Laboratory of Ultrafast Processes Spectroscopy

    The laboratory came out of the Section of Picosecond Spectroscopy which was set up in 1974 under the guidance of Prof. P.G. Kryukov. First researchers of the section were Yu.A. Matveets, D.N. Nikogosyan, A.V. Sharkov. Dr. Yu.A. Matveets has been in charge of the laboratory since 1980.
    The staff of the laboratory includes seven research workers informally subdivided into two groups. One of these groups (headed by Dr. Yu.A. Matveets) are conducting experimental work on the femtosecond spectroscopy of condensed media and the other (headed by Dr. V.M. Farztdinov) are engaged with the theoretical aspects of the spectroscopy of condensed media and ultrafast relaxation processes and also the analysis and interpretation of experimental data.
    The main directions of laboratory activities:
 1) investigation of ultrafast photoinduced processes in the matter in a condensed phase;
 2) selective laser action on the matter by a powerful ultrashort pulse.
    Laser setups with the time duration first in picosecond (30-6 ps), and later in femtosecond (300-50 fs) ranges were created for these works. These setups became the main elements of pico- and femtosecond spectrometers.
    In the first experiments made with the help of these devices the primary photoinduced processes in biological objects (in bacteriorhodopsin, in reaction centers of bacterial photosynthesis, in hematoroporphyrine) have been investigated.
    In the works on the investigation of bacteriorhodopsin [1] it was shown that just after the absorption of light quantum the isomerization of retinal with the characteristic time 0.5 ps takes place which leads to the formation of an intermediate product with the lifetime ~3 ps. These experimental facts contributed much to the understanding of molecular mechanism of light energy transformation by retinal protein complexes [2]. The investigation of primary photoinduced processes in reaction centers of purple bacteria with the time resolution ~10-13 s allowed to measure the velocity of energy migration among the pigments and to detect the primary electron donor in the process of transformation of the energy of absorbed quantum to the energy of separated charges (Fig. 24) [2].

Fig. 24

 a) Simplified energy band diagram of RC pigments;
 b) Schemes of processes occuring in RC at various excitation intensities.

    The investigations on the second direction (selective action) started in 1977; first experiments were the experiments on multistep selective electron excitation of Rhodamin 6G molecule in D2O solution through an intermediate vibrational of H2O in the whole IR range does not allow to make this method effective [3a]. So the next stage of investigations were successful experiments on less selective but more effective multistep electron excitation of biomolecules (bases of nucleic acids) in aqueous solutions through singlet levels [3b]. In the same direction works in multistep ionization and dissociation of H2O and aqueous solutions on the components of nucleic acids [4] and on nonlinear photochemical synthesis of aminoacids were performed [5].
    In 1989, investigations were started on ultrafast processes in solids, such as the relaxation of charge carriers in semiconductors [6, 7], metals [9], high-temperature superconductors [9 16], fullerites [17 27], polymers and other carbon-bearing compounds [28 30], and semiconductor microcavities [31 33].
    Experimental studies are being performed on the laser femtosecond spectrometer developed at Institute of Spectroscopy. The experimental scheme is as follows: a short exciting pulse with a duration of the order of 100 fs and an intensity from 108 to 1013 W/cm2 is used to strongly heat the charge carriers, so that the electron temperature is detached from the lattice temperature. A probe pulse that can be delayed in the range 0 1 ns with respect to the exciting pulse is then employed to observe the kinetics of the nonequilibrium system, i.e., monitor the dynamics of energy transfer from the electronic subsystem to the lattice phonons. The probing can be carried out over a wide spectral range   from 0.45 to 0.9 mm. Both the absorption and reflection coefficients of the sample under study can be measured.
    Compared to the other spectroscopic techniques, femtosecond optical spectroscopy is advantageous in that it allows studying the relaxation dynamics of nonequilibrium charge carriers and discriminating between the contributions from these carriers and phonons to the changes in the optical properties of the sample. As applied to metals, it enables one to discriminate a narrow spectral region corresponding to transitions from some valence band to the neighborhood of the Fermi level and observe the temperature variation dynamics of the charge carriers and the lattice.
    Indeed, the electron-electron collision time in normal metals is usually short   around 10 14 s, so that at times t > 10 13 s a quasiequilibrium is established in the electronic subsystem and one can introduce the notion of electronic temperature. The rise of the electronic temperature, determined by means of delayed probe pulses, causes the following processes:
    (1) The smearing of the distribution function of the carriers in the vicinity of the Fermi level, leading to a reduction of the population of electronic states below the Fermi level and an increase in the population of electronic states above the Fermi level. As a result, the change of the dielectric function e2(w) in the course of excitation will be manifest as an alternating function going to zero at the point wF corresponding to transitions to (or from) the Fermi level. Thus, femtosecond spectroscopy makes it possible to determine the position of the Fermi level proceeding from the condition De2(wF) = 0 [11, 12].
    (2) The shifting of the Fermi level. In good metals this effect is insignificant, but in poor ones it may be substantial.
 At delay times shorter than the electron-photon relaxation time (for normal metals, this time is around 1 ps [8, 10]) these processes are dominant. To observe them, the probe and exciting pulses should have a duration much shorter than the electron-photon relaxation time, and so pulses are required with a duration around 100 fs and even shorter.
    The femtosecond laser spectroscopy technique was used to perform test experiments on the measurement of the electron-phonon interaction parameter in metals. We studied the temporal behavior of the reflection and transmission difference spectra of copper films in the range 505 605 nm following their excitation with high-power 150-fs laser pulse. The value obtained for the parameter l w2  (27 4 meV [8]) agrees within the accuracy of measurement with the results obtained by other techniques.
    Investigations were conducted into the effect of various electron-phonon interaction mechanisms on the energy relaxation rate in polar semiconductors   CdS-CdSe microcrystallites in a glass matrix [6, 7].
    The mechanism responsible for the development of attraction between electrons in high-temperature superconductors is known to remain the subject of heated discussions. Most popular at present are two main opinions as to the mechanism of superconductivity in high-temperature superconductors. The first is that this is a phonon mechanism, probably supplemented by the interlayer tunneling effect. According to the second opinion, it is an entirely different mechanism based on the interaction between electrons and spin fluctuations. These theories predict different types of symmetry for the superconducting order parameter: an s-symmetry for the phonon mechanism and a d-symmetry for the fluctuation mechanism.
    The determination of the value of the electron-phonon interaction constant l (or the Eliashberg parameter l w2 ) can help to reveal the superconductivity mechanism in oxide compounds. The electron-phonon interaction parameter in metals can be determined by various methods: by measuring electronic heat capacity, cyclotron mass, etc. All these methods, however, are indirect. The only direct way to measure the parameter l w2  in high-temperature superconductors would be the study of the energy relaxation of nonequilibrium electrons in real time. But to do this, a method is required that would allow this process to be investigated on a femtosecond time scale. Femtosecond laser spectroscopy could be suitable for this purpose.
    But is there a possibility of measuring electron-phonon interaction constants in the optical region of the spectrum and determining the width of the superconductor gap? A widespread belief as to the optical properties of superconductors is that no differences between the normal and superconducting states can be observed in optical spectra with a photon energy of hw >> D.
    The research workers of the laboratory have got, in cooperation with their colleagues from the laboratory of nanophysics, a positive answer to the above question: femtosecond laser spectroscopy makes it possible not only to directly determine the electron-phonon interaction parameter and the width of the superconductor gap within the framework of a single experiment, but also to observe on a real time (femtosecond) scale the changes in the electronic spectrum associated with the creation or annihilation of a new phase   the superconducting state [10 14, 16].
    With the spectral region corresponding to transitions from some band to the neighborhood of the Fermi level being discriminated at T > Tc, it becomes possible to observe changes in optical spectra upon transition from the normal to superconducting phase (and vice versa): the spectral width of the resonance response observed proves to be directly related to that of the superconductor gap [12 13, 16].
    By studying the dynamics of the reflection and absorption spectra of a superconductor at T > Tc, the spectral region corresponding to interband transitions to the Fermi level has been discriminated. Based on the relaxation dynamics of the spectra, the electron-phonon interaction parameter l w2  has been found to be equal to some 500 (meV)2 (the corresponding l   1). The width D of the superconductor gap has been estimated from optical spectra (hw >> D !) at T < Tc to be around 30 meV [10, 14].
    The data obtained as to the position of the Fermi level, the width of the superconductor gap, and the electron-phonon interaction parameter agree well with the results of other experimental techniques and theoretical works.
    By approximating the experimental curves by the double-exponential fitting functions DDfit(t), the spectral dependence of the optical density relaxation rate t1 has been obtained. In cooperation with the laboratory of nanophysics, a sharp rise has been discovered of the energy relaxation time of electrons in certain spectral regions, which are believed to be associated with transitions to the neighborhood of the Fermi level. The dependence obtained opens up possibilities for a new method of determining the position of the Fermi level [15]. The possibility of employing this method has been demonstrated for high-temperature superconductors (YBa2Cu3O7 d) [15]. Finally, subpicosecond-resolution spectroscopy has made possible the study of the spectrum of coherent low-lying phonon vibrations [15]. This furnishes interesting information on vibrations that interact most strongly with electrons.
    The discovery of the molecule C60 and its subsequent synthesis have opened the Pandora box of carbon structures that could not be imagined previously and demonstrated the versatility of the sixth element in the periodic table. Carbon-bearing compounds are examples of materials demonstrating all the accessible dimensions   from the three-dimensional diamond with sp3 bonds to the two-dimensional layered graphite with sp2 bonds, from the recently discovered unidimensional carbon nanotubes to zero-dimensional fullerens   and types of ordering   from the perfection of the crystalline diamond or graphite to various types of random networks of amorphous carbon, such as glass carbon, diamond-like carbon, and ultrahard fullerite.
    Carbon is also unique as an electronic material. It can be a metal in the form of graphite, a semiconductor in the form of diamond or fullerite, a superconductor, when doped with suitable materials, and a polymer, when bonded with hydrogen and other elements. So unique wealth and variety of carbon materials possessing quite different physical properties is a most strong motive for gaining a deeper insight into the interrelation between structural and electronic, optical, vibrational, and other properties.
    In this connection, the laboratory has been engaged, in collaboration with the laboratory of nonophysics, with investigations into the ultrafast (femtosecond-scale) processes of relaxation of electronic excitations in various carbon-bearing materials   fullerens and fullerites, polymers, and various carbon films.
    Subject to study have been the processes of energy relaxation of nonequilibrium charge carriers in C60 films in the broad spectral range 1.6 3.4 eV [17 27]. A strong photoinduced darkening has been detected in the spectral ranges 1.6 2.4 and 2.9 3.4 eV, as well as photobleaching in the spectral range 2.4 2.9 eV. The femtosecond dynamics of the photoinduced optical density of an ultrathin C60 film has been experimentally studied. The spectral dependence of the relaxation time of the photoinduced response allows one to selectively discriminate the relaxation of electrons in various bands [25].
 It has been demonstrated that on time scales shorter than 1 ps the dynamics of the photodarkening spectra is governed by the energy relaxation of the photoexcited charge carriers on intra- and intermolecular vibrations, accompanied by the formation of excitons, and exciton annihilation at later stages [17 21, 25 27]. The temporal behavior of the spectra in the photobleaching region is due to the development of a random local field, screening, the dimerization of molecules, and energy relaxation [25 27].
    The temporal variations of the optical density of a C60 film have been used to study the excitation of coherent phonons in the frequency range 10 400 cm 1 [22-24, 26]. It has been found that optical excitation gives rise to a nonequilibrium but reversible dimerization of fullerens C60, which vanishes after relaxation. The complete splitting of the Hg(1) intramolecular vibration mode of fullerens C60 points to a substantial deformation of the fulleren molecules consequent upon the absorption of photons by them.
    It has been found that at high excitation intensities I > 1011 W/cm2 there takes place the saturation of the photodarkening growth [19 21, 27] and slowing down of the relaxation of the photoinduced response of the C60 film with increasing pump intensity [27]. This is due to the additional heating of the charge carriers as a result of the internal conversion of electrons from the highly excited bands.
    The photoinduced optical response of carbonated polyacrylonitrile films and its rate of change have been studied in the wide spectral range 1.6 3.4 eV [29-30]. The spectral dependences of the optical response and its rate of change have been compared with the structure of the films.
    The nonlinear optics of thin-film microcavity structures similar to the Fabry-Perot cavity have recently attracted much attention, for devices of this sort can serve as a basis for the development of new information transmission and processing systems using fast-acting optical means. At the same time, the problems of interaction between ultrashort pulses and planar microcavity structures containing nonlinear media have not been adequately studied. In this connection, the investigators at the laboratory are being engaged with research into the effect of nonlinear interaction between thin-film planar resonator structures the type of the Fabry-Perot cavity.
    In collaboration with the laboratory of semiconductor structures, they have studied the excitation and relaxation of natural modes in semiconductor microcavities of the ZnSe semiconductor material in contact with Cr and Cu metal films on quartz substrates [31 33]. It has been demonstrated that the photoinduced variation of the boundary conditions caused by femtosecond laser pulses leads to a shift of the frequency of the natural oscillations of the microcavities and their line broadening. The excitation of coherent photons has been found to take place in the microcavities.
    Recently the femtosecond pulses was used successfully in laser photoelectron microscopy   unique device, which can realize high spatial resolution (a few nm), high spectral (chemical) selectivity and femtosecond temporal resolution (with collaboration with Laser Spectroscopy Laboratory) [34].
    The laboratory is intensely and fruitfully cooperating with both the other laboratories of the institute (Laboratory of Laser Spectroscopy, Prof. V.S. Letokhov, Laboratory of Nanophysics, Dr. Yu.E. Lozovik, Laboratory of Semiconductor Structures, Dr. E.A. Vinogradov) and other institutes in Russia (P.N. Lebedev Physical Institute, Institute of Biochemical Physics of the Russian Academy of Sciences) and other countries (Laser Laboratory and Institute of Biochemical Physics in G ttingen, Germany).

    During the past few years, three Doctor s and seven Ph.D. Thesis have been prepared and defended at the laboratory.

    One monograph has been published on the basis of the investigation results obtained.

Monographs  of Laboratory of Ultrafast Processes Spectroscopy

1. a) V.S. Letokhov, Yu.A. Matveets, A.V. Sharkov and others.  Laser Picosecond Spectroscopy and Photochemistry of Biomolecules . Ed. by V.S. Letokhov. (Nauka, Moscow, 1987), p.253.
b)  Laser Picosecond Spectroscopy and Photochemistry of Biomolecules . Ed. by V.S. Letokhov (Adam Hilger, Bristol, 1987), p. 309.

References

1. Yu.A. Matveets, S.V.Chekalin, A.V.Sharkov. Molecular dynamics of primary photoprocesses in bacteriorodopsin: subpicosecond study of absorption and luminescence kinetics. J. Opt. Soc. Am. B2, 634 (1985).
2. S.V. Chekalin, Yu.A. Matveets, A.P.Yartsev. Study of fast photoprocesses in biomolecules with the aid of femtosecond laser spectrometer. Rev. Phys. Appl. 22, 1761 (1987).
3. a) P.G. Kryukov, V.S. Letokhov, Yu.A. Matveets, D.N. Nikogosyan and A.V. Sharkov. Selective two-step excitation of electron state of organic molecules in aqueous solution by picosecond light pulses. Quantum Electronics 5, 2490 (1978) b) P.G.Kryukov, V.S. Letokhov, D.N. Nikigosyan et al. Multiquantum photoreactions of nucleic aid components in aqueius solution by powerfull ultrashort picosecond radiation. Chem. Phys. Lett. 61, 375 (1979).
4. D.N. Nikigosyan, V.S. Letokhov. Nonlinear laser photophysisc, photochemistry and photobiologyof nucleic acids. Rev. Nuovo Cimento 6, 1-89 (1984).
5. E.V. Khoroshilova, N.P. Kuz mina, V.S. Letokhov, Yu.A. Matveets. Nonlinear photochemical synthesis of biomolecules by powerful UV picosecond pulses. In: Photochemistry and photobiology, ed. by A.H. Zewail. (Harwood Acad. Publ., 1983), vol. 2, p. 1267.
6. a) Yu.E. Lozovik, Yu.A. Matveets, A.G. Stepanov, V.M. Farztdinov, S.V. Chekalin, A.P. Yartsev. Femtosecond relaxation of excited carriers in CdSexS1-x micro-crystallits in glass matrixe under high excitation intensities. Pis'ma v ZhETF 52, 851-854 (1990).
 b) S.V. Chekalin, V.M. Farztdinov, V.V. Golovlev, Yu.E .Lozovik, Yu.A. Matveets, A.G. Stepanov, A.P. Yartsev. Femtosecond relaxation of excited carriers in microcrystallites in glassy matrixe at  excitation intensity 1010-1012 W/cm2. Ultrafast Processes in Spectroscopy 1991. Eds. A.Laubereau and A.Seilmeier. IOP Publish. Ltd., Bristol, 1992. p. 389-392.
7. I.V. Bezel, Yu.A. Matveets, A.G. Stepanov, S.V. Chekalin, A.P. Yartsev. Two-photon absorption of powerfull femtosecond pulse in semiconductor doped glasses at the energies higher that the band gap. Pis'ma v ZhETF 59(6), 376-380 (1994).
8. V.V. Golovlev, Yu.A. Matveets, A.M. Sanov, V.S. Letokhov. Investigation of electron temperature relaxation of gopper film under femtosecond laser excitation. Pis'ma v ZhETF 55, No. 8, 441- 444 (1992).
9. M.E. Gershenson, V.V. Golovlev, I.B. Kedich, V.S. Letokhov, Yu.E. Lozovik, Yu.A. Matveets, E.G. Sil'kis, A.G. Stepanov, V.D. Titov, M.I. Faley, V.M. Farztdinov, S.V. Chekalin, and A.P. Yartsev. Direct measurement of the electron-phonon  interaction  in YBa2Cu3O7-d  by  femtosecond  laser spectroscopy method. Pis'ma v ZhETF 53(11), 1189 (1990).
10. S.V. Chekalin,V.M. Farztdinov, V.V. Golovlev, V.S. Letokhov, Yu.E. Lozovik, Yu.A. Matveetz, A.G. Stepanov. Femtosecond spectroscopy of YBa2Cu3O7-d: electron-phonon interaction measurement and energy gap observation. Phys. Rev. Lett. 67, No. 27, 3860-3863 (1991).
11. S.V. Chekalin,V.M. Farztdinov, V.V. Golovlev, V.S. Letokhov,  Yu.E. Lozovik, Yu.A.Matveetz, A.G.Stepanov. Femtosecond spectroscopy of YBa2Cu3O7-d-: electron-phonon interaction measurement and energy gap observation. Ultrafast Processes in Spectroscopy 1991. Eds. A.Laubereau and A.Seilmeier. IOP Conference Series No. 126, p. 261-266, IOP Publish. Ltd., Bristol, 1992.
12. a) A.L. Dobryakov, V.M. Farztdinov, Yu.E. Lozovik. Energy Gap in the Optical Spectrum of Superconductors. Physical Review B47, No. 17, 11515-11517 (1993).
 b) A.L. Dobryakov, Yu.E. Lozovik, V.M. Farztdinov. Energy Gap in the Optical Spectrum of Superconductors. Soviet Phys.: Sverkhprovodimost' 6,  No. 7, 1343-1351 (1993).
13. A.L.Dobryakov, V.M. Farztdinov, Yu.E. Lozovik and V.S. Letokhov. Energy Gap in the Superconductor Optical Spectrum. Optics Comm. 105, 309-314 (1994).
14. A.L. Dobryakov, V.V. Golovlev,V.S. Letokhov, Yu.E. Lozovik, Yu.A. Matveetz, A.G. Stepanov, V.M. Farztdinov, S.V. Chekalin. Femtosecond spectroscopy of superconductors YBa2Cu3O7-d: the measurement of electron-phonon interaction parameter and energy gap observation. Soviet Phys.: Optika i Spectroscopia 76,  No. 6, 975-983 (1994).
15. I.I. Vengrus, A.L. Dobryakov, C.A. Kovalenko, V.S. Letokhov, Yu.E. Lozovik, G. Marowsky, Yu.A. Matveets, V.M. Farztdinov, N.P. Ernsting. Spectral dependence of relaxation in YBa2Cu3O7-d under femtosecond laser excitation. Pis'ma v ZhETF 66(9), 739-743 (1995).
16. V.M. Farztdinov, Yu.E. Lozovik, Yu.A. Matveets. Femtosecond Optical Spectroscopy of High Tc Superconductors and Fullerites. Brazilian J. Phys. 26,  No. 2, 482-499 (1996).
17. I.E. Cardash, V.S. Letokhov, Yu.E. Lozovik, Yu.A. Matveets, A.G. Stepanov, V.M. Farztdinov. Ultrafast relaxation of photoinduced darkening in fullerites. Pis'ma v ZhETF 58(2), 134-138 (1993).
18. S.V. Chekalin, V.M. Farztdinov, E. Akesson, V. Sundstrom. Relaxation of C60 in solution and films: results of femtosecond investigations. Pis'ma v ZhETF 58(4),  286-290 (1993).
19. V.M. Farztdinov, Yu.E. Lozovik, Yu.A. Matveets, A.G. Stepanov, and V.S. Letokhov. Femtosecond Dynamics of Photoinduced Darkening in C60 Films. J. Phys. Chem. 98, No.13,  3290-3294 (1994).
20. V.M. Farztdinov, Yu.E. Lozovik, V.S. Letokhov. Saturation of the fullerite photodarkening at high laser energy fluences. Chem. Phys. Lett. 224, 493-500 (1994); [Erratum: Chem. Phys. Lett. 233(4), 490 (1994)].
21. I.V. Bezel,  S.V. Chekalin, Yu.A. Matveetz, A.G. Stepanov, A.P. Yartsev, and  V.S. Letokhov. Two-photon absorption of powerful femtosecond pulse in C60 Film. Chem. Phys. Lett. 218 , No. 5, 6, 475-478 (1994).
22. a) A.L.Dobryakov, S.A. Kovalenko, V.S. Letokhov, Yu.E. Lozovik, G. Marowsky, Yu.A.Matveets, V.M. Farztdinov, N.P. Ernsting. Coherent phonons in fullerites under femtosecond laser excitation. Pis'ma v ZhETF 61(12), 957-961 (1995).
 b) A.L. Dobryakov, V.M. Farztdinov, S.S. Kovalenko, V.S. Letokhov, Yu.E. Lozovik, Yu.A. Matveets. Observation of Coherent Phonons Generation in C60 Films. SPIE Proceedings 2797(16),102-107 (1996).
23. A.L.Dobryakov, V.M. Farztdinov, Yu.E. Lozovik, Yu.A. Matveets, A.G. Stepanov, A.P. Yartsev, and V.S. Letokhov. Femtosecond optical spectroscopy of fullerites. SPIE Proceedings 2797(15), 94-101 (1996).
24. A.L. Dobryakov, N.P. Ernsting, V.M. Farztdinov, S.A. Kovalenko, V.S. Letokhov, Yu.E. Lozovik, G. Marowsky, Yu.A. Matveets. Coherent phonons in fullerites under femtosecond laser excitation. Ultrafast Processes in   Spectroscopy,   Ed. by  O. Svelto, S. De Silvestri and  G. Denardo. Plenum Press, New York, p. 95-100, 1996.
25. V.M.Farztdinov, A.L.Dobryakov, N.P.Ernsting, S.A.Kovalenko, Yu.E. Lozovik, Yu.A. Matveets. Spectral dependence of ultrafast relaxation in solid C60. Laser Physics 7(2), 393-396 (1997).
26. V.M. Farztdinov, A.L. Dobryakov, N.P. Ernsting, S.A. Kovalenko, V.S. Letokhov, Yu.E. Lozovik, Yu.A. Matveets. Spectral Dependence of Femtosecond Relaxation and Coherent Phonons Excitation in C60 Films. Phys. Rev. B 56(7), 4176-4185 (1997).
27. V.M. Farztdinov, S.A. Kovalenko, Yu.A. Matveets, N.F. Starodubtsev, and G. Marowsky. The Slowing-Down of Ultrafast Relaxation in C60 Films at High Femtosecond Pump Intensities. Appl. Phys. B  66(2), 225-230 (1998).
28. N.I. Afanas eva, Yu.E. Lozovik, Yu.A. Matveets, A.G. Stepanov, S.V. Chekalin, A.N. Shegolikhin. Femtosecond spectroscopy of polydiatsetilen. Optika i Spektoskopiya 85(2), 808 812 (1997).
29. T.S. Zhuravleva, L.M. Zemtsov, G.P. Karpacheva, S.A. Kovalenko, V.V. Kozlov, Yu.E. Lozovik, Yu.A. Matveets, P.Yu. Sizov, V.M. Farztdinov. Femtosecond spectroscopy of carbon films. Physical Chemistry 17, No. 6, 150-155 (1998) (in Russian) .
30. T.S. Zhuravleva, S.A. Kovalenko, Yu.E. Lozovik, Yu.A. Matveets, V.M. Farztdinov, A.L. Dobryakov, A.V. Nazarenko, L.M. Zemtsov, V.V. Kozlov, G.P. Karpacheva, and G. Marowsky. Ultrafast Optical Responce of IR Treated Polyacrylonitrile Films. Polymeres for Advanced Technologies 9, 613-618 (1998).
31. E.A.Vinogradov, A.L. Dobryakov, V.M. Farztdinov, Yu.E. Lozovik, Yu.A. Matveets, S.A. Kovalenko.  Ultrafast transient phenomena in semiconductor microcavities, SPIE Proc. 3239, 294-301 (1997).
32. a) Yu.E. Lozovik, A.L. Dobryakov, V.M. Farztdinov, S.A. Kovalenko, Yu.A. Matveets, E.A. Vinogradov. Dynamics of Semiconductor Microcavity Modes in Femtosecond Time Scale. Proceedings of International Symposium "Nanostructures 97: Physics and technology". St. Petersburg, Russia, 23-27 June 1997, p. 87-94.
b) E.A.Vinogradov, A.L. Dobryakov, S.A. Kovalenko, Yu.E. Lozovik, Yu.A. Matveets, V.M. Farztdinov. Femtosecond dynamics of microcavity semiconductor modes. Izvesyiya RAN, seriya fizicheskaya, 1998, 62(2), 221-227 (in Russian).
33.  Vinogradov, V.M. Farztdinov, A.L. Dobryakov, S.A. Kovalenko, Yu.E. Lozovik, Yu.A. Matveets. Femtosecond Spectroscopy of Semiconductor Microcavity Polaritons. Laser Physics 8(3), 620-624 (1998).
34.  S.K. Sekatskii, S.V. Chekalin, A.L. Ivanov, Yu.A. Matveetz, A.G. Stepanov, V.S. Letokhov. Journ. Phys. Chem. A102, 4148 (1998).