Manifestation of gyrotropy upon light scattering in paratellurite

Upon scattering of laser radiation at different wavelengths, three optical effects—rotation of the plane of polarization of linearly polarized light, dependence of the specific rotation on the radiation wavelength, and the dependence of the scattered light intensity on the angle between the scattered and incident beams—are simultaneously observed. The intensity distributions of the scattered laser light at 0.488, 0.531 nm, and 0.633 μm along the optical axis of a thick sample of a TeO₂ single crystal are visualized. Based on the patterns obtained, the values of the specific rotation are calculated, and the curve of the optical rotation for the visible range is obtained.     

Dynamic light scattering at the ice water interface during freezing

The 488 nm radiation of an argon laser is scattered quasi-elastically at the [0001]-face (basal plane) of a growing ice crystal. The scattering plane is the basal plane. The Rayleigh-linewidth is proportional to the square of the scattering vector. One measures about 2 krad/s at a scattering angle of 90°. The linewidth does not depend on the growth rate. Scattering is only observed once a critical growth rate ν_crit = 1,5 μ m?s has been exceeded. Then the scattering intensity depends linearly on the growth rate in the range between 0,03 μ m?s and 2,5 μ m?s. Once the surface is molten scattering vanishes and does not reappear until the growth rate has again exceeded the threshold ν_crit. The coherence properties of the scattered light indicate that the thickness of the scattering layer is less than 6 μ m. The observations are interpreted in terms of a fluctuating interface. The decay time of the fluctuations has been calculated. Satisfactory agreement with the observed Rayleigh linewidth is obtained.     

Different angle-resolved polarization configurations of Raman spectroscopy: A case on the basal and edge plane of two-dimensional materials

Angle-resolved polarized Raman(ARPR) spectroscopy can be utilized to assign the Raman modes based on crystal symmetry and Raman selection rules and also to characterize the crystallographic orientation of anisotropic materials.However, polarized Raman measurements can be implemented by several different configurations and thus lead to different results. In this work, we systematically analyze three typical polarization configurations: 1) to change the polarization of the incident laser, 2) to rotate the sample, and 3) to set a half-wave plate in the common optical path of incident laser and scattered Raman signal to simultaneously vary their polarization directions. We provide a general approach of polarization analysis on the Raman intensity under the three polarization configurations and demonstrate that the latter two cases are equivalent to each other. Because the basal plane of highly ordered pyrolytic graphite(HOPG) exhibits isotropic feature and its edge plane is highly anisotropic, HOPG can be treated as a modelling system to study ARPR spectroscopy of twodimensional materials on their basal and edge planes. Therefore, we verify the ARPR behaviors of HOPG on its basal and edge planes at three different polarization configurations. The orientation direction of HOPG edge plane can be accurately determined by the angle-resolved polarization-dependent G mode intensity without rotating sample, which shows potential application for orientation determination of other anisotropic and vertically standing two-dimensional materials and other materials.     

Coherent anti-Stokes Raman scattering and normal anti-Stokes Raman scattering due to LO phonons in GaP using cw diode lasers

Coherent anti-Stokes Raman scattering (CARS) and normal anti-Stokes Raman scattering (NARS) have been measured in (001) GaP at room temperature due to the 403 cm⁻¹ LO phonons using a continuous wave (CW) 785.0 nm fixed-wavelength pump laser and a CW Stokes laser tunable in the 800-830 nm wavelength range. CARS measurements are normally made using pulsed lasers. The use of CW diode lasers allows a more accurate comparison between the measured and calculated values of the CARS signal. The pump and Stokes laser beams were linearly polarized perpendicular to each other, same as the pump and normal Stokes/anti-Stokes scattered light for the GaP sample used in this work. The pump and Stokes laser powers incident upon the GaP sample, located in the focal plane of a 20 mm effective focal length lens, were <20 and 50 mW, respectively. The diameter of the laser beams in the focal plane of the focusing lens was determined to 40±5 μm. The pump and Stokes laser beam intensities incident upon the 0.3 mm thick GaP sample were <2 and 5 kW cm², respectively. The powers of the CARS and NARS signals were measured using a Raman spectrometer. The signal output of the Raman spectrometer was calibrated using the pump laser and several neutral density filters. The Raman linewidth (full-width at half-maximum) of the LO phonons was determined to be 0.95±0.05 cm⁻¹, using the variation of the CARS signal with the wavelength of the Stokes laser. The measured powers of the CARS and NARS signals are about a factor of 5 and 1.5, respectively, smaller than those calculated from the corresponding theoretical expressions.     

Flow measurements based on Rayleigh scattering and Fabry-Perot interferometer

Flow diagnostics based on molecular Rayleigh scattering are discussed along with the results of a feasibility study to non-intrusively measure flow properties in a small supersonic wind tunnel. The technique uses an injection seeded, frequency doubled Nd:YAG laser tuned to an absorption band of iodine. The molecular Rayleigh scattered light is filtered with an iodine cell to block light at the laser frequency. The Doppler-shifted Rayleigh scattered light that passes through the iodine cell is analyzed with a planar mirror Fabry-Perot interferometer used in a static imaging mode. An intensified CCD camera is used to record the images. The images are analyzed at several subregions, where the flow properties are determined. Each image is obtained with a single laser pulse, giving instantaneous measurements.     

3D Flame surface density measurements via orthogonal cross-planar mie scattering in a low-turbulence bunsen flame

Quasi-instantaneous Mie scattering measurements were conducted to determine the flame surface location using two orthogonal planes to obtain the 3D orientation and measurement of 3D flame surface density (FSD) on a stabilised piloted Bunsen burner with turbulence levels of 1 to 2.5 times the laminar flame speed. A double-pulsed 527 nm high-frequency laser (part of a high-frequency particle image velocimetry dual-head setup) was split into two separated laser beams through a polarizer to generate laser sheets at 3 kHz on a vertical plane and a horizontal plane. The vertical plane across the centerline of the Bunsen-stabilized flame was kept constant, whilst the height of the horizontal plane was adjusted from the base of the flame for different heights. The flame edge was defined as the location where droplet tracers disappear, and calculated based on the local change in the number density of the flame edge. Projections of the flame normal vector onto two measurement planes were used to calculate directing angles on each plane, and the 3D FSD was estimated based on the measured angles at the intersecting lines of two planes. A comparison of the 2D to 3D FSD magnitudes was made, for cases with and without mean angle corrections. The results show that the 2D FSD approximations are lower than the 3D measurements by a factor of 20–30% if uncorrected, and up to 28% after corrections.     

Temporal reflectance from a light pulse irradiated medium embedded with highly scattering cores

This paper presents a new approach to utilize ultrashort pulsed laser for optical diagnostics with numerical simulations. The method is based on the use of ultrafast pulses with a pulsewidth selected according to the probed medium's radiative property and/or size. Our previous work in nonhomogeneous media has shown that the resulting time-resolved reflectance signal will have a unique characteristic: it will show a direct correlation of ballistic photon travel time and interface location, which is in between different layers or nonhomogeneous regions. The premise is based on utilizing the medium's structural information carried by the ballistic and snake photons without being masked by the diffuse photons. In this study, the space-time correlation is further explored in the case of minimally scattered photons from a large scattering coefficient core region embedded within a less-scattering medium. Time-resolved reflectance signals of the single scattering core and multiple scattering cores within a three-dimensional medium demonstrate the concept and illustrate the additional effect due to the scattered photons from the core region. A unique temporal signal profile's correlation at various detector positions with respect to the scattering core is explained in detail. The result has important implications. This approach will lead to a much simpler and more precise determination of the probed medium's composition or structure. Due to the large computational requirement to obtain the physical details of the light pulse propagation inside highly scattering multi-dimensional media, the reverse Monte-Carlo method is used. The potential applications of the method include non-destructive diagnostics, optical imaging, and remote sensing of underwater objects.     

微型拉曼光谱仪杂散光分析及抑制方法的改进

作为一个微弱光信号探测系统,拉曼光谱仪中的杂散光分析可以为其设计提供较大帮助。针对微型拉曼光谱仪系统,结合光学设计和三维建模优化了其光机结构,系统分辨率为0.7 nm,体积为110 mm×95 mm,属便携式微型拉曼光谱仪,并基于杂散光分析软件TracePro对系统进行了光线追迹和仿真分析。首先通过优化孔径光阑初步抑制了入射处带来的杂散光,然后针对系统内部的主要杂散光(光栅零级衍射光)抑制装置即光学陷阱进行了详细分析和设计改进,改进后的光学陷阱较改进前更有效地利用了光谱仪内部空间,且分析结果表明改进后的光学陷阱将杂散光线数量减少了50%,杂散光归一化辐照度强度从10-5降低至10-7,在微型化的同时可有效抑制微型拉曼光谱仪系统中的杂散光,将更加有利于微弱信号的探测,为微型拉曼光谱仪的设计和装调提供了参考。     

Determination of surface normal temperature gradients using thermographic phosphors and filtered Rayleigh scattering

Wall temperature as well as the temperature distribution within or close-by the boundary layer of an electrically heated axisymmetric jet impinging on a flat plate were monitored to deduce wall-normal temperature gradients. The radial surface temperature profile of the plate was determined by coating it with thermographic phosphors (TPs), materials whose phosphorescence decay time is dependent on their temperature. The TP was excited electronically by a frequency-tripled Nd:YAG laser (355 nm) and the temporal decay of the phosphorescence intensity was measured zero-dimensionally by a photomultiplier tube. In this case the 659-nm emission line of Mg₃F₂GeO₄:Mn was monitored. The non-intrusive measurement of gas temperatures near the surface was performed two-dimensionally by filtered Rayleigh scattering (FRS). A tunable frequency-tripled single-longitudinal-mode alexandrite laser beam at 254 nm was formed into a light sheet pointing parallel to the surface. The scattered light was imaged through a very narrow linewidth atomic mercury filter onto an intensified charged coupled device (ICCD). The elastic stray light from surfaces was strongly suppressed, whereas Doppler-broadened light was detected. Thermographic phosphors proved to be reliable for the measurement of surface temperatures. Dependent on the specific experimental conditions, problems appeared with signals interfering with the FRS radiation close-by the surface. Results and challenges of this approach are discussed. PACS 07.20.Dt; 32.50.+d; 44.20.+b; 42.65.Es; 33.20.Fb     

527 nm激光辐照盘靶受激布里渊散射光角分布

 在“星光Ⅱ”激光装置上，开展了527 nm激光与金盘靶和铝盘靶相互作用实验，研究了激光辐照盘靶的受激布里渊散射光散射机制，获得了散射光能量角分布。实验结果表明：激光辐照金盘靶时，在入射激光能量大致相同的情况下，受激布里渊散射光能量在匀滑的条件下比没有匀滑时低一个数量级以上（背反能量除外），束匀滑对散射光的抑制作用十分明显。在匀滑条件下，越靠近背反方向，散射光能量越大。散射光能量分布沿方位角的变化不大。激光辐照铝盘靶时，散射光能量分布的离散性较大。     

10−9量级高灵敏度点源透射比测试设备研究

为了实现对光学系统杂散光抑制能力的定量评价,开展了10^?9量级高灵敏度点源透射比测试设备的研究和实验验证。采用脉冲光源、脉冲探测的新测量方法,在保证测试系统具有高灵敏度测量能力的同时,简化了微弱光电信号探测组件的复杂程度,建立了一套最大测试口径为600 mm、测试波长为527 nm的点源透射比测试设备,并利用该设备测试了一台250 mm口径空间光学相机的点源透射比。实验结果表明:60°入射角度时的点源透射比测试结果为1.68×10^?9。证明该设备的测试误差在10^?9或更低的量级,具备10^?9量级高灵敏度点源透射比测试能力。本文研究可以为天文望远镜、星敏感器、空间目标监视载荷等多种类型的光学仪器提供杂光抑制性能评估。     

Superresolution vibrational imaging by simultaneous detection of Raman and hyper-Raman scattering

We have developed a superresolution vibrational imaging method by simultaneous detection of Raman and hyper-Raman scattering. Raman and hyper-Raman images obtained with the same laser spot carry independent information on the sample spatial distribution, owing to different signal dependence (linear in Raman and quadratic in hyper-Raman) on the incident light intensity. This information can be quantitatively analyzed to recover the incident light intensity distribution at the focal plane. A superresolution vibrational image is then derived by the constrained deconvolution of the images by the obtained incident light intensity distribution. This method has been applied to a TiO? nanostructure and the obtained superresolution image was compared with a scanning electron microscopy image. The spatial resolution achieved by the present method is evaluated to be 160 nm, which is more than twice better than the diffraction limited resolution.     

Gaussian beam scattering from arbitrarily shaped objects with rough surfaces

The scattered field of Gaussian beam scattering from arbitrarily shaped dielectric objects with rough surfaces is investigated for optical and infrared frequencies by using the plane wave spectrum method and the Kirchhoff approximation, and the formulae for the coherent and incoherent scattering cross sections are obtained theoretically based on geometrical optics and tangent plane approximations. The infrared laser scattering cross sections of a rough sphere are calculated at 1.06 μm, and the influence of the beam size is analysed numerically. It is shown that when the beam size is much larger than the size of the object, the results in this paper will be close to those of an incident plane wave.     

Taking into account scattering of polarized laser radiation by cuvet walls when probing blood

We have studied the ratio of the intensities of radiation scattered by a layer of blood in a cuvet and by the cuvet walls for normal incidence or incidence at the Brewster angle for probing radiation from a He-Ne laser, linearly polarized in the plane of incidence (plane of observation) or orthogonal to that plane. We have compared it with the intensity of the radiation reflected by the surface of the cuvet wall, as estimated based on the geometric approximation. We have found the optimal conditions for probing and observing scattering for the layer in a cuvet, where such conditions are characterized by a minimal effect from the cuvet walls and the maximum ratio of useful signal to background. Report given at the Fifth International Scientific and Technical Conference on Quantum Electronics, 22–25 November 2004, Minsk, Belarus. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 2, pp. 270–272, March–April, 2006.     

Gaussian beam scattering from arbitrarily shaped objects with rough surfaces

Abstract

The scattered field of Gaussian beam scattering from arbitrarily shaped dielectric objects with rough surfaces is investigated for optical and infrared frequencies by using the plane wave spectrum method and the Kirchhoff approximation, and the formulae for the coherent and incoherent scattering cross sections are obtained theoretically based on geometrical optics and tangent plane approximations. The infrared laser scattering cross sections of a rough sphere are calculated at 1.06 μm, and the influence of the beam size is analysed numerically. It is shown that when the beam size is much larger than the size of the object, the results in this paper will be close to those of an incident plane wave.     

Edge wave on axis behind an aperture or disk having a ragged edge

Diffraction by a circular aperture or disk having a ragged edge is investigated. Theory and measurements are reported. The ragged edge is modeled as N arcs, of differing radii a(i), each of which contributes a scattered signal to the edge wave on axis behind the aperture or disk. The amplitude of each scattered signal is proportional to the angle of the arc, and the corresponding time delay is square root of [(ai)2 + (s0)2/c0], where s0 is the axial distance from the aperture plane and c0 is the sound speed. Kirchhoff theory is used to make the calculation. A formula is derived for the rms pressure of the edge wave in terms of the rms pressure and autocorrelation function of the incident wave. The formula can be evaluated for incident waves that are sinusoidal, random (e.g., noise), or transient. Predictions agree reasonably well with underwater measurements made with a spark-generated pulse incident on various apertures. The main result is that making the edge ragged reduces the rms pressure of the edge wave. Indeed, an edge profile is presented that, for a given frequency and axial observation point, eliminates the edge wave completely.     

Laser scattering with a rapidly tuned dye laser

A facility is described for the measurement of the total spectrum produced by laser radiation scattered from a plasma during one laser pulse. The dye laser is electrooptically tuned by a high frequency voltage applied to a modified Lyot filter within its cavity. The scattered spectrum is registrated together with the laser reference signal by a transient recorder and processed afterwards. Both measuring time and inaccuracy could be reduced drastically. Scattering experiments have been accomplished in a H₂ cascade of 5 mm diameter arc at atmospheric pressure, and at 14 and 20 A, respectively. For this arc very precise spectroscopic results are available for checking the accuracy of the scattering measurements. Both the spectroscopic line and continuum experiments yield a temperature which lies between electron and ion temperature if there is no local thermal equilibrium, whereas the pure electron temperature is provided by the scattered spectrum. The agreement between spectroscopic and scattering results within the high error limits is the experimental proof for the applicability of this new scattering technique which requires only about one hundredth of the measuring time, as compared with the usual shot-to-shot registration.     

HT-7 Nd:YAG激光汤姆逊散射光学系统分析

 光学透过性能对信号微弱的激光汤姆逊散射测量系统极为重要。主要讨论了HT-7 Nd:YAG激光汤姆逊散射系统光学设计中消除杂散光和提高散射信号传输效率的方法，并对系统的光学性能进行了模拟计算。模拟结果显示，干涉滤光片分光谱仪的光学透过性能随电子温度的升高而增强，等离子体电子温度为1 keV时其透过率可达到10%；汤姆逊散射信号非常微弱，等离子体电子密度为1×10¹⁹/m³时每道信号仅有几百到上千个光子；等离子体电子温度在0.3～5.0 keV范围内时，光子量子统计涨落引起的电子温度测量误差在3%以下。     

激光等离子体受激Raman散射光谱的时间分辨测量

采用光学多道谱仪和光学条纹相机耦合,组成时间分辨的Raman散射光谱测量系统,可实现0.5nm的光谱分辨和好于10ps的时间分辨。采用该测量系统,在神光Ⅱ装置上开展了脉宽1ns、波长351nm的激光与两种不同尺寸柱腔靶相互作用的物理实验,获得了时间分辨的SRS光谱实验结果。研究表明,SRS光谱在时间上相对于入射激光有一定的延迟,腔靶尺寸减小时,延迟时间随之减小。通过长、短波截止波长分析电子密度方法,计算得出了Ⅰ型和Ⅱ型腔靶SRS散射光最短波长光谱发生的密度区分别为0.069nc和0.027nc。     

Reflection and scattering from laser-irradiated solid targets at a wavelength of 0.44 μm

Studies of the reflectivity and spectral behaviour of the light scattered from plane solid targets irradiated with the third harmonic of the iodine laser (λ = 0.44 μm) in the intensity range 10¹² to 10¹⁵ W/cm² are reported. The complex spectra of the scattered light observed with normally and obliquely incident laser light provide evidence of stimulated scattering processes. The results are compared with current theories on stimulated Brillouin scattering.