水中气泡上的体散射函数的模拟与计算

基于几何光学的基本原理,推导出一种可以计算水中大尺度气泡上光散射角度与强度的关系式。推导中避免引入衰减因子G,较Davis模型更为简单。该模型可应用于光在水中单个气泡上散射的数值计算。最后,模拟计算了平行光束入射水中气泡的体散射函数曲线,发现水中气泡的前向散射远大于后向散射;当气泡半径在远大于光波波长的前提下变化时,气泡上散射光强分布规律与气泡半径无关;而介质相对折射率的增大会削弱前向散射而增强后向散射光强。     

Light scattering from spherical plasmonic nanoantennas: effects of nanoscale roughness

We have investigated the light scattering properties of smooth and roughened nanoshells with dipolar and quadrupolar plasmon resonances tuned to 830 nm. In the dipole resonant case small but measurable variations in the angle dependent light scattering (ADLS) due to the introduction of surface roughness are observed. In the quadrupole case, the distinctive side lobe scattering characteristic of quadrupolar emission is strongly quenched for roughened nanoshells. PACS 81.07.-b; 78.67.-n     

The contribution of bubbles to high-frequency sea surface backscatter: a 24-h time series of field measurements

Measurements of acoustic sea surface backscattering, wind speed, and surface wave spectra were made continually over a 24-h period in an experiment conducted in 26 m of water near the Dry Tortugus collection of islands off south Florida in February 1995. The backscattering measurements were made at a frequency of 30 kHz and a sea surface grazing angle of 20 degrees; a time series of the decibel equivalent of this variable, called SS20, was studied in terms of its dependence on environmental variables. On occasion reliable estimates of scattering in the grazing range 15 degrees-27 degrees were also obtained during the 24 hours. The scattering data exhibited evidence, in terms of scattering level and grazing angle dependence, of scattering from near-surface bubbles rather than scattering from the rough air-sea interface. The scattering data were compared with a model for sigma(b), the apparent backscattering cross section per unit area due to bubble scattering, that is driven by a parameter, beta1, equal to the depth-integrated extinction cross section per unit volume. Using an empirical model for beta1 based on data from a 1977 experiment conducted in pelagic waters, model predictions agreed reasonably well with the 1995 measurements presented here. Additional model-data comparisons were made using four measurements from a 1992 experiment conducted in pelagic waters. Finally, the 24-h time series of acoustic scattering exhibited a hysteresis effect, wherein for a given wind speed, there was a tendency for the scattering level to be higher if prior winds had been falling. A better understanding of this effect is essential to reduce uncertainty in model predictions.     

Electromagnetic effects in the Surface Enhanced Raman Scattering from a molecule at a liquid Hg surface

In a previous publication the image enhancement effect and the enhanced radiation due to near field excitation of surface roughness were calculated for a finite-size molecule above a metal described by a nonlocal dielectric relation [Phys. Rev. Letters 44 (1980) 1774]. When applied to a roughened Ag surface these calculations led to a predicted Surface Enhanced Raman Scattering (SERS) gain of ~ 10³ due to each effect, yielding an overall gain ~ 10⁶, in agreement with experiment. Here these calculations are extended to the case of a liquid Hg surface, the roughness corresponding to thermally excited ripplons. The SERS gain due to image enhancement is reduced to ~ 10, due primarily to the ~ 24 times greater electron scattering rate in Hg over that in Ag. The roughness gain is reduced to ~ 2 at room temperature, due to the difference between the ripplon spectrum and that of the boss-like surface structures assumed for the solid surface. The predicted overall SERS gain for Hg is ~ 20, far less than reported in recent observations [R. Naanan et al., J. Phys. Chem. 84 (1980) 2692]. The discrepancy raises serious questions about the electromagnetic explanations of SERS.     

Optimum surface roughness for surface enhanced Raman scattering

Surface enhanced Raman scattering from copper phthalocyanine thin films deposited onto Ag films roughened by underlayers of gas-evaporated Si particles was investigated. The surface roughness was systematically varied by varying the average size of Si particles. Results of quantitative intensity measurements indicate that there exists an optimum surface roughness depth for SERS. The maximum enhancement factor obtained is ≈1.5 x 10⁴ and a crude estimate of the optimum roughness depth is of the order of 100 Å. The origin of presently observed enhancement is thought to be purely electromagnetic, involving the excitation of the surface plasmon modes via the surface roughness.     

Sonoluminescence: nature's smallest blackbody

The transduction of sound into light through the implosion of a bubble of gas leads to a flash of light whose duration is delineated in picoseconds. Combined measurements of spectral irradiance, Mie scattering, and flash width (as determined by time-correlated single-photon counting) suggest that sonoluminescence from hydrogen and noble-gas bubbles is radiation from a blackbody with temperatures ranging from 6000 K (H(2)) to 20,000 K (He) and a surface of emission whose radius ranges from 0.1 microm (He) to 0.4 microm (Xe) . The state of matter that would admit photon-matter equilibrium under such conditions is a mystery.     

Estimation of surface roughness effect on spectral characteristics of tone signals propagating along stationary paths in a shallow sea

Scattering of a tonal low-frequency (up to 400 Hz) acoustic signal on a rough surface during its propagation along stationary paths in a shallow water area is considered. A calculation technique of estimating the spectral power density of the signal scattered at wind waves (reverberation) given an arbitrary angular distribution of the wind wave is developed based on the Bragg scattering model. The obtained results are compared with the results of numerical simulation of resonant sound scattering at surface perturbations according to an algorithm proposed by the authors, as well as with the results of experimental investigations.     

Development of a Mathematical Model for Light Scattering by statistically irregular particles

A mathematical model is described to predict Fraunhofer diffraction by statistically irregular particles. It is demonstrated that the particles are characterised by a probability distribution of radius and a correlation function in the surface. The results of calculation show that at angles not too far from the forward scattering direction and for irregularity heights within a limited range, the irregular particles may be represented approximately by spheres with a size distribution equal to the probability distribution of radius. However, the calculations are highly sensitive to the exact nature of the probability functions. Comparisons of the results with those of a simpler model are not satisfactory beyond the first forward scattering lobe.     

空化单气泡外围压强分布

关键词：     

测量金属膜粗糙度的表面等离子激元光谱方法

介绍了确定金属膜表面粗糙度的表面等离子激元光谱方法。测量了二种银膜在表面等离子激元激发条件下的光散射强度分布。通过与理论计算的拟合，得到了描写这二种银膜／空气同粗糙界面的特征参数。     

Hydrodynamic boundary conditions and dynamic forces between bubbles and surfaces

Dynamic forces between a 50 microm radius bubble driven towards and from a mica plate using an atomic force microscope in electrolyte and in surfactant exhibit different hydrodynamic boundary conditions at the bubble surface. In added surfactant, the forces are consistent with the no-slip boundary condition at the mica and bubble surfaces. With no surfactant, a new boundary condition that accounts for the transport of trace surface impurities explains variations of dynamic forces at different speeds and provides a direct connection between dynamic forces and surface transport effects at the air-water interface.     

基于等离子体环量控制的翼型气动特性

为了研究等离子体环量控制对翼型的影响特性,采用基于唯象学的等离子体气动激励数学模型和二维雷诺平均N-S方程,选取NCCR 1510-7067N环量控制翼型,数值模拟后缘半径对升力和效费比的影响规律,并进行优化。设计最佳后缘半径模型进行低速风洞实验,获得迎角-4~12,速度6,10,15 m/s下的压力分布和升力特性。研究表明:后缘半径过大或过小都不利于Coanda效应的产生,确定最佳后缘半径与弦长的比值为0.048,效费比97.69。低雷诺数下,随着迎角的增加,出现了层流长泡分离和短泡分离,等离子体射流不仅改善了尾部流场,还通过环量增加抑制层流分离,提高了升力。     

Real-time two-dimensional imaging in scattering media by use of a femtosecond Cr(4+):forsterite laser

An original femtosecond Cr(4+):forsterite laser source associated with a nonlinear optical correlator was used for imaging through scattering media with 1220-nm light. The system, which operates as an ultrafast optical gate by sum-frequency generation in a nonlinear crystal, was able to detect the light reflected from a resolution chart hidden in a turbid medium, at an attenuation of as much as 15 mean free paths. When the object was illuminated with a collimated beam, real-time two-dimensional images were obtained, with a maximum transverse resolution of ~20 microm.     

Frequency shift in cavities with rough walls

The influence of the roughness of the surface on the resonance frequency of an electromagnetic cavity is calculated. The roughness of the surface is described by a stochastic process and the frequency shift is then found as an expectation value. If the correlation distance of the surface roughness is much smaller than the curvature radius of the cavity wall, a quite simple formula for the frequency shift is found.     

Surface enhanced Raman scattering from mildly roughened surfaces: variation of signal with metal grain size

The intensity of surface enhanced Raman scattering from benzoic acid derivatives on mildly roughened, thermally evaporated Ag films shows a remarkably strong dependence on metal grain size. Large grained (slowly deposited) films give a superior response, by up to a factor of 10, to small grained (quickly deposited) films, with films of intermediate grain size yielding intermediate results. The optical field amplification underlying the enhancement mechanism is due to the excitation of surface plasmon polaritons (SPPs). Since surface roughness characteristics, as determined by STM, remain relatively constant as a function of deposition rate, it is argued that the contrast in Raman scattering is due to differences in elastic grain boundary scattering of SPPs (leading to different degrees of internal SPP damping), rather than differences in the interaction of SPPs with surface inhomogeneities.     

Improvement of sidewall surface roughness in silicon-on-insulator rib waveguides

Silicon-on-insulator (SOI) rib waveguides with residual sidewall roughness were fabricated through inductive coupled plasma reactive ion etching (ICPRIE) process. The sidewall surface morphology was characterized by scan electron microscope (SEM), and the root mean square (rms) roughness of the sidewall surface was directly measured by atomic force microscope (AFM). Sidewall surface roughness is the dominant scattering loss source. The ripples on the sidewall surface could be eliminated by mixed ICPRIE, and the rms roughness could be low down to 0.3 nm after thermal oxidation and hf rinse. According to the scattering theory developed by Payne and Lacey, the scattering loss could be minimized to below 0.01 dB/cm. The results indicated that the scattering loss would be a sharp fall by the combination with these two techniques.PACS 42.82.Et; 42.81.Dp; 52.80.Yr; 68.37.Ps; 81.65.Ps     

The Brewster effect for a one-dimensional random surface

By the use of a unitarity and reciprocity conserving theory for the scattering of p-polarized light from a one-dimensional random dielectric surface the we determine the shift in the Brewster angle toward smaller angles of incidence caused by the surface roughness, and the (non-zero) value of the reflectivity at its minimum. Both quantities may be useful in the characterization of the roughness of such surfaces.     

Optical guiding of atoms through a hollow-core photonic band-gap fiber

We demonstrate the first guiding of atoms through a hollow-core photonic band-gap fiber. Rb atoms from a thermal oven source travel through 6.1 and 4.0 cm long fibers with a guiding potential 1/e(2) radius of 3.0 microm. When the photon scattering rate is small, the observed guiding efficiency is greater than 70%. An unexpected low-speed cutoff is attributed to inefficient optical coupling.     

不同预淀积层银膜表面等离子激元波的辐射效率

测量了棱镜／ＬＢ膜／银膜／空气，棱镜／银膜／空气，棱镜／银膜／空气三种结构在衰减全反射条件下激发的表面等离子激元波由于银膜－空气界面粗糙度而散射以空气中的辐射效率，同时，由散射光强分布计算出表面度粗糙度参数随横向相关长度σ和表面波纹深度δ，再由σ和δ计算出理论辐射效率，理论与实验结果相符合。     

尾流气泡幕光学特性的数理模型

对实验室模拟的尾流以及真实尾流气泡幕中气泡的运动规律进行了分析和计算,并提出了气泡幕中气泡随半径分布规律的数学物理模型,引入了最可几半径的概念从单个气泡对激光散射的影响推导出了气泡幕对激光的散射效应的数学表达式,并利用所得到的表达方式在实验室条件下进行了相关计算所建立的理论模型对于气泡分布的计算结果同实验结果相比,一致性相当好在气泡幕对激光散射的影响中,首次对激光偏振化特性的影响进行了区别计算计算结果表明:激光的两种偏振化状态对气泡幕散射的影响从粗糙结构来看没有明显差异,但其精细结构有所不同对不扩束和扩束两个条件下的计算明确显示出,扩束条件下散射光的强度是不扩束条件下的二倍以上,但二者的精细结构并没有可见的差别.