Small-core As-Se fiber for Raman amplification

We have demonstrated Raman small-core As-Se fiber. More than 20-dB of gain was observed in a 1.1-m length of fiber pumped by a nanosecond pulse of approximately 10.8-W peak power at 1.50 microm. The peak of the Raman gain occurred at a shift of approximately 240 cm(-1). The Raman gain coefficient is estimated to be approximately 2.3 x 10(-11) m/W, which is more than 300 times greater than that of silica. The large Raman gain coefficient coupled with the large IR transparency window of these fibers shows promise for development of As-Se Raman fiber lasers and amplifiers in the near-, mid-, and long-IR spectral regions.     

四氯化碳费米共振的拉曼光谱研究

费米共振现象是一种广泛存在于分子振动光谱中的现象,特别是结构比较复杂的多原子分子.在多原子分子中当振动倍频或组合频位于某一基频附近,由于发生振动耦合,会出现两个新峰,峰的位置向两侧发生移动,二者谱线强度发生变化,把这种现象称为费米共振.费米共振现象不仅存在于红外光谱中,也存在于拉曼光谱中.文章中测量了CCl4的拉曼光谱,利用所得到的谱线峰位和用Originpro7.5软件程序获得积分强度,用费米共振的相关理论计算了C-Cl的a1对称伸缩振动频率v1与C-Cl2的f对称弯曲振动频率v4的组合频(v1 v4)与(某一未知基频)C-Cl的f对称伸缩振动频率v03的费米共振特征参数,进而计算出了耦合系数W和这一未知基频v03.该文对理解费米共振,了解分子振动频率,研究分子结构有很重要的参考价值.     

Concentration of small ring structures in vitreous silica from a first-principles analysis of the Raman spectrum

Using a first-principles approach, we calculate Raman spectra for a model structure of vitreous silica. We develop a perturbational method for calculating the dielectric tensor in an ultrasoft pseudopotential scheme and obtain Raman coupling tensors by finite differences with respect to atomic displacements. For frequencies below 1000 cm(-1), the parallel-polarized Raman spectrum of vitreous silica is dominated by oxygen bending motions, showing a strong sensitivity to the intermediate range structure. By modeling the Raman coupling, we derive estimates for the concentrations of three- and four-membered rings from the experimental intensities of the Raman defect lines.     

Raman effects in a highly nonlinear holey fiber: amplification and modulation

We experimentally demonstrate that a short length of highly nonlinear holey fiber (HF) can be used for strong L(+) -band (1610-1640-nm) Raman amplification and ultrafast signal modulation. We use a pure silica HF with an effective area of just 2.85mum(2) at 1550 nm, which yields an effective nonlinearity ~15 times higher than in conventional silica dispersion-shifted fiber. Using a 75-m length of this fiber, we obtained internal Raman gains of more than 42 dB and a noise figure of ~6 dB under a forward single-pump scheme, and the Raman gain coefficient was experimentally estimated to be 7.6 chi 10(-14)m/W . Also, an 11-dB signal extinction ratio in a Raman-induced all-optical modulation experiment was achieved with the same fiber.     

Low-frequency vibrational motions in proteins: Physical mechanisms and effect on functioning

A possibility for the low-frequency (terahertz) vibrational motions of large parts of protein molecules is considered and the characteristic frequencies are estimated. The problem of damping of these motions is discussed and the effect of solvent molecules on the model low-frequency molecular oscillators is experimentally studied using Raman spectroscopy. The data obtained show that the effect of solvent is not reduced to a decrease in the Q factor and causes variations in frequencies and relative amplitudes of the low-frequency oscillations.     

Steady-state and transient Raman gain in magnetoactive narrow band-gap semiconductors

Using the hydrodynamic model of semiconductor-plasmas and following the coupled-mode approach, a detailed analytical investigation is undertaken to study both steady-state and the transient Raman gain in transversely magnetized narrow band-gap semiconductors arising from electron density perturbations and molecular vibrations of the medium. Using the fact that the origin of stimulated Raman scattering (SRS) lies in the third-order susceptibility of the medium, we obtain an expression for the gain coefficient of the backward Stokes mode in steady-state and transient regimes and study the dependence of the magnetic field and pump pulse duration on its growth rate. The threshold pump intensity and optimum pulse duration for the onset of transient SRS are estimated. An externally applied magnetic field substantially enhances the transient SRS gain coefficient in narrow band-gap semiconductors, which can be of great use in the compression of scattered pulses.     

Subfemtosecond pulse generation and multiplicative increase of pulse spacing in high-order stimulated Raman scattering

We demonstrate a technique for multiplicatively increasing the pulse spacing of subfemtosecond pulse trains generated by high-order stimulated Raman scattering, which uses pairs of off-resonant two-photon excitation of a molecular Raman mode to establish strong molecular modulation in the medium. Every laser pair has two single-mode laser beams tuned slightly off two-photon resonance, and the laser pairs have their corresponding carrier frequencies shifted from each other by one half, one third, or one fourth of the selected molecular Raman mode frequency. Theoretical analyses and calculations based on the fundamental vibrational transition of solid hydrogen show that the pulse spacing of the subfemtosecond pulse trains thus generated can readily be doubled, tripled, or even quadrupled.     

The moment method for fiber Raman amplifier gain ripple minimization

We aim to propose a novel fiber Raman amplifier modeling based on the moment method, which is previously introduced for modeling the inhomogeneous Erbium doped fiber amplifiers and recently employed to analyze the fiber Raman amplifier with continuous pump spectrum. In this model, the number of governing equations is independent of the number of signals and according to the degree of accuracy it is proportional to the number of pumps. This method is employed to analyze the Raman fiber amplifiers with an arbitrary input signal line shape and to minimize the gain ripple of the fiber Raman amplifier with respect to the pump powers and pump frequencies.     

Resonance suppression and electromagnetic shielding effectiveness improvement of an apertured rectangular cavity by using wall losses

The cavity-mode resonance effect could result in significant degradation of the shielding effectiveness （SE） of a shielding enclosure around its resonance frequencies. In this paper, the influence of coated wall loss on the suppression of the resonance effect is investigated. For this purpose, an equivalent circuit model is employed to analyze the SE of an apertured rectangular cavity coated with an inside layer of resistive material. The model is developed by extending Robinson＇s equivalent circuit model through incorporating the effect of the wall loss into both the propagation constant and the characteristic impedance of the waveguide. Calculation results show that the wall loss could lead to great improvement on the SE for frequencies near the resonance but almost no effect on the SE for frequencies far away from the resonance.     

光纤-氮系统的受激拉曼散射

在实验上,对三种不同长度的石英光纤进行了高温高压扩氮制成了光纤氮拉曼增益介质,给出了测试SRS光谱的实验装置、光谱及传输模式照片.在理论上和实验上讨论了阈值条件和谱线宽度与抽运光功率之间的关系,实验结果和理论计算基本符合.同时,在实验上还观测到微弱的氮分子和石英分子间的耦合模式. 关键词： 光纤-氮系统 受激拉曼散射 传输模式 阈值条件     

Fluoride glass Raman fiber laser at 2185 nm

We report on the first Raman laser based on a fluoride glass optical fiber. The Raman fiber laser was pumped by a 9.6?WTm3+:silica CW fiber laser operating at a wavelength of 1940?nm. A maximum output power of 580?mW was measured at 2185.1?nm, corresponding to a frequency shift of 579?cm(-1) (17.37?THz). We observed a threshold power of 3.8?W and a low power slope efficiency of 29% with respect to the launched pump power. Using those results and the known fiber parameters, we estimated a Raman gain peak value of 3.52*10(-14)?m/W, which is lower than the previously reported values.     

Sättigungseffekte und Linienaufspaltung bei der stimulierten Resonanz-Raman-Streuung

Saturation Effects and Line Splitting of the Stimulated Resonance Raman Scattering The gain of the stimulated resonance Raman scattering (SRRS) is calculated for the case of any sharp resonance. It is shown that under steady-state conditions for longitudinal excitation in direction of the cell the saturation of the absorption is a necessary condition to find a sufficient large gain. Therefore, in the case of a Franck-Condon excitation the SRRS with an excited initial level is only possible, whereas for a O—O-excitation the SRRS with the ground or an excited state as initial state is equally probable. Changing of the population by stimulated fluorescence does not influence this assertion.     

Optical phonons in Ge quantum dots obtained on Si(111)

The Raman light scattering from optical phonons of Ge quantum dots grown by molecular beam epitaxy on a Si(111) surface is studied. A series of Raman lines related to the quantization of phonon spectrum is observed. It is shown that phonon frequencies are adequately described in terms of the elastic properties and the dispersion of the optical phonons of bulk Ge. The strain experienced by the Ge quantum dots is estimated.     

Raman response function for silica fibers

The commonly used Lorentzian form of the Raman response function for studying propagation of ultrashort pulses in silica fibers does not properly account for the shoulder in the Raman gain spectrum originating from the Boson peak. We propose a more accurate form of this response function and show that its predictions for the Raman-induced frequency shift should be in better agreement with experiments.     

Ultraviolet resonance Raman spectroscopy of a β‐sheet peptide: a model for membrane protein folding

The partitioning of a hydrophobic hexapeptide, N‐acetyl‐tryptophan‐pentaleucine (AcWL5), into self‐associated β‐sheets within a vesicle membrane was studied as a model for integral membrane protein folding and insertion via vibrational and electronic spectroscopy. Ultraviolet resonance Raman spectroscopy allows selective examination of the structures of amino acid side chains and the peptide backbone and provides information about local environment and molecular conformation. The secondary structure of AcWL5 within a vesicle membrane was investigated using 207.5‐nm excitation and found to consist of β‐sheets, in agreement with previous studies. The β‐sheet peptide shows enhanced Raman scattering cross‐sections for all amide modes as well as extensive hydrogen‐bonding networks. Tryptophan vibrational structure was probed using 230‐nm excitation. Increases in Raman cross‐sections of tryptophan modes W1, W3, W7, W10, W16, W17, and W18 of membrane‐incorporated AcWL5 are primarily attributed to greater resonance enhancement with the B_b electronic transition. The W17 mode, however, undergoes a much greater enhancement than is expected for a simple resonance effect, and this observation is discussed in terms of hydrogen bonding of the indole ring in a hydrophobic environment. The observed tryptophan mode frequencies and intensities overall support a hydrophobic environment for the indole ring within a vesicle, and these results have implications for the location of tryptophan in membrane protein systems. Copyright © 2009 John Wiley & Sons, Ltd.     

Tellurite glasses with peak absolute Raman gain coefficients up to 30 times that of fused silica

An experimental system has been assembled to measure the absolute values of the Raman gain spectrum for millimeter-thick glass samples. Results are reported for two new oxide glasses with Raman gain coefficients as much as 30 times larger than that of fused silica and more than twice its spectral coverage.     

Passively Q-switched fiber laser that uses saturable Raman gain

Operation of a short all-fiber passively Q-switched Raman laser pumped by a continuous-wave laser diode is experimentally demonstrated. The passively switched fiber laser consists simply of a double-clad ytterbium-doped silica fiber that is spliced directly to a moderately germanium-doped silica fiber. The placement of the Ge-doped silica fiber within the fundamental (Raman pump) cavity allows interplay between fundamental and Stokes fields to take place, which leads to saturation of the Raman gain as a result of pump depletion. Pulse widths of 70 and 60 ns at the first and second Stokes wavelengths of 1168 and 1232 nm, respectively, are produced at a stable 588 kHz repetition rate.     

Transform method in resonance Raman scattering with quadratic Franck–Condon and Herzberg–Teller interactions

A new method of considering quadratic vibronic interactions and deviations from the Condon approximation in resonance Raman scattering (RRS) is proposed. The method is used for the generalization of the transform law between absorption and Raman excitation profiles, derived earlier for a basic model. In the case of arbitrary mixing of modes with similar frequencies, a simple generalization of the transform law for first-order RRS is obtained. For an arbitrary change of frequencies without mode mixing the transform law for the nth-order RRS is given. The equations are valid for an arbitrary number of modes and also take temperature effects into account.     

Femtosecond stimulated Raman scattering picosecond molecular thermometry in condensed phases

We demonstrate the capability of femtosecond stimulated Raman scattering (FSRS) data to measure the temperature of condensed matter at the molecular vibrational level. We report the temperature dependence of Raman loss to Raman gain ratios for low frequency modes (<300 cm(-1)) in a CaCO3 single crystal from cryogenic to room temperature, which is shown to be in agreement with theoretical predictions. We also report the measurements of nonequilibrium time evolution of mode specific vibrational temperatures in the CaCO3 single crystal to demonstrate that FSRS can measure temperature on picosecond time scales. Finally, we point out the unique origin of this temperature dependent anti-Stokes to Stokes ratio in stimulated Raman, which is not present in other coherent Raman spectroscopies. These measurements require no material dependent parameters or prior calibration.     

Condensation process of alcohol molecules on mesoporous silica MCM-41 and SBA-15 and fumed silica: a spin-probe ESR study

A few alcoholic solutions of di-tert-butyl nitroxide (DTBN), a spin probe, at a high concentration were condensed on several silica materials, such as MCM-41, two types of SBA-15, and fumed silica, at various amounts in vacuum. At a very low solution dose the electron spin resonance (ESR) spectrum is that of an immobilized nitroxide radical. With increasing solution dose, the spectrum is gradually sharpened and a well-separated three-line spectrum is observed at the dose that is estimated to fill the surface with a monomolecular layer. Thus, the DTBN molecule can make rapid tumbling motion on this solvent layer. With a further increase in the solution dose the ESR spectrum is modified in different ways from system to system: the line width increases approximately linearly with respect to the solution dose for the SBA-15 and fumed silica systems, but it remains almost constant for the MCM-41 system until the solution dose exceeds the total volume of a nanochannel. The line width increase with respect to the solution dose is small for the SBA-15 system but large for the fumed silica system. These results have been interpreted geometrically with the structures of these silica materials and a condensation model for the alcohols on these surfaces. In relation to the present results, a model of the collective molecular flow of the alcohol solutions through the nanochannel of MCM-41 is given.