Raman scattering of light in silicon nanostructures: First-and second-order spectra

First-and second-order Raman scattering spectra in Si nanocrystals have been studied. The shift to lower frequencies and the substantial broadening of first-order Raman scattering lines observed to occur with decreasing nanoparticle size were established to correlate with those in second-order spectra. It is shown that the experimentally observed shifts of peaks and their broadening cannot be predicted based only on the phenomenological model of strong phonon wave function localization. The anharmonic effect originating from the heating of the nanoparticle surface by laser radiation should also be included. Proper fitting of experimental data revealed that the anharmonic constants depend strongly on nanoparticle size. The shape and spectral positions of maxima in second-order Raman scattering spectra have been theoretically described.     

The π-electron spectra of the benzene N-heterocyclics

The π-electron spectra of the nitrogen heterocyclics are discussed on the basis of a perturbation applied to the spectra of the corresponding aromatic hydrocarbons. The frequency of the first π→π band in pyridine is the same as that of the corresponding band in benzene, because of the accidental cancellation of a first-order blue shift by a second-order red shift. The first-and second-order shifts are related to two perturbation parameters, and these are obtained from experiment by examining the spectra of the benzene heterocyclics. These parameters can then be satisfactorily related to one another by assuming a perturbation field of the form exp (?2ηr)/r and by allowing for the C-N bond being shorter than the C-C bond in benzene.     

GaN直纳米线的光学性能

对 Ga N直纳米线的拉曼光谱及光致发光光谱进行了研究。拉曼光谱表明 ,与计算值相比 ,E2 ( high)声子频率在 560 cm- 1有 -9cm- 1的移动 ,这种声子频率显示出向低能带频移及带变宽的特征 ,是由于纳米尺寸效应所引起的结果。体系的光致发光光谱在 3 44 .8nm附近的近带隙发光 ,与文献报道的 Ga N体材料的数值3 65nm相比有一蓝移 ,这是由于量子限制效应造成的     

GaAs/AlAs超晶格的近共振喇曼散射研究

本文介绍GaAs/AlAs超晶格的室温近共振喇曼散射测量结果。由于超晶格中Fr?hlich相互作用的共振增强效应,GaAs LO声子偶模的散射得到了很大的增强。和前人的结果一样,在偏振谱我们观察到了偶模。但和前人的结果不同,在退偏振谱中我们观察到的是奇模,而不是偶模。从而证明了在近共振条件下LO声子限制模仍遵从与非共振时一样的选择定则。二级喇曼散射实验结果表明,在偏振谱中二级谱是由两个偶模组合而成,而在退偏振谱中的二级谱与前人的结果不同,由一个奇模与一个偶模组合而成。上述结果与最近提出的黄朱模型的预言是一 关键词：     

Interplay between phonon confinement effect and anharmonicity in silicon nanowires

Getting light out of silicon is a difficult task since the bulk silicon has an indirect energy electronic band gap structure. It is expected that this problem can be circumvented by silicon nanostructuring, since the quantum confinement effect may cause the increase of the silicon band gap and shift the photoluminescence into the visible energy range. The increase in resulting structural disorder also causes the phonon confinement effect, which can be analyzed with a Raman spectroscopy. The large phonon softening and broadening, observed in silicon nanowires, are compared with calculated spectra obtained by taking into account the anharmonicity, which is incorporated through the three and four phonon decay processes into Raman scattering cross-section. This analysis clearly shows that the strong shift and broadening of the Raman peak are dominated by the anharmonic effects originating from the laser heating, while confinement plays a secondary role.     

The Raman effect in crystals

A review is given of progress in the theoretical and experimental study of the Raman effect in crystals during the past ten years. Attention is given to the theory of those properties of long-wavelength lattice vibrations in both cubic and uniaxial crystals which can be studied by Raman scattering. In particular the phenomena observed in the Raman scattering from crystals which lack a centre of inversion are related to the theory. The angular variations of the scattering by any type of lattice vibration in a crystal having any symmetry can be easily calculated using a complete tabulation of the Raman tensor. Recent measurements of first-order lattice vibration spectra are listed. A discussion of Brillouin scattering is included. The relation of second-order Raman spectra to critical points in the lattice vibration density of states is discussed, and measurements of the second-order spectra of diamond and the alkali halides are reviewed. The theory and experimental results for Raman scattering by electronic levels of ions in crystals are examined, and proposals for Raman scattering by spin waves, electronic excitations across the superconductive gap and by plasmons are collected together. Finally, the prospects for applying lasers as sources for Raman spectroscopy are discussed, and progress in the new technique of stimulated Raman scattering is reviewed.     

The Raman effect in crystals

A review is given of progress in the theoretical and experimental study of the Raman effect in crystals during the past ten years. Attention is given to the theory of those properties of long-wavelength lattice vibrations in both cubic and uniaxial crystals which can be studied by Raman scattering. In particular the phenomena observed in the Raman scattering from crystals which lack a centre of inversion are related to the theory. The angular variations of the scattering by any type of lattice vibration in a crystal having any symmetry can be easily calculated using a complete tabulation of the Raman tensor. Recent measurements of first-order lattice vibration spectra are listed. A discussion of Brillouin scattering is included. The relation of second-order Raman spectra to critical points in the lattice vibration density of states is discussed, and measurements of the second-order spectra of diamond and the alkali halides are reviewed.

The theory and experimental results for Raman scattering by electronic levels of ions in crystals are examined, and proposals for Raman scattering by spin waves, electronic excitations across the superconductive gap and by plasmons are collected together.

Finally, the prospects for applying lasers as sources for Raman spectroscopy are discussed, and progress in the new technique of stimulated Raman scattering is reviewed.     

Investigation of micropipes in 6H-SiC by Raman scattering

The spectra of 6H-SiC crystals including micropipes have been examined for the Si face using Raman scattering. The first-order Raman features reveal that the intensity of the transverse optical phonon band centered at ∼796 cm⁻¹ is sensitive to the micropipes. And the second-order Raman features of the micropipes in bulk 6H-SiC are well-defined using the selection rules for second-order scattering in wurtzite structure. It is found that there are some second-order peaks missing for the micropipe-including sample, which may be induced by the reduction of the incident laser intensity at around the micropipe, especially the uneven surface in the inner wall of the micropipe. These features might also be employed to characterize other structural defects such as screw-dislocations and threading edge dislocations.     

温度对β胡萝卜素结构有序的影响

本文测量了全反式β胡萝卜素在二甲基亚砜中81–25 ℃ 范围的紫外–可见吸收和拉曼光谱. 结果表明, 随温度降低, 紫外–可见吸收光谱、拉曼光谱都发生红移, 拉曼光谱线型变窄, 散射截面增加这些现象的发生是由于随温度降低, β胡萝卜素分子的热无序降低、分子结构有序性增加、π电子离域扩展, 有效共轭长度增加, 分子的电子能隙变窄. 另外, 随着温度的降低, 溶剂密度增加, 由Lorentz-Lorenz 关系得知相伴的折射率增加, 从而引起吸收光谱的红移. CC键键长增加, 使CC 键拉曼光谱红移; 振动弛豫时间变长, 各CC 键之间的键长差减小, 线宽变窄; 但由于声子, π电子耦合加强使CC键拉曼线型不对称程度增加, 低频端"肩"扩展, CC键的弱阻尼相干振动增加, 使拉曼散射截面增加. 关键词： β胡萝卜素 分子结构有序 红移 拉曼散射截面     

Band filling and interband scattering effects in MgB2: carbon versus aluminum doping

We argue, based on band structure calculations and the Eliashberg theory, that the observed decrease of T(c) of Al and C doped MgB2 samples can be understood mainly in terms of a band filling effect due to the electron doping by Al and C. A simple scaling of the electron-phonon coupling constant lambda by the variation of the density of states as a function of electron doping is sufficient to capture the experimentally observed behavior. Further, we also explain the long standing open question of the experimental observation of a nearly constant pi gap as a function of doping by a compensation of the effect of band filling and interband scattering. Both effects together generate a nearly constant pi gap and shift the merging point of both gaps to higher doping concentrations, resolving the discrepancy between experiment and theoretical predictions based on interband scattering only.     

n-SiC的电子拉曼散射及二级拉曼谱研究

研究了利用离子注入法得到的掺氮n-SiC拉曼光谱. 理论线形分析表明,与4H-SiC相比,6H-SiC中LO声子等离子体激元耦合模(LOPC模)拉曼位移随自由载流子浓度变化较小. 5145nm激发光下得到的电子拉曼散射光谱表明,k位处由1s(A₁)到1s(E)的能谷轨道跃迁带来的拉曼谱6H-SiC中有四条,4H-SiC中有二条;高频6303及635cm^-1处观察到的谱线被认为与深能级缺陷有关. 最后,利用纤锌矿型结构二级拉曼散射选择定则指认了6 关键词： 碳化硅 电子拉曼散射 轨道能谷分裂 倍频谱     

Size-dependent photo-induced shift of the first exciton band in CdTe quantum dots in glass prepared by a two-stage heat-treatment process

CdTe nanocrystals were grown from commercially available RG850 Schott filter glass by two-step heat-treatment process which almost doubles the particle to matrix volume fraction. A calculation shows that a quantized-state effective mass model in the strong confinement regime might be used to deduce the average radius for the nanocrystals larger than 2 nm in radius from the energetic position of the first exciton peak in optical absorption spectrum. Size-induced shift of ∼360 meV in the first exciton peak position was observed. The steady state photoluminescence spectra exhibit a broad band red shifted relative to the first exciton band, which indicates the existence of shallow trap states. The non-linear optical properties of CdTe nanocrystals were studied by room temperature resonant photoabsorption spectroscopy. The differential absorption spectra had three-lobed structure whose size-dependent evolution was explained by bleaching of the absorption, red shift and broadening in the Gaussian absorption band used to fit the first exciton peak. A maximum red shift of 2.32 meV for the average nanocrystal radius of 4.65 nm was estimated by fitting the photomodulation spectra with a combination of first and second derivative Gaussian absorption bands. We presume that the red shift is induced by the electric field of trapped charges in surface states. Internal electric field strengths of 23 and 65 kV/cm were predicted for the average nanocrystal radii of 3.95 and 4.65 nm, respectively, with the help of second-order perturbation theory in the strong confinement limit.     

Cascade processes in inelastic light scattering in ZnSe nanowire structures

The resonance Raman scattering of light in MBE-grown structures with ZnSe nanowires (10–20 nm in diameter) with an Au film deposited on the substrate used as a catalyst was investigated. The thicknesses of the Au layers were 2, 10, and 100 Å. The photon energy of the He-Cd pump laser (λ = 441.6 nm) was in excess of the band gap of bulk ZnSe, and the measurements were conducted at room temperature. Under these conditions, the Raman spectra are defined by a cascade process in which the electron interacting with a longitudinal optical phonon transfers between real band states with a certain probability of radiative recombination at each step. The blue shift of the luminescence maximum associated with the quantum confinement of carriers in the nanowire has been observed. The average nanowire diameter derived from the magnitude of this shift agrees well with electron microscopy measurements.     

Raman spectra of IVB and VIB transition metal disulfides using laser energies near the absorption edges

In this paper, we present Raman spectra of ZrS₂, HfS₂, MoS₂ and WS₂ using laser energies near the energies of the absorption edges. The Raman spectra probe the properties of the first-excited electronic state and the nature of the electron-phonon coupling. The spectra of the IVB disulfides are independent of the laser excitation energy, suggesting weak electron-phonon interaction. In contrast, additional Raman bands appear in the spectra of the VIB disulfides as the laser energy approaches the band gap energy. The new modes in the spectra of MoS₂ and WS₂ cannot be assigned as first-order processes nor as combination bands of the phonons with zero momentum. The resonance Raman scattering of MoS₂ is analyzed in terms of second-order scattering due to the coupling of phonon modes of nonzero momentum with an electronic transition associated with excitonic states.     

Fano Resonances for Tilted Linear and Quadratic Band Touching Dispersions in a Harmonically Driven Potential Well

Considering models with tilted linear and quadratic band touching dispersions, the effect of the transverse linear tilt on the transmission spectra is analyzed through a harmonically driven potential well oriented longitudinally. Employing the Floquet scattering matrix formalism, Fano resonances are found as an outcome of matching between the Floquet sidebands and quasi-bound states, where the tilt renormalizes their energies and wave vectors. It is found that the Fano resonance energy decreases (increases) for linear (quadratic) band touchings as the magnitude of the transverse momentum increases, indicating a distinct signature of the underlying band dispersion in the transmission profile. The sign of the product of the transverse momentum and the tilt also determines the relative shift in the Fano resonance energy with respect to the untilted case for both band dispersions, suggesting a possible tunability of the Fano resonance for tilted systems. Importantly, the tilt strength can also be directly determined by measuring the Fano resonance energy as function of the transverse momenta direction. The shot noise spectra and their differential property are further studied where an inflection region and undulation, respectively, is found around the Fano resonance energy. Interestingly, differential shot noise and transmission spectra both qualitatively behave in a similar fashion and might thus serve as important observables for future experiments on driven solid-state systems.     

Magneto-Raman scattering inp-type indium antimonide

We report for the first time stimulated magneto-Raman scattering inp-type InSb. Two different Raman scattering processes were observed. The first one has a Raman shift of about 2cm⁻¹/kG and is observed at magnetic fields up to 30kG. The other one is observable only at high magnetic fields above 30kG and shows Raman shifts between 1.2cm⁻¹ and 3.0cm⁻¹ with a tuning rate of about 0.2cm⁻¹/kG. The first process can be interpreted either as spin-flip Raman scattering by photo-excited electrons in the conduction band or as Raman scattering by holes in the valence band involving transitions from heavy to light hole states. The other Raman shift observed seems to occur on account of transitions between the heavy hole ladders.     

Quantum confinement effect in the vibrational spectrum of CdSxSe1−x quantum dots in a fluorophosphate glass matrix

The vibrational spectra of mixed cadmium sulfoselenide nanocrystals in a fluorophosphate glass matrix are investigated by Raman spectroscopy. The asymmetry of the lines of the fundamental modes of nanocrystals is experimentally observed in the region of lattice vibrations, which is interpreted as a quantum confinement effect. In the framework of the model of confined phonons, the contribution of the band states to the Raman scattering spectrum is calculated and the size of nanocrystalline regions is estimated. The results obtained are in good agreement with the data on the low-frequency Raman scattering in these objects.     

Double resonance Raman scattering of second-order Raman modes from an individual graphite whisker

Resonant Raman scattering of second-order Raman modes from an individual graphite whisker synthesized by a high-temperature heat-treatment method at a special pressure was discussed here. The dependence of phonon frequencies on the incoming laser light and the frequency difference between Stokes and anti-Stokes scattering show their origin from double resonance Raman scattering. Our results show that all the experimental results of second-order Raman modes in graphite whiskers, such as the excitation-energy dependence on the mode frequency, the frequency shift between a second-order Raman mode and its fundamentals, and the frequency discrepancy between Stokes and anti-Stokes components of a second-order Raman mode can be well understood by double resonance Raman scattering.     

Frequency Shift of Spectral Lines Generated by Multiple Dynamic Scattering

The frequency shift of spectral lines generatedby single dynamic scattering is known from Wolf'spioneering work. The multiple scattering effect isdiscussed and for the case of redshift two main results are shown: (1) the multiple scattering effectcauses a larger shift than does single scattering, and(2) m scatterings with scattering angle /m eachproduce a larger shift than that done by n scatterings with scattering angle /n each if m >n. These results might be of particular interest inconnection with the observed spectra ofquasars.     

Spectral Changes of Light Produced by Multiple Scattering from Space-Time Fluctuating Random Media under the Born Approximation

Changes in the spectrum of light scattered by space-time fluctuating random media are investigated theoretically within the accuracy of the second-order Born approximation, as a continuation of the previous paper [Opt. Commun. 123 (1996) 234]. Effects of multiple scattering on the resultant field are manifested by a number of illustrative examples. It is shown, in particular, that the spectrum of the scattered light to be observed under practical circumstances is distorted by the process of multiple scattering. Comparison between spectra of both single and multiple scattering lights is also presented in the framework of the dynamic scattering.