Temperature Dependence of Raman Scattering in 4H-SiC Films under Different Growth Conditions

The microRaman scattering of 4H-SiC films, fabricated by low pressure chemical vapor deposition under different growth conditions, is investigated at temperatures ranging from 80 K to 550K. The effects of growth conditions on E2 （TO）, E1 （TO） and A1 （LO） phonon mode frequencies are negligible. The temperature dependences of phonon linewidth and lifetime of E2 （TO） modes are analyzed in terms of an anharmonic damping effect induced by thermal and growth conditions. The results show that the lifetime of E2 （TO） mode increases when the quality of the sample improves. Unlike other phone modes, Raman shift of A1 （longitudinal optical plasma coupling （LOPC）） mode does not decrease monotonously when the temperature increases, but tends to blueshift at low temperatures and to redshift at relatively high temperatures. Theoretical analyses are given for the abnormal phenomena of A1 （LOPC） mode in 4H-SiC.

An application of radiation boundary condition to scattering problem of acoustic waves in fluids

A numerical method of solving acoustic wave scattering pnblem in fluids is described. Radiation boundary condition (RBC) obtained by factorization method of Helmholtz equation is applied to transforming the exterior boundary value problem in unbounded region into one in a finite region. Combined with RBC and scatterer surface boundary condition, Helmholtz equation is solved numerically by the finite difference method. Computational results for sphere and prolate spheroidal scatterers are in excellent agreement with eigenfunction solutions and much better than the results of OSRC method.

Influence of wave front healing on seismic tomography

Wave front healing is a common natural phenomenon.To further investigate wave front healing,we simulated wave propagation in a spherical anomaly surrounded by homogeneous media using a high-order finite difference solution of the acoustic equation.Furthermore,we analyzed the characteristics of the wave propagation in the anomaly,and found that they are related to the dominant frequency of the seismic wave and the dimensions of the anomaly.Through quantitative comparison of the wave front energy of the diffracted wave and transmitted wave,we summarized the influences of the wave front healing on seismic tomography.We conclude that,under the strong scattering condition,only positive anomalies can be inverted by ray-based tomography,only large anomalies can be inverted by finite-frequency tomography,and small negative anomalies cannot be inverted by any first-arrival traveltime tomographic methods.These conclusions are verified by tomographic experiments based on different theoretical models.Finally,we propose that more information besides the first-arrival traveltime should be used to invert the high wave number components of the media.Besides the above acquisitions of wave front healing on seismic tomography,we explain the banana-doughnut phenomena,and offer a new insight into the wave scattering,which should be important for better understanding the wave propagation and seismic inversion.

Distance Estimation in Ultrasound Images Using Specific Decorrelation Curves

Speckle decorrelation algorithm is a method using decorrelation curves to estimate the distance between two neighbor- ing ultrasound images. In this paper, we propose a new method to obtain specific decorrelation curves for distance estimation. First, several datasets of synthetic ultrasound （US） images are obtained by scanning different scatters. Second, based on the US datasets, we compute low-order moments and the elevational decorrelation curves. Finally, low-order moments are used to classify different scattering conditions. The suitable decorrelation curves can be acquired when the scattering style has been determined. With these steps, the relationship between low order moments and the decor- relation curves is established by the scattering conditions. This relationship proves to be efficient and applicable in the experiment section. The decorrelation curves chosen according to the rela- tionship also perform well in the distance estimation test.