Bursting behaviours in cascaded stimulated Brillouin scattering

Stimulated Brillouin scattering is studied by numerically solving the Vlasov-Maxwell system. A cascade of stimulated Brillouin scattering can occur when a linearly polarized laser pulse propagates in a plasma. It is found that a stimulated Brillouin scattering cascade can reduce the scattering and increase the transmission of light, as well as introduce a bursting behaviour in the evolution of the laser-plasma interaction. The bursting time in the reflectivity is found to be less than half the ion acoustic period. The ion temperature can affect the stimulated Brillouin scattering cascade, which can repeat several ×at low ion temperatures and can be completely eliminated at high ion temperatures. For stimulated Brillouin scattering saturation, higher-harmonic generation and wave-wave interaction of the excited ion acoustic waves can restrict the amplitude of the latter. In addition, stimulated Brillouin scattering cascade can restrict the amplitude of the scattered light.     

ZnO单晶的微区光致发光特性研究

ZnO单晶材料以其优良的综合性能在光电子器件方面掀起了研究热潮,因此对ZnO单晶的研究具有重要的理论和实践意义。采用激光辐照的方式,对ZnO单晶进行了光致发光(photoluminescence, PL)光谱实验,分析研究了ZnO单晶在不同温度(低温)和不同激光能量强度照射下其光致发光特性。研究结果表明,ZnO单晶内存在少量杂质及表面氧缺陷,这些结构对其发光特性有一定的影响;在低温条件下,ZnO单晶具有良好的发光特性,且随着温度的提高,发光光谱峰的位置会向长波长方向移动,但强度会减小;当激光光源的强度增大,ZnO单晶的PL发射光谱的强度也会随之增大,且峰的位置和相对强度不变。结合拉曼(Raman)光谱实验,从分子及原子振动、转动类型验证了纤锌矿ZnO单晶的六方晶系结构;配合X射线衍射(X-ray diffraction, XRD)技术,得出ZnO单晶良好的结晶特性以及晶轴取向。     

Determination of elastic,piezoelectric,and dielectric constants of an R:BaTiO₃ single crystal by Brillouin scattering

From the sound velocity measured using the Brillouin scattering technique,the elastic,piezoelectric,and dielectric constants of a high-quality monodomain tetragonal Rh:BaTiO3 single crystal are determined at room temperature.The elastic constants are in fairly good agreement with those of the BaTiO3 single crystal,measured previously by Brillouin scattering and the low-frequency equivalent circuit methods.However,their electromechanical properties are significantly different.Based on the sound propagation equations and these results,the directional dependence of the compressional modulus and the shear modulus of Rh:BaTiO3 in the(010) plane is investigated.Some properties of sound propagation and electromechanical coupling in the crystal are discussed.     

Influence of atomic order on TA1[110] phonon softening [4] and displacive phase transition in Fe72Pt28 Invar alloys

The acoustic phonon dispersions of two Invar crystals , one ordered with the () structure, the other disordered fcc, have been investigated between 3.4 K and 470 K by inelastic and elastic neutron scattering. For the ordered crystal, pronounced softening of the whole phonon branch is observed on cooling below the Curie temperature. Particularly strong phonon softening at the M-point zone boundary of the structure leads to a displacive, antiferrodistortive phase transition at low temperatures. For the disordered crystal, much weaker softening of the phonons is observed and restricted to the region near the Brillouin zone center, where increasing elastic scattering with decreasing temperature indicates the growth of local tetragonal strain. This strain is considered as a typical precursor of the transformation to bct martensite. Specific heat measurements, performed at low temperatures on both crystals confirm the neutron scattering results and reveal considerable enhancement of the low energy phonon density of states in the ordered crystal. Received 18 January 1999     

Determination of elastic, piezoelectric, and dielectric constants of an R:BaTiO3 single crystal by Brillouin scattering

From the sound velocity measured using the Brillouin scattering technique, the elastic, piezoelectric, and dielectric constants of a high-quality monodomain tetragonal Rh：BaTiO3 single crystal are determined at room temperature. The elastic constants are in fairly good agreement with those of the BaTiO3 single crystal, measured previously by Brillouin scattering and the low-frequency equivalent circuit methods. However, their electromeehanical properties are significantly different. Based on the sound propagation equations and these results, the directional dependence of the compressional modulus and the shear modulus of Rh：BaTiO3 in the （010） plane is investigated. Some properties of sound propagation and electromechanical coupling in the crystal are discussed.     

Acoustic properties of the disordered relaxor ferroelectric PbSc1/2Ta1/2O3

The behavior of acoustic phonons in crystals of a relaxor ferroelectric, namely, the lead scandium tantalate PbSc_1/2Ta_1/2O₃ (PST), is studied in the vicinity of the diffuse phase transition. The behavior of longitudinal and transverse acoustic phonons in a PST single crystal is examined using Brillouin scattering. The phonon subsystem is found to behave anomalously in the vicinity of T = 297 K, which can probably be assigned to the existence of a phase transition. Analysis of the results obtained yields the values of the elastic moduli C₁₁, C₁₂, and C₄₄ for the cubic phase of the crystal over a wide temperature range.     

光子晶体光纤中去极化型声波导布里渊散射频移和散射效率研究

利用矢量有限元法,数值模拟了不同结构参数下光子晶体光纤中空气孔填充率与去极化型声波导布里渊散射频移的关系,以及布里渊散射频移与布里渊散射效率的关系。研究结果表明,空气孔填充率的增大会导致扭转径向模式的去极化型声波导布里渊散射频移下移。空气孔节距或纤芯直径一定时,高阶模式的频移对空气孔填充率的变化更为敏感。扭转径向模式的去极化型布里渊散射效率都随布里渊散射频移的增大而增大。     

Determination of elastic properties of Si/Ge superlattices and Si1-xGex films from surface acoustic modes by Brillouin scattering

Surface acoustic waves (SAW) have been measured by means of Brillouin scattering (BS) both as a function of k×h and the direction of k in the sample plane (k is the wavevector of the surface acoustic mode and h the thickness of the film). The velocity of the Rayleigh wave on sufficiently thick films (h > 4000 Å) has been experimentally found to ve uneffected by the elastic properties of the substrate material. Thus the directional dependence of the hypersonic surface wave is completely determined by the elastic properties of the layer material alone and reflects its crystallographic symmetry. The SL's can be treated as media with effective elastic constants because the wavelength of the thermally excited Rayleigh wave is much longer than the SL period. Furthermore, the angular dispersion of the SAW can be used to calculate the elastic constants of each film separately.     

Stimulated Brillouin scattering in magnetized diffusive semiconductor plasmas

Using the hydrodynamical model and following the coupled mode approach, detailed analytical investigation of stimulated Brillouin scattering is performed in an electrostrictive semiconductor. The total induced current density including diffusion current density and the effective Brillouin susceptibility are obtained under off-resonant laser irradiation. The analysis deals with the qualitative behaviour of the Brillouin gain and transmitted intensity with respect to excess doping concentration and magnetic field. Efforts are directed towards optimizing the doping level and magnetic field to achieve maximum Brillouin gain at pump intensities far below the optical damage threshold level. It is found that by immersing a moderately doped semiconductor in a sufficiently strong magnetic field in transverse direction, one can achieve resonant enhancement of Brillouin gain provided the generated acoustic mode lies in the dispersionless regime.     

Elastic surface wave anomalies near the cluster-to-layer transition of Au on NaCl

The cluster-to-layer transition of Au on NaCl(001) has been investigated by means of Brillouin scattering from elastic surface wave excitations. In the cluster regime the softening of the Rayleigh surface mode with increasing cluster size is paralleled by that of a new acoustic surface mode, which appears in the frequency range above the transverse acoustic (TA) phonon mode of NaCl. The frequency crossing of the new mode and the TA mode marks the percolation threshold.     

Coherent Brillouin spectroscopy in a strongly scattering liquid by picosecond ultrasonics

In a modification of a picosecond ultrasonic technique, a short acoustic pulse is launched into a liquid sample by a laser pulse absorbed in a semitransparent transducer film and is detected via coherent Brillouin scattering of a time-delayed probe pulse. With both excitation and probing performed from the transducer side, the arrangement is suitable for in vivo study of biological tissues. The signal is collected from a micrometer-thick layer next to the transducer and is not affected by the diffuse scattering of probe light deeper in the sample. The setup, utilizing a 33 nm thick single crystal SrRuO(3) transducer film, is tested on a full fat milk sample, with 11 GHz acoustic frequency recorded.     

The study of guided waves in surfaces and thin supported films using surface Brillouin scattering and acoustic microscopy

This paper reviews the use of surface Brillouin scattering (SBS) and acoustic microscopy (AM) in studying the surface dynamics of solids in order to obtain information about the near-surface elastic properties of solids and thin supported films. The vibrational modes that are probed by these means include Rayleigh surface and pseudo-surface acoustic waves, longitudinal lateral waves (surface skimming bulk longitudinal waves) and various thin film guided modes, such as Sezawa and Love waves. SBS is the inelastic scattering of light, mediated by thermodynamic fluctuations in the surface elevation and near surface elastic strains. The scattering cross-section is conveniently expressed in terms of Fourier domain elastodynamic Green's functions. AM depends on the insonification of a surface through a coupling fluid, and the resulting excitation and subsequent decay of the various surface modes. The complex reflectivity of the fluid-loaded surface, and the line and point force surface Green's functions are invoked in the interpretation of different modalities of AM, yielding much the same information about the surface dynamics. The focus in this paper is on the Green's function approach. A number of illustrative examples, drawn from the authors' research, are provided.     

Acoustic properties of proton exchanged LiNbO3 investigated by Brillouin scattering

The propagation of surface acoustic waves at microwave frequencies (10¹⁰ Hz) was studied on proton exchanged LiNbO₃ crystals by means of Brillouin scattering. The proton exchange causes a large velocity reduction for surface acoustic waves propagating in the x–y plane of ay-cut crystal as well as for longitudinal bulk acoustic waves travelling in the proton exchanged sub-surface region. The velocity reduction amounts to about 20% for both types of waves. The corresponding elastic constants are reduced even by about 40% since the density remains almost constant. This softening seems to involve both the shear and compressional elastic constants, but in an anisotropic way.Thus by proton exchange it is possible to build acoustic waveguides adjacent to the surface, similar to the construction of optical waveguides. By a lateral control of the proton exchange rate optical elements for ultrasonic waves, for example, acoustic lenses can be produced without deformation of the flat surface.The absorption of surface acoustic waves on proton exchanged surfaces is stronger than on pure LiNbO₃ indicating a novel absorption mechanism becoming active in the proton exchanged material.     

Brillouin scattering in the layer compound GaSe

Brillouin scattering has been used to obtain the five independent elastic constants in the layer compound GaSe. The results show clear elastic anisotropy of the crystal. Resonant Brillouin scattering near the absorption edge has also been studied, but no resonant enhancement was found.     

Theoretical and experimental study of laser induced damage on GaAs by nanosecond pulsed irradiation

The surface damage experiments of gallium arsenide (GaAs) single crystal irradiated by 1.06 and 0.53 μm nanosecond irradiations are carried out with fundamental and frequency-doubled Nd:YAG laser, respectively. The surface damage thresholds for both wavelengths are experimentally determined and the damaged morphologies and elementary component are analyzed with electron probe microanalyzer (EPM). It is found that the components of Ga and As almost keep constant in our experiments when the irradiated fluence is just around the surface damage threshold and no oxygen is found at all. The theoretical calculations on temperature rise for both wavelengths are carried out using the purely thermal model. It is shown that for irradiation with photon energy above the corresponding band gap the theoretical calculation is in good agreement with the experimental results; however, for that with photon energy just below the band gap, the experimental results cannot be effectively explained by the purely thermal heating mechanism. Combining with the experiment of multi-shot damage from references we finally conclude that the damage by laser irradiation with photon energy below the band gap should be explained by the micro-defect accumulation and consequently enhanced absorption heating mechanism.     

Brillouin light scattering on the two quasi-transverse waves in rotated sample of the LiNbO3 crystal

The paper presents results for the Brillouin light scattering experiment, using a LiNbO₃ crystal, and identifies Brillouin scattering switch from one quasi-transverse acoustic branch to the second one during sample rotation. Measured hypersonic frequencies of quasi-transverse acoustic waves observed in different crystallographic directions are examined. A selection rule for the switching phenomenon is described.     

Brillouin Scattering Study on the single crystals ofl-arginine phosphate (LAP)

This is the first report representing Brillouin Scattering Studies on single crystals of monoclinic LAP, which is a new organic material very useful for secondary harmonic generation. The crystal elastic constants were evaluated from the measured data. The curves of the reciprocal velocity and the acoustic velocity as a function of temperature are given here.     

High reflectivity phase conjugation in magnetized diffusion driven semiconductors

Optical phase conjugation via stimulated Brillouin scattering (OPC-SBS) in magnetized diffusion driven semiconductors under the off-resonant transition regime has been investigated theoretically. The model is based upon the coupled-mode approach and incorporates the effect of pump absorption through the first-order induced polarization. The linear dispersion is found not to affect the reflectivity of the phase conjugate Stokes shifted Brillouin mode. The reflectivity of the image radiation is dependent upon the Brillouin susceptibility and can be significantly enhanced through n-type doping of the crystal and the simultaneous application of magnetic field. Moreover, the threshold of the pump intensity required for the occurrence of SBS in the crystal with finite optical attenuation can be considerably diminished through a suitable choice of the excess carrier concentration and the magnetic field. Consequently, OPC-SBS becomes a possible tool in phase-conjugate optics even under not-too-high power laser excitation by using moderately doped n-type semiconductors kept under the influence of magnetic field. Numerical estimates made for n-InSb crystal at 77 K duly irradiated by nanosecond pulsed 10.6 μm CO₂ laser show that high OPC-SBS reflectivity (70%) can be achieved at pump intensities below the optical damage threshold if the crystal is used as an optical waveguide with relatively large interaction length (L ∼5 mm) which proves its potential in practical applications such as fabrication of phase conjugate mirrors.     

飞秒激光诱导ZnO :Al薄膜周期结构及其光致发光特性

利用单束800 run飞秒激光在掺杂了Al的ZnO薄膜中制备了纳米周期条纹结构.研究了不同能流密度的飞秒激光在照射不同的时间后,表面纳米周期结构的变化规律及其形成机制.利用He-Ge激光器作为激发光源,研究了ZnO:Al薄膜的光致发光特性及其与纳米周期结构的关系.结果表明,近带隙发光增强的主要原因是800 nm飞秒激光...     

Resonant Brillouin scattering near the a exciton in CdS

We have studied resonant Brillouin scattering in CdS by tuning a narrow-band cw dye laser through the A-exciton transition at helium temperatures. We observed besides the resonance enhancement a pronounced asymmetry of the Stokes and anti-Stokes shift and a dramatic increase of the Brillouin shifts as a function of laser frequency. These observations are consistent with a model of exciton-polariton scattering by longitudinal acoustic phonons. For incident frequencies higher than the longitudinal exciton frequency additional features are found which cannot be explained by the two-branch model for exciton-polaritons.