Techniques for coherent phonon excitation and stimulated Brillouin scattering

The basic principle of a rotating chisel-shaped phonon coupling and detection probe, believed to be novel, is presented for the measurement of coherent phonon propagation and interaction characteristics.     

Brillouin scattering by surface acoustic modes for elastic characterization of ZnO films

Brillouin scattering from surface phonons was used for determining the dispersion curves of guided acoustic modes propagating along piezoelectric ZnO films. Measurements were performed on films of different thicknesses in the range between 20 and 320 nm, deposited by RF magnetron sputtering on Si and SiO(2) substrates. Brillouin spectra from Rayleigh acoustic modes are taken in the backscattering geometry at different incidence angles between 30 degrees and 70 degrees . The experimental data for the ZnO/Si films fit the expected theoretical dispersion curves fairly well for film thicknesses greater than 150 nm, while they appreciably depart from the same curves for smaller thicknesses. This behavior is interpreted in terms of a reduction of the effective elastic constants of the film in a layer near the interface, due to the lattice misfit between the film and the substrate. This effect was not observed in the case of ZnO films deposited on fused quartz substrates.     

Computation of Brillouin scattering cross-sections for multilayers

Acoustic Green's function allows the analysis of localisation of acoustic modes and of their possible confinement. Electromagnetic Green's function allows a consistent analysis of both scattering mechanisms (elasto-optic and ripple) and their interference. The computed cross-section is directly comparable with experimental spectra.     

Brillouin scattering measurements of the temperature dependence of sound velocity and acoustic absorption in simple alcohols

A scanning Fabry–Perot interferometer was used to measure Brillouin spectra of methanol, isopropanol and a 95% ethanol–water mixture for temperatures ranging between 285?K and 320?K. The Brillouin frequency shifts and linewidths were used to calculate the velocities and absorption coefficients of hypersonic acoustic waves in these liquids. The temperature dependence of sound speed and acoustic attenuation was determined. For all three materials, both sound velocity and absorption coefficient decreased with temperature.     

New surface integral equations for the light scattering of multi-metallic nanoscatterers

Based on the Stratton–Chu formulation, a set of new surface integral equations of Maxwell's equations was developed to investigate the interactions of an incident EM wave with multi-metallic nanoscatterers for a two-dimensional TM-mode problem. These equations in terms of the surface components of the tangential magnetic field H_z, the normal displacement field D_n and the tangential electric field E_t can be solved systematically along the multi-connected interfaces of the scatterers and the host by using boundary-element method (BEM). Three interesting nanostructures (a dimer, a trimer and a hexagonal lattice) are studied to show the distinct optical responses of the coupling surface plasmon resonance. The numerical results show that even though the distance between the two nanoscatterers is very close (e.g. gap is less than 1 nm), these surface integral equations still can obtain a converged result of the surface components by increasing adaptively the number of the discretized nodes along the adjacent multi-connected boundary. In addition, an asymmetric dimer, which contains two scatterers of different diameters in different orders of magnitude, is studied to demonstrate the ability of BEM for solving multi-scale problems.     

High-order perturbation theory for light scattering from a rough metal surface.

The angular distribution of the mean diffuse intensity scattered from a metal surface with one-dimensional roughness is studied with perturbation theory. From an approach based on the reduced Rayleigh equations in p polarization, exact perturbation terms up to eighth order in the height parameter are developed for surface roughness consistent with a stationary Gaussian process. The theory is evaluated for a number of cases in which surface plasmon polariton excitation is significant and produces effects such as backscattering enhancement. For surface roughness having a wide Gaussian power spectrum, it is found that the high-order terms lead to roughness-induced broadening of the backscattering peak. For rectangular spectra, two cases are studied in which backscattering effects are due to sixth- and eighth-order terms; both cases provide good comparisons with previously unexplained experimental results. Further, because of an eighth-order term, the diffuse intensity is shown to contain a specular peak that also relies on polariton excitation. This new effect is studied in detail and is found to arise from the constructive interference of contributions produced by multiple-scattering processes, although the time-reversed paths that produce backscattering enhancement are not essential to the specular effect.     

Simulation of light scattering from a particle upon a wafer surface

We simulated light scattering from a particle located on a smooth surface. We developed a new approach utilizing the discrete sources method based on a strict mathematical model for this scattering problem. The main features of the corresponding numerical algorithm are presented. The results of modeling and comparisons with other theoretical results and experimental data are shown as well.     

Heavy fluorocarbon liquids for a phase-conjugated stimulated Brillouin scattering mirror

Heavy fluorocarbon liquids were found to be an ideal medium for high-power stimulated Brillouin scattering experiments. Fluorinert FC-75 gives high reflectivity, near 98%, and excellent fidelity in phase conjugation of 14-J, 18-ns optical pulses. The complete removal of microparticle impurities was found to be essential for avoiding laser damage in a high-energy operation. A method of purification based on ultrafiltration has been developed.     

Confined optical phonon modes in aligned nanorod arrays detected by resonant inelastic light scattering

We investigated the optical phonon excitations of laterally aligned nanorod arrays by resonant Raman scattering. We observed a strong suppression of the surface-optical phonon modes in the closely packed aligned arrays and a small asymmetry in the longitudinal-optical phonon peak with respect to the rod alignment orientation. These observations can be explained by the spatial distribution of the potential of the different phonon modes derived from the first principles calculations.     

多模增益光纤受激布里渊散组束中散射效果的研究

提出了在计算多模增益光纤有效受激布里渊增益系数时,以优化的高斯模场面积作为光纤有效截面积的方法,并基于光强耦合微分方程,研究了多模增益光纤稳态受激布里渊散射的效果.结果表明,无论是一阶还是二阶受激布里渊散射,随着泵浦功率的增加,残余的泵浦功率都将达到饱和,而Stokes功率都线性增加;二阶Stokes功率相对较弱;光纤的有效长度随泵浦功率的增加逐渐减小.     

Atomic structure display of a real silicon surface under light scattering

Angular dependencies of the scattered light intensity were measured on Si wafers that have different crystallographic orientations by using a He-Ne laser (λ = 632.8 nm, 80 μm spot diameter). During the experiment the Si wafer was fixed relative to the incident beam. Regular patterns were found in the azimuthal-angle-resolved scattering curves. Such patterns seem to be caused by the faceted shallow atomic structures of the surface.     

Geometric-optics-integral-equation method for light scattering by nonspherical ice crystals

A new geometric-optics model has been developed for the calculation of the single-scattering and polarization properties for arbitrarily oriented hexagonal ice crystals. The model uses the ray-tracing technique to solve the near field on the ice crystal surface, which is then transformed to the far field on the basis of the electromagnetic equivalence theorem. From comparisons with the results computed by the finite-difference time domain method, we show that the novel geometric-optics method can be applied to the computation of the extinction cross section and single-scattering albedo for ice crystals with size parameters along the minimum dimension as small as ~6. Overall agreement has also been obtained for the phase function when size parameters along the minimum dimension are larger than ~20. We demonstrate that the present model converges to the conventional ray-tracing method for large size parameters and produces single-scattering results close to those computed by the finite-difference time domain method for size parameters along the minimum dimension smaller than ~20. The present geometric-optics method can therefore bridge the gap between the conventional ray-tracing and the exact numerical methods that are applicable to large and small size parameters, respectively.     

Multi-parameter regularization method for particle sizing of forward light scattering

The matrix equation in the inversion of particle sizing based on forward light scattering is an ill-posed problem. To solve such an inversion problem, a number of algorithms have been proposed. The single parameter regularization is effective for retrieving the particle size distribution, but the solution is usually oscillatory in the presence of measurement errors. In this work, a multi-parameter regularization is presented to diminish the oscillations of the solution, which is verified with simulations and experiments.     

Comparison between an objective and a psychophysical method for the evaluation of intraocular light scattering

In this study we present the comparison of the performance of two systems to measure intraocular scattering. Measurements were made by using a psychophysical system based on a brightness comparison method that provides a glare index and a physical system based on the double-pass technique, which gives an objective scatter index by measuring the optical quality of the eye. Three external diffuser filters that simulated different grades of intraocular scattering were used in subjects with normal vision. The two measured indexes showed a graded rise with increasing level of scattering. The discrimination ability obtained for both systems showed that they were able to distinguish among conditions ranging from normal to early cataracts.     

光纤拉曼放大器中级联的受激布里渊散射

在以25kmG652单模光纤为增益介质的光纤拉曼放大器中,观察到了级联的受激布里渊散射。光纤中的双瑞利折射作用导致了二阶布里渊散射光子的出现,加快了二阶布里渊散射的出现。在拉曼增益的作用下,随着二阶布里渊散射的饱和会出现级联的多阶布里渊散射,形成梳状光谱。该梳状光谱也具有明显的阈值特性。在拉曼泵浦功率是1200mW的时候,出现梳状光谱的阈值在1.5到2.5dBm之间。在传输系统中,级联的受激布里渊散射会导致放大器的增益饱和,严重恶化系统的信噪比,因此必须采用增加信号光的带宽或者对信号光进行调制等措施加以控制。但该现象也有可能被用来制造梳状光源。     

Analysis of light scattering from a cutting tool edge

The scattering of light from cutting tools is studied. The contribution of cutting tool edge parameters (height and width) to scattering patterns and the influence of side surface roughness on scattering patterns are investigated. An angle-limited integrated scattering method is developed and analyzed for fast determination of edge parameters.     

Comparative study of the elastic properties of polycrystalline aluminum nitride films on silicon by Brillouin light scattering

Brillouin light scattering (BLS) has been used to investigate the elastic properties of polycrystalline AIN films, about 1 μm thick, grown by DC-reactive magnetron sputtering on Si₃N₄ coated (100)-Si substrates. Taking advantage from the detection of a number of different acoustic modes, a complete elastic characterization of the films has been achieved. The elastic constants c₁₁ and c₆₆ have been selectively determined from detection of the longitudinal and of the shear horizontal bulk modes, respectively, travelling parallel to the film surface. The three remaining elastic constants, namely c₄₄, c₃₃ and c₁₃, have been obtained from detection of the Rayleigh surface wave and of the longitudinal bulk wave propagating at different angles from the surface normal. The values of the elastic constants of these sputtered AIN films depend on the deposition conditions and on the microstructural properties of the films, especially oxygen contamination and quality of texture. In the case of the films with the best degree of texture and the lowest oxygen content, the values of the elastic constants are rather close to those previously determined in epitaxial A1N films grown at high temperature by MOCVD. This demonstrates that sputter deposition at relatively low temperature can be used to grow high quality A1N films on Si and is of great importance in view of the integration of these films in the technology of IC semiconductors.     

Characterization of milled Si3N4 powder using X-ray peak broadening and surface area analysis

Si₃N₄ powder has been milled using a planetary ball mill. The specific surface area, crystallite size and lattice distortions were studied as a function of milling time using the Brunauer-Emmett-Teller (BET) technique and X-ray powder diffractometry. The crystallite size decreased rapidly during the first 50 h of milling. Above 170 h no further decrease of the crystallite size occurred. The smallest crystallite size obtained was 0.074 m. Lattice distortions were small and decreased slightly during the first 50 h of milling. Specific surface area increased linearly with time. Rapid wear of the milling parts occurred during the first 50 h. Increasing the milling time produced only minor wear. The oxygen content increased linearly with milling time. Reaction with the milling fluid produced an increase in carbon content.     

Online monitoring of a belt grinding process by using a light scattering method

Industrially ground surfaces often have a characteristic surface topography known as chatter marks. The surface finishing is mainly monitored by optical measurement techniques. In this work, the monitoring of an industrial belt grinding process with a light scattering sensor is presented. Although this technique is primarily applied for parametric surface roughness analysis, here it is shown that it enables also the measurement of the surface topography, i.e., the chatter marks occurring during the belt grinding process. In particular, it is proven that the light scattering method is appropriate to measure online the topography of chatter marks. Furthermore, the frequency analysis of the data reveals that the wavelength of chatter marks strongly depends on process parameters, such as the grinding speed.