Goldstone mode of a magnon Bose−Einstein condensate in MnCO<Subscript>3</Subscript>

The dependence of the enhancement of the Raman scattering on the size of a dielectric column is measured in structures with the spatial modulation of the height and lateral sizes of the dielectric coated with a thick metal layer (10–80 nm). It is established that, in the case of a thick metal coating (silver, gold, and copper coatings are used) at dimensions of the dielectric column close to the laser pump wavelength, considerable enhancement of the Raman signal oscillating upon the variation of the geometrical dimensions of the structure is observed. It is shown that the observed resonance enhancement of the Raman signal is associated with the transformation of the electromagnetic radiation into localized plasmon–polariton modes, and the efficiency of such transformation is determined by the commensurability of the wavelength of the plasmon–polariton mode and the planar size of the metal film. For different metal coatings, the dependence of the enhancement of the Raman scattering on the laser wavelength is measured.     

The influence of surface roughness on the Raman scattering of pyridine on copper and silver surfaces

The surface enhanced Raman scattering (SERS) has been measured of pyridine adsorbed on silver or copper with various surface roughness. The surface has been controlled by electron microscopy. In addition the incident laser wavelength has been varied. By taking into account the dielectric constants of silver and copper the maximum enhancement factors for an average roughness of 1000 Å and 500 Å for silver and copper, respectively, could be quantitatively accounted for by the antenna resonator model.     

生物分子拉曼散射测定过程中荧光背景的抑制

拉曼共焦技术、表面拉曼增强技术以及降低入射激光强度的方法，被运用于SOD、DNA等生物分子拉曼散身实验中，以抑制其荧光背景，改善光谱质量。     

Raman scattering enhancement by dielectric spheres

Raman scattering experiments were performed on Si(60 nm)/metal/substrate structures with and without silica microspheres (with a diameter between 0.5 and 5 µm) on top. Raman scattering from the thin Si layer exhibits enhancements (~20) due to the dielectric spheres, where the enhancement factors depend on the diameter of the spheres. The interaction between light and dielectric spheres has been simulated by finite difference time domain calculations (FDTD), wherein particularly the electric energy density (ED) distribution in the thin Si layer was of concern. For microspheres with a diameter less than ~3 µm, the transverse ED distribution (perpendicular to the incident light direction) within the Si layer is characterised by a single peak centered on the optical axis. For larger diameters, a multimodal transverse ED distribution develops where the maximum is not centered on the optical axis. Using an ad‐hoc approach for surface enhanced Raman scattering in combination with the FDTD calculations, the experimental Raman observations are well accounted for. Copyright © 2013 John Wiley & Sons, Ltd.     

Raman Scattering From Water Vapor

Raman scattering from water vapor has been observed using a thermally-tuned TEM₀₀ ruby laser as excitation for a range of incident wavelengths from 6943.1 Å to 6944 Å covering the strong water absorption line centered at 6943.8 Å with a linewidth of 0.04 Å. Raman scattering cross-section relative to nitrogen has been determined (for the first time to our knowledge) to be 0.39±0.12. No significant enhancement of Raman signal was observed even though the incident ruby was tuned to the absorption line. Possible reasons for the lack of resonance Raman effect are given.     

Effect of shape and interstice on surface enhanced Raman scattering (SERS) of molecules adsorbed on gold nanoparticles in the near‐dipole and quadrupole regions

Surface enhanced Raman scattering (SERS) of adsorbed molecule on colloidal gold nanoparticles of different shapes, namely nanospheres (NSs), nanorods (NRs), and nanoprisms (NPs) as well as the three NPs arrays of different interstice prepared by NS lithography, are studied with incident wavenumbers in the near‐dipole and near‐quadrpole regions of the nanoparticles. In the colloidal gold nanoparticles, the SERS enhancement is the largest for the sharp tip followed by the truncated tip NPs, then the NRs and least enhancement for the NSs. This decreasing order of enhancement occurs although the incident wavenumber was near the dipole resonance of NSs and the quadrupole resonance for the NPs. These varied enhancements are explained in part as due to the binding energies of the nanocrystal facets, but the larger contribution results from the plasmon electromagnetic fields. A parallel finite difference time domain (FDTD) calculations were carried out, which corporate the experimental results and show agreement with ratios of the SERS enhancement for the different shapes. The normalized SERS intensity for NPs of different interstice distances show a sharp rise with the decrease of the interstice distances because of interparticle dipolar and quadrupolar coupling as evidenced also by FDTD calculations. Furthermore, these calculations show that the enhancement is polarization independent for an incident wavelength near quadrupole resonance but polarization dependent for an incident wavelength near the plasmon dipole transition. In the last case, the enhancement is larger by an order of magnitude for a polarization parallel to the NPs bisector than for polarization normal to the bisector with no hot spots for the relatively large interstice dimensions used. Copyright © 2012 John Wiley & Sons, Ltd.     

Chemical contribution to surface-enhanced Raman scattering

We present a new mechanism for the chemical contribution to surface-enhanced Raman scattering (SERS). The theory considers the modulation of the polarizability of a metal nanocluster or a flat metal surface by the vibrational motion of an adsorbed molecule. The modulated polarization of the substrate coupled with the incident light will contribute to the Raman scattering enhancement. We show that for a metal cluster and for a flat metal surface this new chemical contribution may enhance the Raman scattering intensity by a factor of approximately 102 and approximately 104, respectively. The new SERS process is determined by the electric field parallel to the surface of the metal substrate at the molecular binding site.     

Orbital ordering in LaMnO3 investigated by resonance Raman spectroscopy

Orbital ordering leads to an unconventional excitation spectrum that we investigate by resonance Raman scattering using incident photon energies between 1.7 and 5.0 eV. We use spectral ellipsometry to determine the corresponding dielectric function. Our results show resonant behavior of the phonon Raman cross section when the laser frequency is close to the orbiton-excitation energy of 2 eV in LaMnO3. We show an excellent agreement between theoretical calculations based on the Franck-Condon mechanism activating multiphonon Raman scattering in first order of the electron-phonon coupling and the experimental data of phonons with different symmetries.     

Theory of Raman scattering in Europium-chalcogenides

The intensity of the spin assisted Raman scattering in the Eu-chalcogenides is calculated using the excited states which were used in the analysis of the optical absorption. Various mechanisms are examined for the Raman scattering. In these mechanisms, the cross effect of the 4f spin-orbit interaction and the exciton-phonon interaction is found to be the most important for the scattering intensity. The characteristics of the Raman scattering from this mechanism are as follows: When the frequency of the incident light is in the tail region of the absorption peak, the polarization of the scattered light is perpendicular to that of the incident light; when the light in the middle region of the absorption peak is applied, the polarization of the incident light is depolarized in the scattered light; the scattering intensity decreases when the spin fluctuation is suppressed by an application of magnetic field or by lowering temperature through the Curie temperature. These characteristics in the Raman scattering have been observed in the experiments.     

任意椭圆偏振激光场非线性汤姆逊散射的一般表述与X射线产生的优化条件

基于相对论条件的电动力学,解析求解了任意椭圆偏振条件下激光场的非线性汤姆逊散射的一般表示.利用解析结果得到了背向非线性汤姆逊散射高次谐波的极值条件.结果表明对于基频背向汤姆逊散射,在相同条件下,圆偏振激光具有最大值而线偏振是最小值,如果激光偏振态从圆偏振连续的过渡到线偏振,背向汤姆逊散射的角功率随之单调递减.这一效应对高强度入射激光尤为重要,当 a² >5时,圆偏振情形的贡献几乎是线偏振的2倍.这对基于汤姆逊散射机制的X射线源实验研究具有重要参考意义. 关键词： 非线性汤姆逊散射 偏振度 极值     

Effect of changing incident wavelength on Raman features of optical phonons in SiC nanorods and TaC nanowires

Novel Raman scattering in polar semiconductor SiC and TaC one-dimensional materials have been carried out. With increasing incident laser wavelength from 488 to 633 nm there is a huge difference in Raman intensity enhancement for the LO/IF peaks and the TO peak. This has been interpreted as due to Fröhlich interaction and abundant defects in polar nano-scale semiconductor materials.     

Optical phase change in bismuth through structural distortions induced by laser irradiation

ABSTRACT

Semimetal bismuth (Bi) is known to possess a wide range of peculiar properties, owing to its unique electronic band structure. Its electronic band can easily be distorted by structural changes, and thereby undergo transitions between semimetal to either semiconductor or metal states. Utilising a focused laser beam, one can easily introduce structural defects, along with phase changes, oxidation, and morphological modifications. Confocal Raman microscopy indicated that the as-fabricated Bi droplets inhibit the Raman signal from the underlying silicon (Si) substrate. After a laser flash heating step, the intensity of Si optical phonons was strongly enhanced at the positions of Bi droplets, and exceeding the intensity from the bare Si substrate. Thus, such laser irradiating step on the Bi droplets induces an optical phase change. The optical phase change was detected as going from inhibition to strong enhancement of the underlying Si substrate Raman signal. From the observed Bi optical phonon modes (E_g and A_1g), alterations in the Raman peaks due to laser exposure indicated that the ordered crystallinity in pristine Bi droplets became deteriorated. The effects of atomic displacements and loss of structural order in Bi droplets impacts its dielectric response. The observed Si Raman signal enhancement is similar to the surface-enhanced Raman scattering effect typically known for noble metals.     

Polarization effect of femtosecond pulse breakdown in subwavelength antireflective relief grating

The transmittance property and the near-field distribution of subwavelength broadband antireflective grating directly patterned into the wide bandgap dielectric material as a function of the surface period and groove depth are performed by a rigorous Fourier modal method. It is found that the transmittivity is insensitive for TE and TM polarization, but the near-field distribution associated with laser damage resistance ability is strongly dependent on polarization state of incident light. What's more, the femtosecond pulse laser damage threshold of surface structure taking into account local maximum electric field enhancement was calculated numerically using a theoretical ionization mechanism model. The higher threshold on the surface period, pulse duration and incident wavelength for TM polarization than that for TE wave is demonstrated quantitatively.     

Plasmonic laser nanoablation of silicon by the scattering of femtosecond pulses near gold nanospheres

We present the fabrication of nanostructures ablated on silicon(100) by the plasmonic scattering of 780 nm, 220 fs laser pulses in the near-field of gold nanospheres. We take advantage of the enhanced plasmonic scattering of ultrashort laser light in the particle near-field to ablate well-defined nanocraters. Gold nanospheres of 150 nm diameter are deposited onto a silicon surface and irradiated with a single laser pulse. We studied the effect of laser polarization on the morphology of ablated nanostructures and estimated the minimum fluence for plasmonic nanoablation. When the polarization of the incident radiation is directed at a 45° angle into the substrate surface, a near-field enhancement of 23.1±7.6 is measured, reducing the required silicon ablation fluence from 191±14 mJ/cm² to 8.2±2.9 mJ/cm². Enhancements are also measured for laser polarizations parallel to the substrate surface when the substrate is angled 0° and 45° to the incident irradiation, giving enhancements of 6.9±0.6 and 4.1±1.3, respectively. Generated nanocrater morphologies show a direct imprint of the particle dipolar scattering region, as predicted in our theoretical calculations. The measured near-field enhancement values agree well with the maximum field enhancements obtained in our calculations. The agreement between theory and measurements supports that the nanocraters are indeed formed by the enhanced plasmonic scattering in the near-field of the nanoparticles. PACS 42.62.-b; 52.38.Mf; 81.65.Cf; 81.16.-c; 78.67.Bf     

Self-focusing of Gaussian laser beam in collisionless plasma and its effect on stimulated Raman scattering process

This paper presents an investigation of self-focusing of Gaussian laser beam in collisionless plasma and its effect on stimulated Raman scattering process. The pump beam interacts with a pre-excited electron plasma wave thereby generating a back-scattered wave. On account of Gaussian intensity distribution of laser beam, the time independent component of the ponderomotive force along a direction perpendicular to the beam propagation becomes finite, which modifies the background plasma density profile in a direction transverse to pump beam axis. This modification in density affects the incident laser beam, electron plasma wave and back-scattered beam. We have set up the non-linear differential equations for the beam width parameters of the main beam, electron plasma wave, back-scattered wave and SRS-reflectivity by taking full non-linear part of the dielectric constant of collisionless plasma with the help of moment theory approach. It is observed from the analysis that focusing of waves greatly enhances the SRS reflectivity.     

Raman scattering studies of individual polar semiconducting nanowires: phonon splitting and antenna effects

Results from Raman scattering experiments on individual crystalline GaP nanowires are presented which indicate that the shape of the nanowire, i.e., the high aspect ratio, may be responsible for two new phenomena involving optical phonons: (1) a shape-inducedsplitting of both the longitudinal optical (LO) and transverse optical (TO) phonons at the center of the Brillouin zone (q=0), and (2) a Raman scattering “antenna” effect which masks the normal Raman polarization selection rules. We suggest that (1) stems from the asymmetry in the long range dipolar sums that control the electromagnetic LO-TOsplitting, and we identify the Raman antenna effect (2) with the internal electric field created by Mie resonances in the nanowire driven by the incident laser field. Although these effects are reported here for GaP, they are expected to be general effects observable in many semiconducting nanowire systems. PACS 78.67.-n; 78.67.Lt; 78.30.-j; 78.30.Fs; 72.10.Di     

In-plane optical anisotropy of two-dimensional VOCl single crystal with weak interlayer interaction

Transition-metal oxyhalides $MOX$ ($M = {\rm Fe}$, Cr, V; $O ={\rm oxygen}$, $X= {\rm F}$, Cl, Br, I), an emerging type of two-dimensional (2D) van der Waals materials, have been both theoretically and experimentally demonstrated to possess unique electronic and magnetic properties. However, the intrinsic in-plane anisotropic properties of 2D VOCl still lacks in-depth research, especially optical anisotropy. Herein, a systematic Raman spectroscopic study is performed on VOCl single-crystal with different incident laser polarization at various temperatures. The polarized-dependent Raman scattering spectra reveal that the ${{ A}}_{{\rm g}}$ mode of VOCl show a 2-lobed shape in parallel polarization configuration while a 4-lobed shape in vertical configuration. In addition, the temperature-dependent and thickness-dependent Raman scattering spectra confirm a relatively weak van der Waals interaction between each layers among VOCl single crystal. These findings might provide better understanding on the in-plane anisotropic phenomenon in VOCl layers, thus will accelate further application of 2D single crystals for nanoscale angle-dependent optoelectronics.     

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.     

Optically produced arrays of planar nanostructures inside fused silica

Linearly polarized femtosecond light pulses, focused inside fused silica to an intensity that leads to multiphoton ionization, produce arrayed planes of modified material having their normal parallel to the laser polarization. The planes are < or = 10 nm thick and are spaced at approximately lambda/2 in the medium for free space wavelengths of both 800 and 400 nm. By slowly scanning the sample under a fixed laser focus, order is maintained over macroscopic distances for all angles between the polarization and scan direction. With the laser polarization parallel to the scan direction we produce long-range Bragg-like gratings. We discuss how local field enhancement influences dielectric ionization, describe how this leads to nanoplane growth, why the planes are arrayed, and how long-range order is maintained.     

Surface-enhanced Raman spectroscopy of semiconductor nanostructures

We review our recent results concerning surface-enhanced Raman scattering (SERS) by confined optical and surface optical phonons in semiconductor nanostructures including CdS, CuS, GaN, and ZnO nanocrystals, GaN and ZnO nanorods, and AlN nanowires. Enhancement of Raman scattering by confined optical phonons as well as appearance of new Raman modes with the frequencies different from those in ZnO bulk attributed to surface optical modes is observed in a series of nanostructures having different morphology located in the vicinity of metal nanoclusters (Ag, Au, and Pt). Assignment of surface optical modes is based on calculations performed in the frame of the dielectric continuum model. It is established that SERS by phonons has a resonant character. A maximal enhancement by optical phonons as high as 730 is achieved for CdS nanocrystals in double resonant conditions at the coincidence of laser energy with that of electronic transitions in semiconductor nanocrystals and localized surface plasmon resonance in metal nanoclusters. Even a higher enhancement is observed for SERS by surface optical modes in ZnO nanocrystals (above 10⁴). Surface enhanced Raman scattering is used for studying phonon spectrum in nanocrystal ensembles with an ultra-low areal density on metal plasmonic nanostructures.