Stimulated raman scattering of fuel droplets

The strong stimulated Raman scattering (SRS) from diesel fuel droplets has the potential of providing the relative concentration of multicomponent fuel and the absolute size of individual droplets. The morphology-dependent resonances (MDRs) of a sphere cause the droplet to act as an optical resonator which greatly lowers the SRS threshold. The number density, quality factor, and frequency shift of several MDRs are calculated as a function of the ratio of the index of refraction of the liquid and the surrounding gas, which approaches unity at the thermodynamic critical condition for the fuel spray. The SRS spectra of monodispersed droplets of toluene, pentane, Exxon-Aromatic-150, and Mobil D-2 are presented. The exponential growth region of the SRS intensity I _1S as a function of the input laser intensity I _input is investigated for the toluene carbon ring breathing mode v ₂ and the pentane C-H stretching region. The I _1S ratio of toluene and pentane is measured as a function of the ratio of the toluene and pentane concentration for monodispersed droplets. The reduced fluctuation in I _1S when I _input is changed from multimode to single-mode is displayed as a histogram of the I _1S of the v ₂ mode of toluene droplets.     

Simultaneous acquisition of all four forms of circular polarization Raman optical activity: results for α‐pinene and lysozyme

A new circularly polarized (CP) Raman spectrometer is described that demonstrates simultaneous acquisition of all four forms of circular polarization Raman optical activity (ROA). The instrument is a design extension of a commercially available back scattering circular polarization (SCP) ROA spectrometer. Circular polarization of the incident beam is introduced with a quarter‐wave plate, and a half‐wave plate alternately positioned in and out of the beam controls the modulation between right circular polarization (RCP) or left circular polarization (LCP) states. Combining this modulation with the simultaneous detection of RCP and LCP scattered Raman radiation allows the measurement of incident circular polarization (ICP), SCP, in‐phase dual circular polarization(DCP_I) and out‐of‐phase DCP_II‐ROA. In addition, three different forms of backscattered Raman spectra, namely unpolarized, highly polarized, and depolarized Raman spectra, as well as a degree of circularity spectrum are obtained. The performance of the new all‐CP ROA spectrometer is evaluated with neat α‐pinene and aqueous hen lysozyme solution. Copyright © 2011 John Wiley & Sons, Ltd.     

Polarization behaviors of twisted carbon nanotube fibers

Polarization behaviors of carbon nanotube (CNT) fibers with different twisting were reported. Scanning electron microscope and polarized Raman spectroscopy were used to investigate the prepared samples. Results indicate that surface twisting angle affects greatly the polarization angle and I_///I_⊥ ratio of twisted CNT fibers. Raman depth profile measurements imply that the twisted fibers consist of non‐uniform CNT alignments. A simplified two‐CNT‐alignment geometric model was proposed to illuminate the experimental observations. The results suggest that polarized Raman depth profile measurement would be a very useful approach for determining the distribution of CNT alignments in CNT fibers. Copyright © 2012 John Wiley & Sons, Ltd.     

DNA-wrapped carbon nanotubes aligned in stretched gelatin films: Polarized resonance Raman and absorption spectroscopy study

We present the study of DNA-wrapped single-walled carbon nanotubes (SWNTs) embedded in the stretched gelatin film by the polarized resonance Raman spectroscopy and visible-NIR optical absorption. The polarized dependent absorption spectra taken along and normal to the stretching direction demonstrate a comparatively high degree of the alignment of isolated SWNTs in the gelatin matrix. The analysis of Raman spectra of isolated SWNTs in the gelatin stretched films showed that the degree of the alignment of carbon nanotubes along the stretching direction is about 62%. The dependence of the peak position of G⁺-band in Raman spectra on the polarization angle θ between the polarization of the incident light and the direction of the stretching of films was revealed. This shift is explained by the different polarization dependence of the most intensive A and E₁ symmetry modes within the G⁺-band. The performed studies of embedded DNA-wrapped nanotubes in the gelatin film show the simple method for obtaining the controlled ordered biocompatible nanotubes inside a polymer matrix. It can be used for manufacturing sizable flexible self-transparent films with integrated nanoelectrodes.     

Probing crystallographic orientation of a single GaN nanotube using polarized Raman imaging

We report the study of crystallographic orientation of a highly anisotropic square‐shaped single wurtzite GaN nanotube (NT) probed by polarized Raman imaging. The polarization‐dependent Raman intensities are collected in two mutually orthogonal directions with respect to the NT long axis. Variation in intensities of symmetry allowed A₁(LO) mode of GaN with two different polarization directions reveals the possible crystalline orientations in the NT. The polarized Raman spectral imaging illustrates the fact of inhomogeneous crystalline orientations along the edges of the NT facets owing to its finite wall thickness. The deviation of polarization selection rule for GaN is attributed to the geometrical anisotropy of the NT. Electron microscopic structural data confirms the spectral imaging analyses. Copyright © 2013 John Wiley & Sons, Ltd.     

用CARS技术确定拉曼退偏比的一种数据处理新方法

Yuika等人利用偏振CARS技术可以准确地确定分子的拉曼退偏比.其方法是，首先对不同检偏角_d所对应CARS谱峰的频率分布进行数学模拟，然后由所得系数随检偏角φ_d的变化求得使CARS信号中共振项消失的偏振角φ^０_d，最后由消失条件ρ＝-1/(tanθtanφ⁰_d)求出退偏比ρ，θ为产生CARS光的Pump光与Stokes光偏振方向的夹角.本文提出的数据处理方法，即交点法.同Yuika等人处理数据的方法相比，交点法毋需关于谱峰频率分布的知识，做法也更为简便.     

Optical properties of lithium niobate and lithium tantalate crystals with impurities and defects

The photoinduced and Raman scattering in lithium niobate and lithium tantalate crystals with impurities and defects have been studied. An exciting laser beam propagated either along the ferroelectric Z axis or perpendicular to it. The conditions for exciting transverse and longitudinal polar optical modes in Raman spectra are established. The regularities of the excitation of Raman spectra in several polarization geometries (X(ZZ)Y, Z(XX, Y Y)Z, Z(XX, Y Y)Z, X(ZX)Y, X(ZX)X and X(ZX)X) have been investigated. Additional (extra) spectral lines are interpreted as a manifestation of a biphonon enhanced by the Fermi resonance and the result of violation of selection rules for pseudoscalar modes of the A ₂ type due to the reduction of the point symmetry group caused by the presence of impurities and defects in real crystals. The conditions for exciting coherent longitudinal and transverse modes in lithium niobate and lithium tantalate single crystals upon stimulated Raman scattering are analyzed. The temperature evolution of the spectra recorded in the X(ZZ)Y geometry near the ferroelectric phase transition point is explained based on the concept of effective soft mode and analysis of the isofrequency opalescence effect. Strong photoluminescence is found in copper-doped lithium niobate crystals.     

单量子点在连续外场激发下发射光子性质的理论研究

用最近所发展的产生函数(generatingfunction)理论方法研究了单量子点在连续外场作用下光子发射的性质:光子发射强度,MandelQ参数以及X与Y偏振光子之间的关联〈NxNy〉在不同激发场强下随激发光场偏振方向的变化性质:在弱场激发下,激发场的强度决定了发射光子的特性;在强光场激发下,发射光子,MandelQ参数和X偏振以及Y偏振光子的关联〈NxNy〉都表现出比较复杂的非线性现象.     

Molecular orientation evolution during low‐density polyethylene blown film extrusion using real‐time Raman spectroscopy

Real‐time polarized Raman spectroscopy was used in this study to measure the molecular orientation evolution during blown film extrusion of low‐density polyethylene (LDPE). Spectra were obtained at different locations along the blown film line, starting from the molten state near the die and extending up to the solidified state near the nip rolls. The trans C C symmetrical stretching vibration of polyethylene (PE) at 1132 cm⁻¹ was analyzed for films possessing uniaxial symmetry. For the given peak, the principal axis of the Raman tensor is coincident with the c‐axis of the orthorhombic crystal, and was used to solve a set of intensity ratio equations to obtain second (〈P₂(cosθ)〉) and fourth (〈P₄(cosθ)〉) moments of the orientation distribution function. The orientation parameters (P₂, P₄) were found to increase along the axial distance in the film line even past the frost‐line height (FLH). The P₂ values also showed an increasing trend with crystalline evolution during extrusion, consistent with past observations that molecular orientation takes place even after the blown film diameter is locked into place. It was also found that the integral ratio (I₁₁₃₂/I₁₀₆₄) obtained from a single, ZZ‐back‐scattered mode can provide a reasonable estimate of molecular orientation. These results indicate the potential of real‐time Raman spectroscopy as a rapid microstructure monitoring tool for better process control during blown film extrusion. Copyright © 2008 John Wiley & Sons, Ltd.     

Precise measurement of the depolarization ratio from photoacoustic Raman spectroscopy

A new method for the accurate determination of the Raman depolarization ratio is reported with an improved setup for photoacoustic Raman spectroscopy (PARS). The precise measurement is achieved by measuring the dependence of the acoustic signal intensity on the cross‐angle between the polarizations of two incident laser beams. We demonstrate this sensitive and simple method with several gaseous molecules, such as CH₄ and H₂. The measured results of depolarization ratios agree well with the theoretical values with an upper error limit of ± 0.005, which is comparable to that with polarization‐resolved CARS spectroscopy. Copyright © 2007 John Wiley & Sons, Ltd.     

Optimization of the design for beamline with fast polarization switching elliptically polarized undulators

Fast switching of X‐ray polarization with a lock‐in amplifier is a good method for acquiring weak signals from background noise for X‐ray magnetic circular dichroism (XMCD) experiments. The usual way to obtain a beam with fast polarization switching is to use two series of elliptically polarized undulators (tandem twin EPUs). The two EPUs generate two individual beams. Each beam has a different polarization and is fast switched into the beamline. It is very important to ensure that the energy resolution, the flux and the spot size at the sample of the two beams are equal in XMCD experiments. However, it is difficult in beamline design because the distances from the two EPUs to the beamline optics are different and the beamline is not switchable. In this work, a beamline design without an entrance slit for fast polarization switching EPUs is discussed. The energy resolution of the two beams can be tuned to be equal by minor rotation of the optics in the monochromator. The flux of the two beams can be balanced through separation blades X, Y in the exit slit, and by adjusting the position of the X blades along the beam. The spot size of the two beams can be adjusted to be equal by shifting the sample as well.     

Polarization-Dependent Light-Gated Phase Conjugation in Methyl-Orange Doped Polymer Film

We have investigated the polarization state of a gated phase conjugate beam using a methyl-orange doped poly-vinyl alcohol (MO/PVA) film in 16 different combinations of linear and circular polarization states of a probe beam and pump beams. The experimental results agree with the theoretical prediction based on the Jones matrix method. Of the third-order nonlinear susceptibility tensors X_ijkl of the MO/PVA film, the X₁₂₂₁ at the 647 nm wavelength in degenerate four-wave mixing under exposure of a gating 515 nm light takes a very small value compared with X₁₁₁₁ and X₁₁₂₂. On the other hand, in the configuration of a probe beam and pump beams having linearly crossed polarizations and without gating light, the MO/PVA film generates a strong linearly polarized PC beam, so that X₁₂₂₁ takes a large value at 515 nm.     

Polarized ARPES spectra of undoped cuprates

The spectral density (SD) in the ARPES spectra of antiferromagnetic (AFM) dielectrics Sr₂CuO₂Cl₂ and Ca₂CuO₂Cl₂ along the principal symmetry directions of the Brillouin zone was studied by the generalized tight binding method. At the valence band top of these undoped cuprates in the AFM state, there is a pseudogap of magnetic nature with E _s(k)～0–0.4 eV between a virtual level and the valence band proper. The observed similarity of dispersion along the Γ-M and X-Y directions can be explained by the proximity of the ³ B _1g triplet and the Zhang-Rice singlet levels. The value of parity of the polarized ARPES spectra at the Γ, M, and X points calculated for the AFM phase of undoped cuprates with an allowance for the partial contributions is even. The conditions favoring observation of the partial contributions in polarized ARPES spectra are indicated. Due to the spin fluctuations, the virtual level acquires dispersion and possesses a small spectral weight. Probably, this level cannot be resolved on the background of the main quasi-particle peak as a result of the damping effects.     

拉曼效应对低双折射光纤偏振态的影响

在准连续情况下, 通过求解低双折射光纤中含有拉曼效应的右旋与左旋圆偏振光所满足的耦合非线性薛定谔方程,着重研究了低输入功率和高输入功率情形在加入拉曼效应前后, 有效偏振拍长和输入功率的变化关系. 结果表明: 无论入射光的偏振是沿着慢轴或沿着快轴, 拉曼效应均改变了入射光偏振态演变的周期, 同时改变了入射光的传输距离.     

Second-and third-harmonic generation by carbon nanotubes irradiated with femtosecond laser pulses

Femtosecond pulses of a Cr:forsterite laser are used to study second-and third-harmonic generation in a layer of single-wall carbon nanotubes produced by low-velocity spraying. The harmonic amplitude in our experiments scales as (I _p)ⁿ as a function of the pump intensity I _p, with n=2 and 3 for the second and third harmonics, respectively. This scaling law holds up to pump intensities on the order of 10¹²W/cm². The ratio of the maximum signal to the averaged background in the spectra of the second and third harmonics is estimated as 50 and 30, respectively. The second and third harmonics produced by a linearly polarized pump field are also linearly polarized, with their polarization vectors oriented along the polarization direction of the pump field. The capabilities of nonlinear-optical methods for structural and morphological analysis of carbon nanotubes are discussed, as well as ways to create solid-state carbon-nanotube generators of optical harmonic.     

高速沉积微晶硅薄膜光发射谱的研究

采用PR650光谱光度计对高速沉积微晶硅薄膜的生长过程进行了在线监测研究,并对所对应的材料进行了Raman谱和红外吸收谱(FTIR)的测试.结果表明：能反映材料晶化程度的I_［SiH^*］/I_［Hβ^*］比值在沉积时间为100 s之内有下降的趋势,且反应气体总流量T_fl越小下降趋势越明显,这与拉曼散射光谱对材料的结构测试结果一致;沉积5 min时I 关键词： 高速沉积 微晶硅薄膜 电子温度     

基于二维结构薄膜的偏振选择相位光栅的研究

利用二维结构薄膜构建了具有偏振选择特性的新型相位光栅,借助严格耦合波分析(RCWA)方法计算了光栅各级衍射强度随入射光波长及入射角的变化,发现在垂直入射情况下,波长600—640 nm范围内,相位光栅对横向电学(TE)模主要产生0级衍射,而对横向磁学(TM)模产生±1级衍射,在波长633nm处,0级衍射光的偏振消光比为109.8,±1级衍射光的偏振消光比为334.6.利用时域有限差分方法对这种相位光栅的偏振分束现象进行了模拟,偏振分离角在玻璃基板内可以达到10°左右,最后模拟了入射角为23°时光栅对不同偏     

Multipeak fitting analysis of Raman spectra on DLCH film

The hydrogenated diamond‐like carbon (DLCH) film with 1‐µm thickness is deposited by direct hydrocarbon gas ion beam method on silicon wafer and annealed at 400 °C. Detailed Raman spectra feature are fitted from nine sets of different peak fitting functions, including Gaussian, Lorentzian and Breit‐Wigner‐Fano (BWF) functions. These fitting results obtained from a two‐peak combination show some specific variances on the G peak position, FWHM_G and I_D/I_G ratio for as‐deposited and as‐annealed DLCH films. The most popular two‐peak fitting method with full Gaussian function tends to exhibit a higher ratio of the G peak position shift and higher I_D/I_G ratio than others fitting methods, the drastic difference among the most popular G (G) & G (D) and B (G) & L (D) schemes also have brought out in I_D/I_G ratio. However, for a more complex four‐peak Gaussian function fitting Raman spectra, the I_D/I_G ratio is close to that of a two‐peak fitting function with a mixture functions of BWF (G) and L (D). Furthermore, a series of systematic peak fitting procedures and comparisons of Raman spectra have been discussed in this study. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of Thermal Treatment on the Structure of Multi-walled Carbon Nanotubes

The effects of vacuum annealing and oxidation in air on the structure of multi-walled carbon nanotubes (MWCNTs) produced by a large-scale catalytic chemical vapor deposition (CCVD) process are studied using Raman spectroscopy and transmission electron microscopy (TEM). A detailed Raman spectroscopic study of as-produced nanotubes has also been conducted. While oxidation in air up to 400°C removes disordered carbon, defects in tube walls are produced at higher temperatures. TEM reveals that MWCNTs annealed at 1,800°C and above become more ordered than as-received tubes, while the tubes annealed at 2,000°C exhibit polygonalization, mass transfer and over growth. The change in structure is observable by the separation of the Raman G and D′ peaks, a lower R-value (I _D/I _G ratio), and an increase in the intensity of the second order peaks. Using wavelengths from the deep ultraviolet (UV) range (5.08 eV) extending into the visible near infrared (IR) (1.59 eV), the Raman spectra of MWCNTs reveal a dependence of the D-band position proportional to the excitation energy of the incident laser energies.     

All-solid-state Nd:Gd<Subscript>0.18</Subscript>Y<Subscript>0.82</Subscript>VO<Subscript>4</Subscript>-LBO green laser at 532 nm

We report a continuous-wave (CW) coherent green radiation at 532 nm by intracavity frequency doubling generation of 1064 nm Nd:Gd_0.18Y_0.82VO₄ laser. With incident pump power of 18.2 W, output power of 1.08 W at 532 nm has been obtained using a 5 mm-long KTP crystal. The optical conversion efficiency was up to 5.9%. At the output power level of 1.08 W, the output stability is better than 5%. The beam quality M² values were equal to 1.26 and 1.12 in X and Y directions, respectively.