Spontaneous Raman scattering spectra of ADP and DADP crystals in different polarization schemes

Spontaneous Raman-scattering spectra of light in ADP and DADP crystals were obtained in four polarization schemes, the widths of some Raman lines were measured. The presence of a strong and sharp Raman line corresponding to the totally symmetric vibration of the phosphate group and a strong diffuse Raman satellite corresponding to the totally symmetric vibration of the ammonium group in the spontaneous scattering spectra of both (ADP and DADP) crystals was observed. According to the theory, such lines are preferable for the stimulated Raman scattering.     

氘含量对DKDP晶体横向受激拉曼散射增益系数的影响

本文测量了不同氘含量K(H_1-xD_x)₂PO₄晶体(DKDP晶体)在Z(XX)Y散射配置下的自发拉曼散射光谱, 并详细分析了氘含量对与横向受激拉曼散射(TSRS)增益系数有关的拉曼频移、半峰宽和散射强度的影响. 然后通过与去离子水拉曼散射对比得出了不同氘含量DKDP晶体的TSRS增益系数. 结果表明随着氘含量的增加DKDP晶体的TSRS增益系数先减小至KDP晶体的40.1%, 后增大至68.9%; 本文认为掺氘后拉曼半峰宽的变化是引起TSRS增益系数随氘含量变化的主要原因.     

Nonlinear-optical properties of photonic crystals

The results of the experimental study of nonlinear-optical effects in photonic crystals, i.e., synthetic opal matrices and nanocomposites (matrices with voids filled with different nonlinear liquids) are presented. The following nonlinear-optical effects were observed under experimental conditions: the photonic flame effect (PFE), stimulated globular scattering (SGS), and stimulated Raman scattering (SRS). The dependence of these effects on the excitation conditions, nanocomposite refractive index contrast, and sample temperature was studied. PFE lines were detected in the Stokes spectral region. SGS spectra at the temperature of liquid nitrogen were studied.     

Picosecond stimulated Raman scattering in crystals

The comparative values of the peak and integral cross sections of spontaneous Raman scattering and the optical dephasing time of molecular vibrations were determined for several oxide crystals by spontaneous Raman spectroscopy. The spectral, time, and energy parameters of stimulated Raman scattering (SRS) were measured for ten crystals using picosecond YLF: Nd laser pumping with a radiation wavelength of 1047 nm. An analysis of the experimental dependence of the threshold energy of pumping SRS on the integral and peak cross sections of spontaneous Raman scattering showed that the SRS gain increment explicitly depended on the integral cross section and was independent of the peak cross section of spontaneous Raman scattering as the ratio between the pumping pulse width (11 ps) and the time of optical dephasing of molecular vibrations changed from 0.42 to 9.3. The gain coefficients of steady-state stimulated Raman scattering under threshold stimulated Raman scattering conditions were determined for all the crystals studied on the basis of the measured threshold SRS pumping energies, the duration and width of the spectrum of pulses, the nonlinear interaction length, the intensity of pumping, and the theoretical dependences that relate the steady-state and transient SRS gain increments. The steady-state SRS gain coefficients obtained in this work fitted well a linear dependence on the peak cross sections of spontaneous Raman scattering, which substantiated the correctness of our analysis and measurements.     

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.     

Hyper-Raman scattering by vibrational excitations in crystals,glasses and liquids

Results of experimental study of hyper-Raman scattering by phonons and polaritons in centrosymmetric and noncentrosymmetric crystals, inorganic oxide glasses and molecular liquids are discussed. Experimental techniques developed for investigation of this weak scattered light are described. We discuss a number of interesting results such as (a) detection of silent modes forbidden in Raman scattering and infrared absorption, (b) determination of the soft mode frequency damping at various temperatures, (c) the study of vibrational polaritons and their resonances with vibrational excitations in centrosymmetrical media, and (d) existence of the vibrational spectra dependence on both value and orientation of momentum transferred to vibrational system in the scattering process in glasses and liquids.     

Determination of the composition of mixtures of organic liquids by CARS spectroscopy

Two methods of realization of coherent anti-Stokes Raman scattering (CARS) spectroscopy of organic liquids are described: with a tunable dye laser and by means of biharmonic laser pumping based on stimulated Raman scattering. The second method is applied to liquids for the first time. The possibilities of the two methods are compared. CARS spectroscopy is applied for the first time to the analysis of a mixture of two organic liquids.     

KDP晶体受激拉曼散射特性

详细比较了磷酸二氢钾(KDP)晶体的自发拉曼散射和受激拉曼散射光谱,在受激拉曼散射(SRS)中观察到了自发拉曼散射中最强的振动模的三阶Stokes光(559.43,589.74,623.50nm),由于其他振动模的受激拉曼散射增益系数较小,其SRS光谱未观察到。另外,比较了传统生长的未退火和退火后的KDP晶体及快速生长的锥区和柱区KDP晶体的受激拉曼散射增益系数,结果表明生长方法和热退火对KDP晶体的受激拉曼散射增益系数无明显影响。     

Raman and Hyper-Rayleigh Scattering in Lithium Tetraborate Crystals

Experimental studies of Raman scattering and the infrared reflection spectra of lithium tetraborate crystals were carried out within a broad temperature range with various polarization geometries. The crystals studied are characterized by record values of radiation resistance, transparence within a broad spectral range including the ultraviolet region, and nonlinear optical properties. A group-theoretical analysis of the vibrational spectra of this crystal was performed and fundamental vibrational terms were assigned to the symmetry types of the point group as well as the polarizations of the corresponding modes. The effective Raman cross section was measured and was found to be one order of magnitude higher than those of the known crystals, in which the stimulated Raman scattering (SRS) was observed. The nonlinear optical and electrooptical coefficients were evaluated and were found to be consistent with the results of independent measurements. The effect of a drastic increase in the intensity of quasi-elastic light scattering at 253 K was registered. It was associated with the phase transition that consists of the disordering of lithium ions with respect to the rigid skeleton. The formation energy for Frenkel defects in the lithium sublattice and the activation energy of the ionic conduction when heating the sample were calculated from the obtained temperature dependences of the intensity of quasielastic and hyper-Rayleigh light scattering.     

Raman Scattering in Three-Dimensional Photonic Crystals

The optical properties of three-dimensional photonic crystals associated with the form of reflection spectra from their surface (appearance of a forbidden gap in the energy spectrum) and with the specific features of Raman scattering are analyzed. Idealized models of the energy band structure of photonic crystals are studied. Expressions for the group velocity of photons with energy close to the forbidden gap are derived. Experimental results on the Raman scattering in photonic crystals based on artificial opal as well as in fused silica are discussed. Bands due to quantum-size effects (presence of nanoclusters in fused silica and nanoglobules forming the lattice of globular photonic crystals) were manifested in the spectra of inelastic light scattering. It is proposed to use photonic crystals for the creation of sensitive sensors of organic and inorganic substances using modern Raman techniques.     

Stimulated Raman scattering of light in artificial opal filled by water

We investigate stimulated scattering of light in globular photonic crystals infiltrated by water. Excitation of stimulated scattering of light is realized using powerful ultrashort (70 ps) laser pulses with an energy of 35 mJ and a frequency repetition of 15 Hz. We use the second-harmonic generation (532 nm) of the master oscillator and amplifier with a wavelength of 1064 nm. The photonic crystals under study are artificial opals filled by water or ethanol. We characterize the sample structures employing an electronic microscope along with the fiber-optics reflectance-spectroscopy technique. Photonic crystals have a stop band near the spectral positions of the exciting line (532 nm) and the first satellite of stimulated Raman scattering of light in water (649 nm). We observe a substantial reduction of the threshold of stimulated Raman scattering of light in water-infiltrated artificial-opal matrices in comparison with that of pure water. Such a reduction is explained as the result of a sharp increase in the photonic density of states near the stop-band edges of investigated photonic crystals. The reduction in the threshold of stimulated Raman scattering of light in water-infiltrated artificial-opal matrices opens up the opportunity to observe stimulated Raman scattering in numerous water media, including water solutions, biological and medical samples, heavy waters, and others.     

Fundamental Studies of Stimulated Raman Scattering and Adaptive Optics on the Basis of Stimulated Light Scattering

Some results associated with the fundamental investigations of stimulated scattering of light in various media are reported. A number of problems of adaptive optics based on stimulated scattering is studied. Concrete results of experimental investigations of physically important characteristics of conventional (spontaneous) and stimulated Raman scattering are presented and discussed. Among them are the scattering indicatrix and the relations between the phases of laser radiation incident on a substance and stimulated scattering. These results were first obtained in the G. S. Landsberg Optical Department of the P. N. Lebedev Physical Institute of the Russian Academy of Sciences. Data on excitation sources and the radiation detection technique are systematized. The main characteristics of spontaneous Raman scattering and forward and backward stimulated Raman scattering (energy parameters, pulse duration, pulse shape, divergence, brightness, spectral width, far- and near-field intensity distributions) are presented. The results on dynamic holography, phase conjugation of light in stimulated scattering, and reconstruction and recording of information (images) with the help of amplifying dynamic holograms are presented. The physical mechanisms responsible for the interference field recording and the phase conjugation of pump radiation in stimulated light scattering are discussed. The data on phase conjugation for stimulated scattering excited by picosecond optical pulses and the results associated with the effect of excitation geometry on the optical phase conjugation in the case of stimulated scattering excited by nanosecond pulses are presented.     

Effect of crystal orientation on picosecond-laser bulk microstructuring and Raman lasing in diamond

In the paper we report on picosecond-laser bulk microstructuring and stimulated Raman scattering (SRS) in type IIa single-crystal diamond in the course of multipulse irradiation at λ=532 nm wavelength using an advanced ps-laser system equipped with additional setups for on-line video imaging and photoluminescence spectra measurements. The effect of crystal orientation (relative to the incident laser beam) on (i) optical breakdown thresholds, (ii) character of bulk modifications, and (iii) generation of stimulated Raman scattering in diamond during irradiation with picosecond pulses of different durations (τ ₁=10 ps and τ ₂=44 ps) is studied. It is shown that the processes of laser-induced breakdown in the bulk of diamond (at the backside of the crystals) and bulk microstructure growth are governed by the dielectric breakdown mechanism. It is found that generation of high-order stimulated Raman scattering in diamond crystals has a considerable effect on the threshold of laser-induced breakdown and bulk microstructuring. Conditions of the efficient SRS lasing are determined, depending on the pulse duration and the direction ([100] and [110]) of the laser beam incidence. A method of local temperature measurements in the bulk of diamond based on the Stokes-to-anti-Stokes intensity ratio in the recorded SRS spectra is proposed, its applicability to determine a “pre-breakdown” temperature of diamond during multipulse ps-laser irradiation is discussed.     

Neutron Scattering and Vibrational Spectra of Molecular Crystals

Infrared and Raman spectra of a number of molecular crystals have been measured for studying molecular vibrations and the intermolecular and intramolecular force fields. The infrared absorption bands arise from interaction of the electric wave with the oscillating dipole moment of the crystal. Raman scattering covers inelastic photon scattering processes and accordingly Raman lines arise from the oscillating polarizability of the crystal. Thus, the vibrational modes observed in infrared absorption or Raman scattering spectra are k = 0 modes, for which translationally equivalent molecules vibrate in phase.     

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.     

Frequency shifts in stimulated Raman scattering

The nonresonant contributions to the nonlinear susceptibility χ⁽³⁾ produce a frequency chirp during stimulated Raman scattering. In the case of transient stimulated Raman scattering, the spectrum of the generated Stokes pulse is found at higher frequencies than expected from spontaneous Raman data. The frequency difference can be calculated from the theory of stimulated Raman scattering.     

近年国内非弹性光散射研究的一些进展

本文介绍了近二年来我国在用喇曼散射和布里渊散射研究半导体、金属及半导体超晶格,高温超导体、磁性物质和介电晶体等方面的进展,以及表面增强喇曼和受激光散射研究的进展情况。     

Stimulated globular scattering of laser radiation in photonic crystals: Temperature dependences

The characteristics of stimulated globular scattering such as the frequency shift, threshold, and conversion efficiency are studied in photonic crystals (synthetic opal matrices and opal nanocomposites) at different temperatures. The results are compared with the study of stimulated Raman scattering in calcite single crystals. In both cases, a decrease in temperature from +20° C to −196° C resulted in an increase in the energy of stimulated scattering energy and its redistribution into the higher-order components.     

DL-丙氨酸晶体粉末的拉曼光谱研究

拉曼光谱可作为研究分子晶体中分子振动和晶格振动的重要手段.DL-丙氨酸晶体和L-丙氨酸晶体晶胞参数相近,结构上同属于正交晶系,但它们的空间群不同,L-丙氨酸晶体的空间群是P212121,而DL-丙氨酸晶体则是Pna21.前人的工作详细研究了L-丙氨酸晶体的拉曼光谱.文章通过对DL-丙氨酸晶体粉末的拉曼光谱研究,和相同条件下D-和L-丙氨酸晶体粉末的拉曼光谱比较,获得了有关DL-丙氨酸晶体中氢键作用和分子构象的信息.