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.     

Scattering matrix for Raman effect in cubic crystals

The scattering matrix for Raman effect in cubic crystals for any general orientation of the crystal and any angle of scattering is given. The averages of the matrix over all possible orientations is also given and checks with the well known value of 6/7 for the depolarisation of degenerate lines for transverse scattering. The scattering matrix for backward scattering in terms of polar angles and for some special cases are also given.     

The Raman surface scattering in crystals

The influence of the crystal surface on the first order Raman spectrum has been theoretically examined, using the microscopic theory of the Raman scattering of the light with finite wavelength. It is shown that the Raman spectrum of the first order consists in this case of continuous bands. In addition to these bands some frequencies that belong to the surface localized modes out of these bands can appear. In the case of the Rosenstock model of the rocksalt structure with equal masses of ions, the solution is carried out analytically.     

Raman scattering in TTF crystals

Polarized Raman spectra at 2, 80 and 300 K are reported for lattice and intramolecular modes in single crystals of tetrathiofulvalene, the donor molecule of the charge transfer complex TTF—TCNQ. In addition to these Raman features, a low-temperature emmision spectrum with associated sharp line structure is reported.     

Theory of the influence of optical activity on Raman effect in crystals

The theory of the “banding” of Raman lines observed in optically active crystals has been fully worked for the different classes of Raman lines. In the case of cubic crystals the Raman lines belonging toE andF classes can exhibit “normal” “reverse” or any intermediate type of banding depending on the crystal orientation, and further the perfection of the banding can vary forE lines. The crystal axes and the sign of optical activity of a cubic crystal can be deduced from Raman effect studies.     

Pre-resonant Raman scattering in antracene crystals

The pre-resonant Raman scattering (PRRS) of anthracene crystals (T = 4.2?30 K) has been studied. A tunable dye laser (v_exc = 25000?25100 cm^-1 was used to produce an excitation close to the lowest exciton band bottom (v₀ = 25097 cm^-1). A sharp increase of the PRRS intensity when approaching the excitation light frequency v_exc to v₀ and then the not less sharp drop of the PRRS intensity close to resonance (v₀ ? v_exc ≈ 35?15 cm^-1) where observed. An extremely high PRRS sensitivity to temperature and the excitation light intensity was found. The results are explained within the framework of stimulated PRRS in anthracene crystals.     

Raman scattering in defect chalcopyrite crystals

Summary Raman-scattering measurements on the defect chalcopyrite compounds CdGa₂Se₄, CdGa₂S₄, ZnGa₂S₄ are reported. The phonon picture is consistent with the space group. A comparison between corresponding phonons suggests a simple model for the vibrational dynamics. Preliminary Raman results in the mixed compound Cd _x Zn_1−x Ga₂S₄ and in resonance conditions in CdGa₂Se₄ are presented. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Raman scattering of x-rays in crystals

The quantum theory for Raman scattering of x-rays by K electrons in crystals is given. The electrons ejected into the conduction band are described by plane waves. The expression obtained for the differential cross section has a Compton-like angular dependence in contrast to the results of Japanese physicists who used the dipole approximation in their work.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 9, pp. 26–28, September, 1971.     

Raman scattering in impurity-induced anharmonic crystals

The theory of first-, second-, and third-order Raman scattering is investigated for isotopically disordered anharmonic crystals. The theory of time-dependent thermodynamic Zubarev Green's functions is adopted to obtain the Raman tensor, intensity of Raman lines, and differential cross sections of various orders of scatterings. It is observed that each class of scattering can be separated into diagonal and nondiagonal parts. The first-order and nondiagonal parts are absent in the case of chemically pure crystals. The diagonal parts are separated into anharmonic and interference terms. The interference terms arise due to the interactions of anharmonic phonons with the local phonons. The temperature and defect dependencies are discussed in detail along with the nature of continuous and line spectra. It is proposed that very high-power laser sources will reveal the third-order spectra, and that the resulting structure can be explained with the help of temperature-dependent one-, two-, and three-phonon density of states.     

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.     

Low-threshold parametric Raman generation of high-order Raman components in crystals

The theoretical and experimental study of the axial transient multiwave parametric Raman generation via four-wave mixing of arbitrary neighboring Stokes components in simple tungstate crystals is carried out. In agreement of the theoretical modeling with the experimental results, we have found that because of the four-wave mixing of arbitrary Stokes components in the short crystals, the stimulated Raman scattering (SRS) generation thresholds of the high-order Stokes components are significantly reduced relative to the values defined by the cascade-like mechanism of SRS.     

Raman frequencies of fluorite crystals

The frequency of th Raman active mode in CaF₂, EuF₂, PbF₂, SrCl₂, BaF₂ and BaCl₂ has been measured at 300 and 77°K.     

New crystals for Raman lasers

Steady-state and transient stimulated Raman scattering (SRS) in crystals is analyzed. The basic laws of an increase in the SRS gain in crystals are revealed, and methods for searching and creating new SRS laser materials are developed. New crystals for picosecond and nanosecond SRS lasers are proposed, fabricated, and characterized. These materials have the highest SRS cross section, a low SRS threshold, and a wide spectral range of operation.     

Defects in crystals studied by Raman scattering

Raman studies of crystal defects are reviewed. Raman spectroscopy is a powerful technique and has been used widely for investigating disordered structures. The degree of disorder in a crystal is quantitatively evaluated in terms of the phonon correlation length. The asymmetric Raman line shapes in defective crystals such as microcrystals, ion-implanted semiconductors are well reproduced by the spatial correlation (SC) model. The effect of alloying induced-potential fluctuations on Raman scattering is also explained within the framework of the SC model. In disordered graphite, the in-plane phonon correlation length is obtained from the relative intensity ratio of the disorder induced peak. The initial lattice disordering rates and the relaxation rates of disorder are determined, using real-time Raman measurements during ion irradiation in a scale of seconds. In this way, the phonon confinement due to the local defects is observed in the kinetic manner. Localized vibrational modes of defects in crystals are also described. In particular, Raman observation of the hydrogen molecule in crystalline semiconductors is discussed in detail.     

Nonstationary Raman amplification of superluminescence in crystals

The sequential excitation of LiF crystals with F ₂ ^? color centers and KGd(WO₄)₂ crystals by 1047-nm laser pulses with a duration of 22 ps is investigated. Broadband (275 cm^?1) Stokes superluminescence appearing in the fluoride crystal undergoes nonstationary Raman amplification in the oxide crystal. The amplified pulses with a Stokes frequency shift of 767 cm^?1, a duration of 7 ps, and a power up to ～1.1 MW have high spatial and time coherence and exhibit time delay increasing from ～1 to 4.5 ns with a decreasing pump power. The results are discussed in terms of the cooperative Raman scattering.     

Stimulated Raman scattering in three-dimensional photonic crystals

The results of experimental studies of stimulated Raman scattering of light (SRS) excited in three-dimensional photonic crystals — synthetic opal matrices infiltrated with Raman active media are presented. It is shown that the SRS threshold in such structures decreases with respect to the SRS threshold in Raman active bulk materials. The influence of the photonic-band structure of the active materials used on the SRS properties is estimated.     

The CIDNP effect in plastic crystals

The CIDNP effects in the photolysis reactions of some aldehydes and ketones (including linear, aromatic and cyclic) in plastic crystals of cyclohexane have been discovered and studied. In going from liquid to solid solutions, the change of polarization sign is observed for some substances investigated. The CIDNP effect for a simple model for a relative motion of atoms of radicals possessing spin density (translational diffusion in a restricted volume) has been calculated. The results observed are interpreted in the framework of the radical pair mechanism of CIDNP effect formation.     

Raman phonons in multiferroic FeVO_4 crystals

Multiferroic materials are promising candidates for next-generation multi-functional devices, because of the coexistence of multi-orders and the coupling between the orders. FeVO4 has been confirmed to be a multiferroic compound,since it exhibits both ferroelectricity and antiferromagnetic ordering at low temperatures. In this paper, we have performed careful Raman scattering measurements on high-quality Fe VO4 single crystals. The compound has a very rich phonon structure due to its low crystal symmetry(P- 1) and at least 47 Raman-active phonon modes have been resolved in the low and hightemperature spectra. Most of the observed modes are well assigned with aid of first-principles calculations and symmetry analysis. The present study provides an experimental basis for exploring spin-lattice coupling and the mechanism of multiferroicity in FeVO4     

Raman scattering of light in N-benzylaniline crystals

Single crystals of N-benzylaniline (C₆H₅-CH₂-NH-C₆H₅) have been grown, X-ray structural analysis of these crystals has been performed, and their symmetry has been identified as corresponding to the space group C _2h ⁵ . Raman spectra of N-benzylaniline single crystals in polarized light for different geometries of the experiment have been analyzed. Frequencies and symmetry of lattice phonons have been determined.