The department was organized in 1985 (Head Prof. G.N. Zhizhin) and it included two laboratories: Laboratory of Crystal Spectroscopy created in 1969 (Head Prof. G.N. Zhizhin) and Laboratory of Disordered Structures Spectroscopy (Head Prof. E.A. Vinogradov) changed from the group “Spectroscopy of High Resolution” in 1984 split from Laboratory of Crystal Spectroscopy in 1982.

At present the head of the department is Prof. E.A. Vinogradov (his Deputy is Prof. B.N. Mavrin). Now the department includes two laboratories: Laboratory of Condensed Matter Spectroscopy (Head Prof. B.N. Mavrin), Laboratory of Semiconductor Structures Spectroscopy (Head Prof. E.A. Vinogradov), and Center for Fourier Spectroscopy (Head Prof. M.N. Popova). The structure of the department changed since 1985, but its basis was Laboratory of Crystal Spectroscopy aimed on the development of the infrared and far infrared (FIR) techniques, the Raman spectroscopy (RS) of the organic and inorganic compounds, the study of the physical principles of coupling of the spectroscopic parameters with the structure and properties of crystals and their components (molecules and molecular ions), the spectroscopy of surface states of both solid and the surface formations on it (adsorption, chemosorption, corrosion and al.), and also the development of new automatic facilities maintaining the broad range of wavelengths (1-500 μm) in combination with high resolution (0.001 cm^-1), high photometric accuracy (0.3 %) and very high ability to the detection of spectra of monolayer and submonolayer of organic and inorganic compounds on metals or dielectrics. The second main laboratory of the department was Laboratory of Molecular Spectroscopy created in 1969 (Head Dr. Kh.E. Sterin ) and changed in Laboratory of Vibrational Spectroscopy of Condensed Matter in 1987 (Head Prof. B.N. Mavrin). The Raman and hyper-Raman spectroscopy of crystals, new carbon materials, porous semiconductors, superconducting and superionic crystals, the film structures, amorphous materials and liquids is the main goal of this Laboratory.

The Laboratory of Condensed Matter Spectroscopy was created after the merger of Laboratory of Crystal Spectroscopy (Head Prof. G.N. Zhizhin) and Laboratory of Vibrational Spectroscopy of Condensed Matter (Head Prof. B.N. Mavrin) in 1999. In view of the change-over of Prof. G.N. Zhizhin to the Director Deputy of STC of Unique Instrumentation of RAS in 2001 now the Laboratory headed by Prof. B.N. Mavrin. The main fields of its investigations are: spectroscopy of optical phonons at the phase transitions in organic, nonlinear, superconducting and superionic crystals; spectroscopy of surface and bulk polaritons; hyper-Raman spectroscopy of centrosymmetrical media: crystals, glasses and liquids; spectroscopy of carbon materials under high pressure; simulation of lattice dynamics; development of new spectral equipment

Spectroscopy of phase transitions

The study of phase transitions in organic and inorganic crystals was one of the main subjects of research work in the Laboratory for many years. New, previously unknown, phase transitions were found [1-6] being confirmed later by X-rays and calorimetric studies. It was proposed for the first time to use the temperature dependence of the intensity of phonon lines (FIR and RS [2-4]), or Davydov and/or Bethe splitting of intermolecular vibrations [5] as the order parameter in the vicinity of phase transition. The ferroelectric phase stabilization near the surface of TlGaSe₂ crystal was observed [6]. The effect of the proton dynamics on Raman spectra and the manifestation of the protons and deuterons tunneling in Raman spectra of the nonlinear crystals Rb(D_xH_1-x)₂PO₄[7].

The separate series of works are forming the RS studies of phonon spectra and their transformations with the change of hydrostatic pressure up to 12 Kbar in semiconducting crystals of the types A^IIIB^Vand A^IIIB^IIIC₂^VI. Nearly 50 compositions were studied, new phase transitions have been discovered in some of them.

After discovery of high-temperature superconductivity (HTSC) the spectroscopy of HTSC and related materials has been as a new trend in the Laboratory. We stated a problem correctly to obtain and intepret the Raman spectra (Fig. 1), to study both the interaction of the electron continuum with the phonon subsystem and the correlation between the spectra and the superconducting properties of HTSC [8,9].

Fig. 1. The frequency dependence of A g -modes on the oxygen content in YBa 2 Cu 3 O x [9].

We have studied the temperature dependences of the Raman spectra of the lithium-containing superionic crystals. It is found, that the widths and the frequencies undergo anomalies at the superionic transition in the LiNbGeO₅ crystal. It is revealed that an interference of one-phonon optical low-frequency modes (an antiresonance) appears with the temperature increase of the γ-Li₃PO₄ crystal. The numerical analysis of the spectrum in the interaction region has shown the strong temperature dependence of the coupling constant defined by the anharmonic coupling of optical and acoustical modes [10].

The computational methods of the vibrational spectra of crystals based on the use of the empiric potentials of the interatomic interaction (rigid-ion model and al.) are developed to intepret the spectra [11].

The solution of many important problems of crystal physics required higher photometric precision of spectral measurements and higher resolution of IR spectrometers. With that aim in view a complex of automated IR spectrometers with a wide spectral range from 0.8 to 50 μm (further broadening is possible up to 500 μm) with a resolution up to 0.005 cm^-1 , including Fourier-spectrometer UFS-02, was created under the direction of Prof. G.N. Zhizhin and E.A. Vinogradov.

The Laboratory took an active part in creation of first commercial Raman spectrometers with laser excitation (DFS-24 and DFS-42) (Dr. Kh.E. Sterin and Prof. B.N. Mavrin). The first original design of the LIDAR and the setup for multichannel and fast detection of Raman spectra (Prof. V.B. Podobedov) were worked out in the Laboratory.

In 1976 several researchers left the laboratory and created the Department of Spectral Instrument Design in Central Design Bureau of Unique Instrumentation of the USSR Acad. of Sci. (now STC UI RAS), which started to develop Fourier-spectrometers for Acad. of Sci. needs. The scientific and technical collaboration between ISAN and STC UI RAS is continued (Prof. G.N. Zhizhin, Dr. N.Y. Boldyrev).

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Surface states spectroscopy. Surface electromagnetic waves

Physics of surfaces and two-dimensional systems as well as applied problems both stimulated the development experimental studies of surfaces and interfaces. The choice of IR and Raman spectroscopy for these purposes is explained by its nondestructive character, by the possibility of applications not only ultrahigh vacuum (as is the case with electron spectroscopy methods) but also at high gas pressure above the surface.

The surface phonon-polaritons in more than 30 crystals of semiconductors and dielectrics were studied, the influence of thin conducting and dielectric layers on the dispersion curves was analyzed [13]. The quenching of surface polaritons by a metal film was discovered, for the first time the theoretically predicted surface polariton line splitting in the resonance with transition layer phonon was confirmed, the root-square dependence of the splitting value on thickness was shown. On this basis was proposed a new method of film's thickness determination, when it is less than visible light wavelength. The spectroscopy of surface polaritons was used for optical constants determination of thin films and transition layers covering the dielectric crystals (metallic and “artificial dielectrics “ films [13], layers formed by the ion implantation of quartz surface).

For the first time the phenomenon of thermostimulated emission of surface polaritons was discovered and an experimental method for surface polaritons dispersion curves determination (ZnSe single crystal and its films on metals [14]) and for investigations of phase transitions [15] was proposed on the basis of this phenomenon.

We begin to use practically at the same time with American scientists the surface plasmons of metals - surface electromagnetic waves (SEW) - as a spectroscopic mean for thin films on metals, using the CO₂ laser with frequency tuning. Later the prisms - elements for SEW excitation - we substituted by “razor blade” and that allowed us to develop interferometric method for determination optical constants of conducting materials, high temperature superconductors as well as dielectrics in the “reststrahlen” range, where the phonon polaritons exist.

In the Laboratory the Raman spectroscopy of the surface polaritons was developed. The dispersion, the frequency dependence of intensity and the polarization properties of the surface polaritons in the Raman spectra of GaP were studied [16]. For the first time were found families of wave-guide modes of of p- and s-type below the TO frequency (Fig. 2) and above the LO frequency and also one of low branches of interference modes in the GaP film [17].

Fig. 2. The dependence of Raman scattering by the wave-guide p-modes of the low branch in GaP on the angle scattering.

For the first time were investigated the Raman spectra of ultrafine amorphous superlattices Si-SiO₂ . The localization strengthening of TO mode were revealed with the decrease of the SiO 2 thickness. The extra features in spectra were displayed that were assigned to the formation of interface modes on the layer boundaries and effects of the dimensional quantization (the folding of acoustical branches).

New bulk and surface phonon-plasmon modes were found in Raman spectra of porous semiconductor InP [18]. It is shown that their appearance may be explained in approximation of the effective medium theory (modified Maxwell-Garnett theory). It is found that the surface electric field in porous GaAs is responsible for the breakdown of the selection rules not only for LO modes due to Frohlich mechanism, but also for TO modes due to the lowering of total symmetry of near-surface layer in pores in presence of the surface electric field. An identification of side products on the surface of the porous GaAs and GaSb obtained with electrochemical etching in the solution of various acids was implemented by means of Raman spectroscopy.

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Spectroscopy of bulk polaritons

The Raman scattering is an effective technique to study the bulk vibrational polaritons, which enable one to obtain the dispersion behavior in the range of wavevectors of 103-105cm-1, to restore the tensor of dielectric constant, to study the interaction between polaritons and multiparticle phonon states in noncentrosymmetrical crystal. In the Laboratory were worked out methods both ω -, and к -spectroscopy of polaritons [19,20], was measured the polariton dispersion in series of crystals, analyzed the polariton widths and lineshapes and investigated the anharmonicity and influence of two-particle states on the polariton dispersion [21].

Fig. 3. a – Raman doublets of Fermi-resonance of polariton with two-phonon zone in LiNbO₃ crystal; в – the ω(θ) dependence in the region of Fermi-resonance .

Raman scattering by polaritons was used to analyze the effects of strong phonon anharmonicity leading to the formation of bound states (biphonons) near the phonon two-particle zones. If this anharmonicity is enough and the biphonon state is formed, the gap proportional to the phonon anharmonic constant appears at crossing the polariton branch and two-particle phonon zone. We observed doublet of Fermi-resonance of polariton with the sum mode in LiNbO₃ [22].

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Hyper-Raman scattering

The hyper-Raman scattering (HRS) from materials is observed in the spectral range of the doubled frequency 2ω_I at an intense irradiation of the ω_I frequency. The HRS intensity is proportional to the square of the pumped intensity and hence it is a nonlinear process concerned with the cubic nonlinearity of material. According to the selection rules in HRS may be active vibrations that are forbidden in Raman and IR spectra. The important feature of HRS is an activity of dipole modes in all media.

Fig. 4. Hyper-Raman spectra (left) and dispersion of E 1 -polaritons (right) in the region of two-phonon states of calcite. [28].

HRS is unique technique to study bulk polaritons and related effects in centrosymmetrical media. Using HRS we investigated low-frequency spectra of dipole soft modes in centrosymmetrical crystals, that are forbidden in Raman scattering and complex for the study in IR. In HRS became first possible to study the dependence of spectra on the magnitude and the direction of the wavevector of vibrations in glasses and liquids. With use of HRS was proposed and proved a new interpretation of vibrational spectra of glasses and liquids based on an average symmetry of media [29].

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Spectroscopy of carbon materials under high pressure

Fig. 5. Diamond anvil cell: 1 – diamond anvils, 2 – tungsten-carbide supports, 3 – moved piston, 4 – thrust-bearing, 5 – clamping nut, 6 – holder, 7 – springs, 8- adjusting systems.

The application of the shear deformation decreases the structure-transition hysteresis and allows one to obtain more homogeneous phase. Using DACS and Raman scattering first several metastable phases of the C₆₀ fullerite were found with the pressure increase. At pressure above 20 GPa the metastable phase C₆₀ has high elastic properties that enabled one to suppose polymerization of C₆₀ molecules [30]. X-ray diffraction and Raman spectra confirmed irreversible transformation of the molecular C₇₀ structure into superhard amorphous phase under high-pressure and shear-deformation condition what lowered pressure of phase transition [31]. With use of DACS and Raman scattering we have studied the phase transitions in SiC. The appearance of new SiC phase is observed by disappearance of LO and TO phonons at 104 GPa. The structure of this phase is a cubic type of NaCl.

At present the investigations of structure and properties of new materials produced by polymerization of C 60 and C 70 under HPHT in situ and quenchable states are developed. A set of new structures are revealed and included to International Powder Diffraction Data. The Raman, IR and luminescence spectra of new phases are studied [32]. The P-T maps are designed for C 60 and C 70 phases. The compressibility measurements of 3D-polymerized C 60 by X-ray diffraction allowed one to determine bulk modulus of B_o≈540 ± 80 GPa that is comparable to bulk modulus of diamond.

Fig. 6. The density of vibrational states (1) and the Raman spectrum (2) of 3D-polymerized C₆₀ phase [33].

New carbon phase that corresponds to “intermediate” structure between graphite and diamond with sp 2 - and sp 3 -bonds were revealed in diamond after irradiation by fast neutrons. The computed vibrational spectrum for this structure was in agreement with the Raman spectrum. The structure of this carbon phase was studied by the high-resolution electron microscopy and it is found its conversion into graphite under electron beam.

Laboratory of Semiconductor Structures Spectroscopy (head of the laboratory professor Е . А . Vinogradov) was founded in April 1990. Initially the staff of the laboratory was formed from the researchers worked in cooperation with E. A. Vinogradov in the Laboratory of Crystals Spectroscopy, ISRAS. Later the group of prof. D.N. Nikogosyan (3 members), which originally belonged to the laboratory of Ultrafast Processes Spectroscopy, joined the laboratory to conduct the research on kinetics of electronic excitations in superlattices. In 2001 the laboratory also included the group of Spectroscopy of Solid State Surfaces headed by prof. V. A. Yakovlev.

The first years' achievements of the laboratory are based on the results obtained by its members during their work in the Solid State Spectroscopy Department. The main scientific achievements are listed below.

An experimental technique to study angular dependence of thermally stimulated IR emission spectra of crystals and films was developed together with the methods of extraction the dispersions of bulk and surface polaritons from the obtained spectra. The developed technique allowed conducting studies of electromagnetic field eigenstates (vibrational polaritons and plasmon- polaritons) in thin-film structures “vacuum – semiconductor film – metallic substrate” and “ATR prism – vacuum gap - semiconductor film – metal” [13-15,20,34-36] both experimentally ( Е . А . Vinogradov and G.N. Zhizhin) and theoretically (k.ph.-m.n Т . А . Leskova, d.ph.-m.n. V.I. Judson, d.ph.-m.n. А .G. Malshukov and d.ph.-m.n. V.I. Rupasov, Theoretical Department ISAN). Using the attenuated total reflection (ATR) technique the t hermostimulated luminescence at the frequencies of nonradiative surface polaritons in the crystalline semiconductors, metals, and semiconducting thin films deposited on metallic substrates was observed [14,20,34-36]. N onradiative surface plasmons of a metal in the structures “ATR prism - gap – metal” and “semiconductor film on a metal” were found to be converted into radiative states, which electromagnetic field is determined by dipole-active excitations of all constituent materials, and depends on a particular geometry of the experiment [20,35].

The results of experimental and theoretical investigations of optical phonons in crystals show that light absorption in the finite sized crystals is not a one-step process, as it was thought before, in which the absorbed photons are transformed into phonons. Instead, the photons penetrating the crystal first turn into an intermediate state of electromagnetic field in the crystal - radiative polaritons - and then the polaritons interact with the whole set of dipole-active states of the crystal. As a result of such interaction the polaritons are transformed into phonons, multiphonon excitations, excitons or free electrons (holes), or into plasmons of the metallic substrate. Light absorption is determined then by the probabilities of radiative and anharmonic decays of the polaritons [35].

Effects of strong resonance interaction between interference modes (cavity modes) in Cd_0.05Zn_0,95Te films and dipole-active vibrations of Cd impurity atoms (atoms in the cavity), substituting Zn atoms in ZnTe, were discovered. The resonance interaction may result in a more than 10⁵ times increase of the integrated absorption by local vibrations of impurity atoms in the cavity [20,35]. The effects of interaction between the excitations in adjoining media, and interaction with the interface plasmons [20,35] were observed.

a	b

Figure 7. а - Spectra of p-polarised reflection-absorption of Cd_0.05Zn_0,95Te films on Al, light incidence angle is φ=20⁰ , film thickness for Cd_0.05Zn_0,95Te is: 1 – d = 0,9 μm, 2 – d = 1,7 μm, 3 – d = 2,3 μm, 4 - d=5 μm, 5 - d=700 μm. Points show the experimental results, solid lines are the results of calculations [20,35]. b –The dependence of p-polarized absorption of the films Cd_0.05Zn_0,95Te at φ=20⁰ upon the film thickness (calculations [20,35]).

Using the vibrational spectroscopy methods Prof. Е . А . Vinogradov and d.ph.-m.n V. М . Burlakov studied ferro-electric phase transitions in layered semiconductors TlInS₂ и TlGaSe₂ . A new feature related to the influence of quasistatistic fluctuations of the order parameter near the phase transition temperature onto vibrational spectrum of the specimen was observed. Such influence results in the Gaussian shape of phonon absorption bands and Raman lines [6, 37-39]. It was also discovered that the phase transition near the surface of the layered semiconductor occurs at a temperature much higher than that in the bulk of the sample, suggesting that the surface stabilizes low-temperature phase in the adjacent layer of considerable thickness ( ~ 1 μm) [6,39].

Experimental studies of charge density wave (CDW) dynamics in (TaSe₄)₂I crystals, and numerical simulations of dynamics of kink-type solitons allowed understanding the nature of giant IR vibration observed in many inorganic systems with CDWs [40,41]. Analytical and numerical studies of the dynamics of anharmonic lattices excited with a uniform harmonic force revealed a remarkable feature – formation of dissipative structures (patterns), resembling one- or two-dimensional lattices of the so-called anharmonic localised vibrations. It was shown that the pattern generation and stability are due to modulation instability of the excited nonlinear mode [42,43], and destructive interference of modulation instabilities of the nonlinear modes composing the pattern [44], respectively.

Е . А . Vinogradov and k.ph.-m.n. V. А . Zayats in cooperation with the Solid State Physics Department of PhIAN (k.ph.-m.n. F. А . Pudonin) studied optical properties of superlattices based on the ultra thin ( ~ 1 nm) alternating layers of amorphous silicon and silicon dioxide. Using photoluminescence spectroscopy and photoluminescence excitation spectroscopy (in cooperation with d.ph.- м .n. D.N. Nikogosyan und Yu.A. Repeev), Raman spectroscopy (in cooperation with d.ph.-m.n.. B.N. Mavrin and k.ph.-m.n. B.N. Denisov), and λ - modulated light absorption [45] a structure of size quantized electronic levels in the superlattices was determined. It was shown that the short-period superlattices possess strictly two-dimensional structure of size quantized subbands of silicon, and that the recombination processes of non-equilibrium charge-carriers in these subbands are determined in the first place by probabilities of resonance charge transfer from silicon sublevels to deep impurity levels of germanium in the barrier layers of silicone dioxide [46-48].

Further activity of the laboratory was concentrated on three main directions:

• Experimental and theoretical studies of cavity modes, near-field effects and the processes of transformation of near-field into the bulk electromagnetic radiation;
• Experimental studies of optical phonons in new materials;

Development of Fourier-spectroscopic technique for near- and middle- IR spectral range.

The first direction involves investigations of cavity modes and their interaction with external electromagnetic radiation using time-resolved spectroscopy in femto-second time scale with sub-wavelength spatial resolution.

The spectra on Fig. 8b show ultrafast photoinduced evolution of the cavity modes resulting in a shift of absorption spectrum in the time scale of the order of the pump pulse duration. The latter phenomenon provides an opportunity of detecting ultrafast photovoltaic effects in the structures with Schottky barrier [52].

A combination of optical spectroscopy with tunneling microscopy and optical near field microscopy developed in the laboratory (k.ph.-m.n. V.N. Konopskyi) allows investigating fundamental questions related to formation of localized states of electromagnetic field in a medium, and an interaction of these states with light. Studies of such localized states are important for development of new apetureless near field microscopy techniques [53-55]. A new method to increase the spatial resolution of the near field apertureless microscope was proposed and experimentally realized. An increase in resolution was achieved by exciting the local plasmons in the system STM tip – sample surface when the tip and the surface are made of the noble metals. Generation of the local plasmons was observed by registering optical signal at the double frequency of modulation of the distance between the tip and the sample surface. The achieved optical resolution turned to be better than the tip radius [54]. Processes of surface plasmon scattering on a rough silver surface involving enhanced back scattering were registered [55] using home-made scanning plasmon near-field microscope, and implementing spatial Fourier- transformation of the near-field images.

The second direction of the laboratory activity includes experimental studies of peculiarities of the optical phonons in various crystalline materials and crystalline solid solutes, as well as the studies of optical phonons in the vicinity of structural phase transitions in low-dimensional systems (layered crystals and thin films). Optical and structural studies of ferro-electro-ceramics, irradiated by high intensity pulse of electrons (in cooperation with JINR, Dubna) [56,57] are being carried out.

Besides basic studies the laboratory is permanently involved in the development of new spectroscopic methods and techniques. Currently a Fourier-spectrometer for visible and IR spectral range developed and manufactured in the laboratory (k.ph.-m.n N.Yu. Boldyrev together with V.M. Krivzun, laboratory of High Resolution Molecular Spectroscopy and Analytical Spectroscopy) is being successfully tested. The spectrometer will be used for studies of multilayered thin film structures and new crystalline materials.

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Group of Solid State Surface Spectroscopy

SEW study had started in the 10 μm spectral region (CO₂ -laser) [13], then SEW propagation on metals was studied at discrete frequencies in near infrared and visible spectral regions (He-Ne-laser) [61]. Recently the spectral range was greatly increased due to employment of Fourier-transform spectrometers [62] and a free electron laser tuned in wide IR region: 5-100 μm (in collaboration with the Institute of Plasma Physics , Nieuwegein, the Netherlands ). Using free electron laser high quality spectra of monolayer Langmuir-Blodgett films on the gold surface were measured, the complex dielectric function of thin (nanometer thickness) films of “carilon” in a wide spectral region was obtained from the SEW propagation parameters [63]. Subsurface layers of the crystals of CaF₂ , BaF₂ , MgO, LiNbO₃ were studied by phase SEW spectroscopy in the far IR region near the phonon resonances. SEW propagation parameters were measured and the optical constants (anomalous dispersion) were calculated on them [64].

High peak intensity of the free electron laser allowed to study nonlinear effects at SEW propagation. The second harmonic generation (SHG) at 4.5 μm from the quartz surface was obtained in the direction that was perpendicular to the sample plane with counter-propagating surface phonon-polaritons excited by a grating made at the quartz surface. The frequency dependence and the influence of a thin organic film on the efficiency of SHG were studied [65]. It was studied visible (0.5235 μm) - infrared (10 μm) sum frequency generation (SFG) from smooth and periodically corrugated silver surfaces. The SFG output from the grating was enhanced by a factor of 10⁴ in comparison with SFG signal from smooth surface due to the simultaneous excitation of surface plasmon-polaritons at visible laser and sum frequencies [66,67]. In such conditions restricted spectral ranges of the SFG spectra of a few nm thick films of Cu-phthalocyanine, fullerene and hexamethylenetetramine were obtained. The spectra of the sum frequency generation in the thiopeptide monolayer CH-stretching vibrations region as far as of thiol molecules adsorbed on the metal surface were measured too. The thickness dependence of the vibrational spectra on the ultrathin film (fullerene and phtahlocyanine) on silver was studied using reflection-absorption spectroscopy and SFG with surface polaritons. It was shown that only the film-metal interface gives SFG signal, as indicates the symmetry in the bulk of the films [68].

The study of the optical and electrical properties of prepared by radio-frequency sputtering ultrathin metal (Nb, Cu, Ni, W, Mo, Ti) films showed the evidence of the quantum-size oscillations of conductivity and optical constants with film thickness change. The oscillation period depends only on metal and is of the order of the Fermi electron wavelength [69,70].

Some other structures interesting from the point of view of quantum effects: superlattices and porous silicon are under study too [71,72]. The spectral dependence of the second harmonic generation intensity was experimentally studied in the porous silicon based photonic crystal. The giant enhancement of the second harmonic was found near the resonance mode and near the photonic gap as a result of both radiation localization due to the multibeam interference and of the two-photon resonance. It was shown that creating of the “doping” levels in the gap in the photonic crystal spectrum leads to the electromagnetic field concentration near the defect and, hence, to the enhancement of linear (absorption and Raman) and non-linear (second harmonic, sum- and difference-frequency generation) optical effects in such structures. SHG spectra of SiH vibrations of the hydrogen monolayer in the porous silicon based photonic crystals was obtained [73-75].

An experimental and theoretical study of the photonic band gap in the propagation of surface plasmons (SPs) on periodically corrugated surfaces was done in the case when the band gap width is larger than the energy distance between the SP dispersion curve for a flat surface and the light line. By using the interferometric measurement we have studied, for the first time, the SP propagation parameters (real and imaginary parts of the SP wave vector) in the vicinity of the photonic band gap. A physical model of the interaction of light waves with SP was introduced and an analytical expression for the SP wave vector near band gaps was derived based on the coupled-mode approach involving three interacting modes. The analytical expression obtained on the basis of this model can be used to determine the form of the SP dispersion curve near the band gap for gratings with the actually used amplitudes of the corrugations. The predictions of this theory are in good agreement with our experimental data and allow to describe the additional minimum in the angular distributions of the SP transmission of periodically corrugated surfaces obtained by another authors [76].

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Comparison of the experimentally measured (from high-resolution spectra) widths of rare-earth spectral lines with those calculated in the framework of the exchange charge model for the crystal field and electron-phonon interaction allowed to improve the theory of relaxation processes for rare-earth ions in dielectric matrices and to obtain a number of important results. Thus, on the example of CsCdBr₃:Pr³⁺ , it has been shown that the phonon density of states may redistribute essentially in crystals activated by rare earths. This

circumstance alone leads to new peculiarities of electron-phonon interaction that are to be taken into account in a description of relaxation processes. Besides, a local enhancement of elastic forces in activated crystals and a respective enhancement of correlations between displacement of a rare-earth ion and its neighbours result in a strong suppression of effective electron-phonon coupling [80].

Discovery of high-T_c superconductors in 1986 revived the interest of scientist to mixed oxides of a rare-earth and a d-element (as, e.g., Cu or Ni). Intensive investigations began of not only high-T_c superconductors themselves but also of related compounds. As Cu-O chains and/or planes are essential elements of high-T_c superconductors, low-dimensional magnets have attracted a considerable attention as model systems. Various compounds containing chains, or ladders, or planes of magnetic d-ions with half-integer (Cu²⁺ ) or integer (Ni²⁺) spin values were synthesized and, in several cases, big single crystals of a good quality were grown. This, in particular, opened a possibility to study fundamental quantum effects, for example, the spin-Peierls transition in a system of half-integer-spin antiferromagnetic Heisenberg chains, the Haldane gap for chains of integer spins, spin gaps for spin ladders, etc.

	By the method of a high-resolution spectral probe, we have investigated magnetic compounds related to high-Tc superconductors of the 1-2-3 type: the so called “blue” phases R2Cu2O5 , “green” and “brown” phases R2BaCuO5 , as well as the chain nickelates R2BaNiO5 (here R states for a rare-earth or yttrium). The main results of these studies, a list of references, and a discussion of methodological aspects can be found in the review article [86].   Another direction of the research conducted by the Center is the study of quantum effects in quasi-one-dimensional magnets with half-integer and integer spin values. We communicated the first observations of folded infrared modes due to a doubling of the primitive crystal cell at the spin-Peierls transition (S=1/2, CuGeO3 [87]) and at the charge-ordering magnetoelastic transition (S=1/2, NaV2O5 [88]) and of a spectral manifestation of the spin-gap opening at such kind of transitions [88]. Fig.15 illustrates these findings (see also Ref.[89]).
	Our subsequent studies have shown that in NaV2O5 charge, magnetic, and phonon excitations strongly interact [90]. As a result of this interaction, the lattice dimerizes, the spins couple forming a nonmagnetic state, and charges order in a single phase transition [91]. A zigzag charge distribution over the “rungs” of the ladder is settled [92]. As for Haldane quasi-one-dimensional magnets with integer spin values, it is worth mentioning our recent study of the system of chain nickelates (ErxY1-x)2BaNiO5. Spectroscopically, we observed the crossover from the one-dimensional quantum (Y2BaNiO5) to three-dimensional classical (Er2BaNiO5) behavior. The staggered magnetization function for Haldane chain in a staggered field directed along the chain and created by an ordered rare-earth subsystem was constructed from the spectral data [93].   The work of the Center was supported by grants from the Russian Foundation for Basic Research, the Russian Academy of Sciences, the Ministry of Industry and Science, as well as by joint programs with foreign institutions INTAS and RAS-CNRS (France). The research was performed in a close cooperation with other scientific institutes. Among them are: M.V.Lomonosov Moscow State University, Kazan State University, P.L.Kapitza Institute for Physical Problems RAS, L.V.Kirensky Physical Institute (Siberian branch of RAS, Krasnoyarsk), A.V.Shubnikov Institute for Crystallography RAS, Institute of Radiotechnics and Electronics RAS, P.N.Lebedev Physical Institute RAS, the Institute of General Physics RAS, Moscow Institute of Energetics, Laboratory of Applied Solid State Chemistry (CNRS-UMR 7475, Superior School of Chemistry of Paris), Laboratory of Solid State Physical Chemistry of the Paris University, the University of Groningen (the Netherlands), the University of Turku (Finland), the Institute of Low Temperature and Structure Research Polish Academy of Sciences, Material Science Institute of Madrid, the University of Madrid, the University of Canterbury (New Zealand), the Institute for Solid State Physics of the Tokio University (Japan).

Monographs

1. Kh.E.Sterin, V.T.Aleksanian, G.N.Zhizhin. Raman spectra of hydrocarbons. A Data Handbook. Pergamon Press. Oxford , New York , Toronto , Sydney , Paris , Frankfurt , 1980, 358 p.
2. G.N.Zhizhin, B.N.Mavrin, V.F.Shabanov. Optical vibrational spectra of crystals. Moscow , Nauka, 1984, 232 p.
3. G.N.Zhizhin, V.A.Vagin, M.A.Gershun, K.I.Tarasov. Highs throughput spectral instruments. Moscow , Nauka, 1988, 263 p.
4. E.A.Vinogradov, I.I. Khammadov. Spectrocopy of bulk and surface phonons in crystals. Tashkent , FAN, 1989, 166 p
5. G.N.Zhizhin, E.I.Mukhtarov. Optical spectra and lattice dynamics of molecular crystals. “Vibrational spectra and structure” series (ed. J.R.Durig), Amsterdam , Elsevier, 1995, vol.21, 447 p.
6. A.N.Kuptsov, G.N.Zhizhin. Handbook of Fourier Transform Raman and Infrared Spectra of Polymers, “Physical Sciences Data” series, Amsterdam, Elsevier, 1998, vol.45, 570 p.

References

1. G.N.Zhizhin. Spectroscopy of low temperature crystal phases of organic substances. Crystalography, v.18(4), 769 (1973).
2. N.I.Afanasyeva, V.M.Burlakov, G.N.Zhizhin. Phonon spectrum in the vicinity of phase transition «crystal-nematic» and the ordering parameter. JETP, v. 23(9), 508 (1976).
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