier GmbHstatus: publishe

LOW-THRESHOLD SUBHARMONIC GENERATION IN COMPOSITE STRUCTURES WITH CANTOR-LIKE CODE

We show experimental evidence of extremely low thresholds for subharmonic generation of ultrasonic waves in one-dimensional artificial piezoelectric plates with Cantor-like structure, as compared to the corresponding homogeneous and periodical plates. The origin of this apparent anomaly is theoretically investigated by studying anharmonic coupling between normal modes. We demonstrate that the large enhancement of nonlinear interaction results from the more favorable frequency and spatial matching of coupled modes (fractons and phonons) in the Cantor-like structure, with no need to invoke anomalous modifications of the nonlinear elastic constants