Laboratory of the Spectroscopy of Ultrafast Processes

 

 

The Laboratory of the Spectroscopy of Ultrafast Processes was organized in 1974, first as the Sector of Picosecond Spectroscopy under the guidance of Dr. P.G. Kryukov. The first research workers at the Laboratory were Yu.A. Matveets, D.N. Nokogosyan, and A.V. Sharkov. From 1980 till 2002 the Laboratory was headed by Dr. Yu.A. Matveets. The present chief of the Laboratory is Dr. S.V. Chekalin. The staff of the laboratory includes five investigators.

From the very beginning the work at the Laboratory was mainly centered on two lines of inquiry: (1) the study of ultrafast photoinduced processes in matter in condensed phase and (2) investigations into the selective action of high-power ultrashort laser pulses on matter.

To perform these works, pulsed laser setups were created to be capable of pulse durations first in the picosecond (30-6 ps) and then femtosecond (330-20 fs) range. These setups became the key elements of pico- and femtosecond spectrometers.

The development of lasers forming the base of such setups required special investigations in the physics of generation and amplification of ultrashort pulses. The most important result obtained at the Laboratory along these lines was the establishment of the self-focusing effect in active media on the generation and amplification processes [1-2]. It was demonstrated in particular that it was exactly self-focusing and not some other nonlinear processes that was the principal limiting factor in amplification. In the work [1] on the origin of the time structure of generated pulses, it was demonstrated, both experimentally and theoretically, that the main reason for the splitting of pulses into shorter fragments was also the self-focusing of the radiation being amplified in the active element, leading to practically inertialess variation of divergence along the time profile of the pulse with increasing intensity. This process, observed experimentally and modeled in [1], was used almost twenty years later to create a new generation of femtosecond lasers (the so-called KLM lasers) most widespread today.

In the experiments conducted with the aid of the super-high-speed spectrometers developed at the Laboratory [3], subject to study were the primary photoinduced processes in various biological objects, such as photosynthesis reaction centers in bacteria [3–5], hematoporphyrin [6], and bacteriorhodopsin [7–8]. The specific features of the X-ray spectra of the plasma typical of such heating conditions were discovered and modeled [9]. Also found were anomalies in the intensities of some lines of NaIX and CVI ions in the VUV and X-ray regions, when observed along and across the plasma column formed in two-step heating by ultrashort pulses. The anomalies observed are due to the amplification taking place under nonstationary heating conditions [10, 11].

A number of works were performed at the Laboratory on studies into the photodesorption of the ions of chromophore-containing molecules by pulses of picosecond [12, 13] and femtosecond [14, 15] duration by way of the mass spectrometry and visualization of the desorbed ions by means of a laser photoion microscope. The authors of these works were the first to discover the selectivity of photodetachment of dye ions and obtain ion-selective images of their escape sites with a magnification of the order of 1000 diameters [12, 13]. An important result of these investigations was the discovery of the reduction of the ion photodetachment threshold at pulse durations under a picosecond (Fig. 1)

 

 


Fig. 1. Chromophore (tryptophan) photodetachment threshold as a function of the exciting-pulse duration.

 

In the cycle of works on the study of bacteriorhodopsin, it was demonstrated [7] that immediately following the absorption of a quantum of light, there takes place the process of isomerization of retinal with a characteristic lifetime of 0.5 ps, leading to the formation of an intermediate product having a lifetime of 3 ps. These experimental facts made an important contribution to the understanding of the molecular mechanism responsible for the conversion of light by retinal-protein complexes [7–8]. The investigations conducted into the primary photoinduced processes in the reaction centers of purple bacteria at a time resolution of some 10–13 s made it possible to measure the rate of migration of energy between pigments and determine the primary electron donor in the process of conversion of the energy of the absorbed quantum into the energy of separated charges [3–5].

The studies into selective action on matter were started in 1977 with experiments on the stepwise selective electronic excitation of the Rhodamine 6G molecule in a D2O solution via an intermediate vibrational overtone transition [16a]. The extremely high radiation absorption power of water all over the IR region of the spectrum prevents this method from being made effective. For this reason, the next research stage involved experiments on the less selective but more efficient multiple-step electronic excitation of biomolecules (nucleic acid bases) in aqueous solutions via singlet states [16b]. Several cycles of experiments were conducted on the multiple-step ionization and dissociation of water and aqueous solutions of nucleic acid components [16b] and on the nonlinear photochemical synthesis of amino acids [17].

In 1989, investigations were launched into ultrafast processes in solids, such as the relaxation of excited charge carriers in semiconductors [18–19], metals [20–22], high-temperature superconductor (HTSC) materials [23–26], fullerites [27–36, 45], polymers and other carbon-containing materials [37, 38], and semiconductor microcavities [39]. Since 1996 experiments have been conducted in collaboration with Scientific and Technological Centre of Unique Instrumentation RAS on achromatic wavefront reconstruction by means of femtosecond laser pulses [4]. As a result of these works, the Laboratory in 2000 was awarded a Yu.I. Ostrovsky Prize of Ioffe Leningrad Physics-Technical Institute for the best publication on holographic interferometry. Successful experiments were also carried out on nanolithography with the aid of femtosecond laser pulses [41] (in cooperation with the Laboratory of Nanophysics), on the measurement of coherent polarization relaxation times in condensed media by the femtosecond interference spectroscopy technique [42], and also on projection photoelectron microscopy of ultrahigh spatial resolution [43] and nonperturbing visualization of light in the near field by means of femtosecond pulses [44] (in collaboration with the Laboratory of Laser Spectroscopy).

Our experimental investigations are being carried out with the laser femtosecond spectrometer developed at the Institute of Spectroscopy [3]. The experimental scheme (Fig. 2) is as follows. A short exciting pulse less than 100 fs in duration and 108-1013 W/cm 2 in intensity causes a strong heating of charge carriers and detachment of the electron temperature from the lattice temperature. A probe pulse, delayed by 0 to 1 ns relative to the exciting pulse, is then used to observe the kinetics of the nonequilibrium system, i.e., to monitor the dynamics of the energy transfer between the electron subsystem and the lattice phonons. The probing can be carried out over a wide spectral region, from 0.26 to 1 μm. Both the absorptivity and reflectivity of the sample under study can be measured in the process.


 

Fig. 2. Schematic diagram of a pump-probe-type experiment.

 

Compared to other spectroscopic methods, femtosecond optical spectroscopy has the advantage that it allows one to study the relaxation dynamics of nonequilibrium charge carriers and separate the contributions from the nonequilibrium charge carriers and the lattice phonons to changes in the optical properties of the material under study. As applied to metals, it makes it possible to isolate the narrow spectral region corresponding to transitions from some valence band into the neighborhood of the Fermi level and observe the dynamics of variation of the temperature 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 during the course of time t > 10–13 s a quasi-equilibrium is established in the electron subsystem and one can introduce the notion of electron temperature. The rise of the electron temperature, governed by the time-delayed probe pulses, gives rise to the following processes.

(1) The smearing of the distribution function of the carriers near the Fermi level, leading to the reduction of the population of the electronic states above the Fermi level. As a result, the change of the dielectric function e2(ω) will manifest itself as an alternating function going to zero at the point ωF corresponding to transitions to (or from) the Fermi level. Thus, femtosecond spectroscopy enables one to determine the position of the Fermi level such that Δe2(ωF)=0 [22, 23, 25].

(2) The shift of the Fermi level. In good metals, this effect is very small, but in bad ones it may be substantial.

With delay times shorter than the electron-phonon relaxation time (for normal metals this time is around 1 ps [22]), these processes are decisive. To observe them, the exciting and the probe pulse must have their duration much shorter than the electron-phonon relaxation time. This calls for pulses with a duration of around 100 fs or shorter still.

Using femtosecond laser spectroscopy techniques, test experiments were conducted on the measurement of the electron-phonon interaction parameter in metals. Studied was the time behavior of the differential reflection and transmission spectra of copper films in the range 505-605 nm following the excitation of the sample by a high-power 150-fs laser pulse. The value obtained for the parameter, λ<ω2>=27±4 MeV, [21, 22] agrees within the accuracy of measurement with the results obtained by other methods.

Studies were carried out into the effect of various electron-phonon interaction mechanisms on the energy relaxation rate in polar semiconductors – CdS–CdSe microcrystallites in a glass matrix [18–19].

It is well known that the mechanism responsible for the development of attraction between electrons in high-temperature superconductors still remains the subject of heated discussions. The establishment of the superconductivity mechanism of oxides can be facilitated by the determination of the magnitude of the electron-phonon interaction constant λ (or of the Eliashberg parameter λ<ω2>). The only direct method for measuring the parameter λ<ω2> for HTSC materials would be the study of the energy relaxation of nonequilibrium electrons in real tome. But this requires a method allowing this process to be studied on a femtosecond time scale. Femtosecond optical spectroscopy makes it possible, within the framework of a single experiment, not only to directly determine the electron-phonon interaction parameter and the magnitude of the superconductor gap, but also to observe on a real ( femtosecond ) time basis the changes in the electronic spectrum associated with the creation or annihilation of a new phase – the superconducting state [23–26]. As a result, it proves possible to obtain unique information on the nature of the superconducting state in these materials, specifically the role of the electron-phonon interaction mechanism therein.

Once the spectral region corresponding to transitions from some band to the neighborhood of the Fermi level has been isolated at a temperature T>Tc , it becomes possible to observe the changes of the optical spectra consequent upon transitions from the normal phase to the superconducting one and vice versa: the spectral width of the difference response observed proves to be directly related to the magnitude of the superconductor gap [23].

Investigations into the dynamics of the reflection and transmission spectra of a superconductor at T>Tc helped us to isolate the spectral region corresponding to band-to-band transitions to the Fermi level. Based on the relaxation dynamics of the spectra, we determined the electron-phonon interaction parameter λ<ω2> to be around 500 (MeV)2 (corresponding to λ≈1) [23–25]. Proceeding from the optical spectra ( žω>>Δ !) at T<Tc, we estimated the magnitude of the superconductor gap Δ at around 30 MeV [23].

The data obtained on the position of the Fermi level, the magnitude of the superconductor gap, and the electron-phonon interaction parameter agree well with the results of experimental and theoretical works performed by other methods.

By approximating the experimental curves with the double-exponential fitted functions ΔDfit( t ) we obtained the spectral dependence of the optical density relaxation rate t1 . In collaboration with the Laboratory of Nanophysics, we predicted and revealed the sharp increase of the electron energy relaxation time in certain spectral regions that we associated with transitions to the neighborhood of the Fermi level. The spectral dependence obtained opens the way to a new method for determining the position of the Fermi level [22]. That such a method is applicable was demonstrated for metals (Au) [22] and high-temperature superconductors (YBa2Cu3O7-δ) [25]. Finally spectroscopy with a subpicosecod-high time resolution made it possible to study the spectrum of coherent low-lying phonon vibrations [53]. This provides interesting information on the vibrations that interact most intensely with electrons.

The processes of the energy relaxation of nonequilibrium excitations in C 60 films were studied within the limits of a wide spectral region between 1 and 3.6 eV [27–32, 34, 36]. The properties of fullerenes – molecules consisting of carbon atoms forming an enclosed spherical or spheroidal shell – have attracted special attention in recent years. The C60 molecule of Ih -type symmetry is the most spherically symmetric one among the other fullerene molecules known. The intensive studies of this molecule are due not only to its geometrical perfection, but also to a number of its remarkable properties. To reveal the specific features of charge transfer in fullerene-containing materials requires thorough investigations into the mechanisms responsible for photoconductivity in pure fullerene films, and so a great enough number of works have been devoted to this matter. However, the contradictory character of the results obtained to date in the study of the relaxation of photoexcited molecules in C60 films by the pump-probe technique with a femtosecond-high time resolution is mainly due to the fact that a substantial number of experiments were performed with the excitation and probing effected at one and the same wavelength. With the experiment staged in this way, it is in principle impossible to reveal the multiple-component nature of the system of excited molecules under study. When studying relaxation in pump-probe-type experiments, different components of the system (and their relaxation mechanisms) can manifest themselves in the course of probing in different regions of the spectrum. In that case, the specific features of these mechanisms can be studied easily enough, even if relaxation in the different components occurs on the same time scale. However the strong overlapping of the wide spectral lines of the photoproducts formed, which is typical of solids, can greatly complicate the studies. In experiments using wide-band probing, we studied the femtosecond dynamics of the optical density induced in an ultra-thin C60 film and revealed the relationship between the relaxation time of the photoinduced response and the wavelength of the probe pulse [27].

The analysis of the difference spectrum dynamics [36] has shown that several regions can be singled out within the limits of the spectral range studied, inside which the decay kinetics are equal, the kinetic curves recorded from different regions differing materially in character (Fig. 3). This means that there exist at least three different components, each relaxing with kinetics of its own.

Our investigations have shown that when exciting C60 films with 100-fs pulses at 645 nm (1010-1011 W/cm2 and at 367 (345) nm (109 W/cm2 ), there are formed, during the course of the pulse, both the primary charge carriers – conduction electrons and localized cations that give rise to local electric fields in the sample – and neutral molecules in an excited state.

 

Fig. 3. Kinetic curves of various bands of C60 obtained with wide-band probing [36].

 

The fastest relaxation processes proceed with the participation of the most mobile components – the conduction electrons. These are the capture of the electrons by the C60 molecules for form excited anions, the electron–cation recombination, and (if oxygen is present) the capture of the electrons by deep traps. In the case of excitation above the mobility threshold (367 nm), the primary charge carriers are produced as a result of direct optical excitation, and in the case of excitation below the mobility threshold (645 nm), as a result of two-photon absorption, the quantum yield here being for this reason much lower. No perceptible manifestation of the singlet-singlet annihilation has been observed in either case, and so annihilation processes play no material part in the production of charge carriers.

Based on the time variations of the optical density of C60 films, we have studied the excitation of coherent phonons in the frequency range 10-400 cm–1 [30]. The total splitting of the H g (1) molecular vibration mode in C 60 fullerenes points to a substantial deformation of the fullerene molecules upon their absorption of photons.

It has also been found that at high excitation intensities. I > 1011 W/cm2 , there take place, as the pumping intensity is increased, the saturation of the growth of the photodarkening [29–30] and retardation of the relaxation of the photoinduced response of the C60 films. This is due to the additional heating of the charge carriers because of the processes of internal conversion of electrons from highly excited bands.

Subject to investigation were also the processes of photoinduced polymerization and photoinduced diffusion of molecular oxygen in thin C 60 films upon their irradiation with femtosecond laser pulses. Comparison of the Raman spectra and exposure doses necessary to observe photopolymerization in the case of CW radiation with their counterparts in the case of femtosecond pulses showed a substantial reduction of the efficiency of both photoinduced processes in the latter case [35].

Composite fullerene-containing films have been actively investigated in recent years in connection with the need to create high-efficiency solar energy converters and suitable materials for nonlinear optical elements and optical computers. Studied at the Laboratory was the femtosecond dynamics of the difference spectra of C 60 –tin composite materials subject to photoexcitation. These materials showed a strong dependence of the observed relaxation on the proportion and spatial distribution of the fullerene and metal. The initial relaxation stages were interpreted as the instantaneous charge transfer from the metal atom to the covalently bonded fullerene molecule in polymer chains and as electron exchange and excitation energy transfer in the metal nanocrystallite- C60 anion system, attended by the relaxation of the excited electron subsystem of the metal [45].

The photoinduced optical response of carbonized polyacrylonitrile films and its variation rate were studied in the wide spectral range 1.6-3.4 eV [38]. The spectral dependences of the optical response and its variation rates were placed in correspondence with the film structure.

The nonlinear optics of thin-film microresonator structures the type of Fabry-Perot resonators have recently attracted close attention because such devices can serve as a basis for developing new high-speed optical data transmission and processing systems. At the same time, the interaction between ultrashort pulses and planar microresonator structures containing nonlinear media of finite thickness is as yet extremely imperfectly understood. In this connection, investigations are being conducted at the Laboratory into the nonlinear interaction of ultrashort light pulses with thin-layer planar resonator structures similar to Fabry-Perot resonators.

In collaboration with the Laboratory of Semiconductor Structures, we studied the excitation and relaxation of natural modes in semiconductor microcavities of the ZnSe semiconductor material adjacent to Cr and Cu films on quartz substrates [39]. It was demonstrated that the alteration in the boundary conditions induced by femtosecond laser pulses caused the natural oscillation frequency of the microcavities to shift and their spectral lines to broaden. Coherent phonons were found to be excited in the microcavities.

In collaboration with Scientific and Technological Centre of Unique Instrumentation RAS and the State Optical Institute, we performed experiments on the study of specific features of volume holograms produced by means of femtosecond laser pulses [40]. We observed the achromatic reconstruction of the wavefront from such a hologram. In that case, the geometric-optical reflection mechanism dominates over the mechanism of radiation diffraction by the periodic interference structure, thanks to the number of the recorded periods of the latter being small because of the short duration of the pulses (Fig. 4).

Fig. 4. Diffraction and achromatic wavefront reconstruction mechanisms.

 

A number of experiments were carried out (in cooperation with the Laboratory of Laser Spectroscopy and Lausanne University , Switzerland ) using femtosecond radiation to observe images in a photoelectron microscope. When irradiating silicon nanotips with the second-harmonic radiation of a Ti-sapphire femtosecond laser, we obtained an ultrahigh spatial resolution of two-photon photoelectron images (up to 3 nm, which is the best spatial resolution attained to date with a photoelectron microscope). In addition, we measured the absolute value of the coefficient of the two-photon extrinsic photoeffect [43].

The two-photon photoelectric effect induced in the material of the tip by short laser pulses passing through it was used for nonperturbing optical near-field measurements. Use was made of pointed optical fibers deposition-coated with a metal (aluminum with a small admixture of nickel) so as to obtain a transparent aperture 50-200 nm in diameter at the point of the tip. These sample fibers were inserted into the vacuum chamber of the laser photoelectron microscope [44] (Fig. 5). Observed in the experiment was the quadratic relationship between the photocurrent and the laser radiation power, which clearly pointed to the two-photon character of the extrinsic photoelectric effect. Also presented in Fig. 5 are the field-emission images of an optical fiber tip and a plot of the square root of the photocurrent intensity as a function of the distance r to the center of the aperture, i.e., the distribution

E 2 ( ρ ). Such a relationship is a nonperturbing measurement of the electric field distribution in a sub-wavelength aperture.

Fig. 5. Optical near-field measurement experiment [44].

 

Fig. 6.

 

 

A new femtosecond center of joint use (Fig. 6) is now being built at the Laboratory. It is nearing completion, and its laser equipment (a Spectra Physics generator, amplifier, and converters (Fig. 7)) and two registration systems for conducting pump-probe-type experiments (a high-sensitivity single-channel system for work with a fixed probing length (developed in collaboration with Avesta-Proekt Co.) and a unique wide-band multichannel system for work at a frequency of 1 kHz (developed in collaboration with the company “Klub Delovykh Partnerov”) are being tuned up.

 

 

 

 

 

Fig. 7. Schematic diagram of the femtosecond laser spectrometer.

 

In its investigations, the Laboratory actively and fruitfully collaborates with both the other laboratories of the Institute of Spectroscopy (Laboratory of Laser Spectroscopy, Dr. V.S. Letokhov, Laboratory of Nanophysics, Dr. Yu.E. Lozovik, Laboratory of Semiconductor Structures, Dr. E.A. Vinogradov, Laboratory of the Vibrational Spectroscopy of Condensed Media, Dr. B.N. Mavrin) and other institutes in Russia (P.N. Lebedev Physical Institute, Institute of Biochemical Physics of the Russian Academy of Sciences, the Institute of General Physics of the Russian Academy of Sciences, Scientific and Technological Centre of Unique Instrumentation of the Russian Academy of Sciences) and other countries (Laser Laboratory and Institute of Biochemical Physics, G o ttingen, Germany, Lund University. Lund , Sweden , Brigham Young University , Provo , USA , Lausanne University , Switzerland ).

Three doctoral and eight Ph.D. dissertations were written and defended at the Laboratory over the past years.

A monograph was published on the basis of the results of investigations performed at the Laboratory.

 

Monograph

(a) V.S. Letokhov, Yu.A. Matveets, A.V. Sharkov et al . Laser Picosecond Spectroscopy and Photochemistry of Biomolecles (Nauka, Moscow ), 1987; (b) V.S. Letokhov, ed. Laser Picosecond Spectroscopy and Photochemistry of Biomolecules (Adam Hilger, Bristol ), 1987.

 

References

  1. A.N. Zherikhin, P.G. Kryukov, Yu.A. Matveets, S.V. Chekalin. About the Origin of Time Structure of Ultrashort Laser Pulses. Kvantovaya Elektronika 1 , 956–959 (1974).
  2. A.N. Zherikhin, Yu.A. Matveets, S.V. Chekalin. Brightness Limitation Due to Self-Focusing in the Course of Amplification of Ultrashort Laser Pulses in Nd glass and YAG. Kvantovaya Elektronika 3 , No. 7, 1585–1590 (1976).
  3. 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).
  4. Yu.A. Matveets, S.V. Chekalin, A.P. Yartsev. Femtosecond Energy Transfer in the Physical Stages of Photosynthesis. Pis'ma v ZhETF 43 , No. 11, 546–548 (1986).
  5. S.V. Chekalin, Yu.A. Matveets, A.Ya. Shkuropatov, V.A. Shuvalov, A.P. Yartsev. Femtosecond Spectroscopy of Primary Charge Separation in Modified Reaction Centers of Rhodopseudomonas Sphaeroides (R26). FEBS Lett. 216 , No. 2, 245 (1987).
  6. S.V. Chekalin, A.P. Yartsev. Differential Spectra of Hematoporphyrin in the Visible Region in Conditions of Excitation by Powerful Subpicosecond Pulses. Biofizika XXXIII , No. 5, 751–754 (1988).
  7. S.V. Chekalin, A.V. Sharkov, A.V. Pakulev, Yu.A. Matveets. Primary Events in Bacteriorhodopsin Probed by Subpicosecond Spectroscopy. Biochimica et Biophisica Acta 808 , 94–102 (1985).
  8. S.V. Chekalin, O.M. Brekhov, V.Yu. Rootskoy, A.V. Sharkov, Yu.A. Matveets, A.V. Konyaschenko. Fluorescence of Bacteriorhodopsin under Subpicosecond Light Excitation. Photochemistry and Photobiology 38 , No. 1, 109–111 (1983).
  9. E.V. Aglitsky, A.N. Zherikhin, P.G. Kryukov, S.V. Chekalin. Peculiarities of the X-Ray Spectra of Plasma Produced by Subpicosecond Laser Pulses. ZhETF 73 , No. 10, 1344–1351 (1977).
  10. A.N. Zherikhin, K.N. Koshelev, P.G Kryukov, V.S. Letokhov, S.V. Chekalin. Observation of Anomalous Intensity in the Range 58-78 ? in the Transition C1 V11 During the Two-Step Heating of Plasma by Ultrashort Laser Pulses. Pis'ma v ZhETF 25 , No. 7, 325–328 (1977).
  11. A.N. Zherikhin, K.N. Koshelev, P.G. Kryukov, V.S. Letokhov, S.V. Chekalin. Search of Amplification on Far VUV Transitions in Multiply Charged Ions in a Laser Plasma Column. Kvantovaya Elektronika 8 , No. 1, 88–97 (1981).
  12. V.S. Letokhov, V.G. Movshev, S.V. Chekalin. Ultrashort-Laser-Pulse Photodetachment of Rhodamin 6G Ions From a Surface .ZhETF 81 , 480–485 (1981).
  13. S.V. Chekalin, V.S. Letokhov, V.S. Likhachev, V.G. Movshev. Laser Photoion Projector. Appl. Phys. B , B33 , 57–61 (1984).
  14. S.V. Chekalin, V.V. Golovlev, A.A. Kozlov, Yu.A. Matveets, A.P. Yarzev, V.S Letokhov. Femtosecond Laser Photoionization Mass Spectrometry of Tryptophan-Containing Proteins. Journ. Phys. Chem. 92 , No. 24, 6855–6858 (1988).
  15. A.A. Kozlov, S.V. Chekalin, Yu.A. Matveets, A.P. Yartsev, V.S. Letokhov. Desorption of Ions Created at the Surface of Molecular Crystals by means of a Femtosecond Laser Pulse. Pis'ma v ZhETF 47 , No. 6, 294–296 (1988).
  16. ( ΰ ) P.G. Kryukov, V.S. Letokhov, Yu.A. Matveets, D.N. Nikogosyan, A.V. Sharkov. Selective Two-Step Excitation of an Electronic State in Organic Molecules in an Aqueous Solution by Picosecond Light Pulses. Kvantovaya Elektronika 5 , 2490 (1978). (b) P.G. Kryukov, V.S. Letokhov, D.N. Nikogosyan et al . Multiquantum Photoreactions of Nucleic Acid Components in an Aqueous Solution by Powerful Ultrashort Picosecond Radiation. Chem. Phys. Lett. 61 , 375 (1979).
  17. E.V. Khoroshilova, N.P. Kuz'mina, V.S. Letokhov, Yu.A. Matveets. Nonlinear Photochemical Synthesis of Biomolecules by Powerful UV Picosecond Pulses. In: A.H. Zewail, ed. Photochemistry and Photobiology (Harwood Acad. Publ., 1983), vol. 2, p. 1267.
  18. YU. E. Lozovik, Yu.A. Matveets, A.G. Stepanov, V.M. Farztdinov, S.V. Chekalin, A.P. Yartsev. Femtosecond Relaxation of Excited Carriers in CdSe x S 1-x Microcrystallites in a Glass Matrix at High Excitation Intensities. Pis'ma v ZhETF 52 , 851–854 (1990).
  19. I.V. Bezel, Yu.A. Matveets, A.G. Stepanov, S.V. Chekalin, A.P. Yartsev. Two-Photon Absorption of Powerful Femtosecond Pulses in Semiconductor-Doped Glasses at Energies above the Band Gap. Pis'ma v ZhETF 59(6), 376–380 (1994).
  20. V.V. Golovlev, Yu.A. Matveets, A.M. Sanov, V.S. Letokhov. Investigation into the Electron Temperature Relaxation in a Copper Film Subject to Excitation by Femtosecond Laser Pulses. Pis'ma v ZhETF 55 , No. 8, 441–444, (1992).
  21. Yu.E. Lozovik, S.A. Kovalenko, A.L. Dobryakov, V.M Farztdinov, Yu.A. Matveets, S.P. Merkulova, N.P. Ernsting. Fermi Liquid Study on Femtosecond Scale. Laser Phys. 9 , No. 2, 557–563 (1999).
  22. A.L. Dobryakov, S.A. Kovalenko, Yu.E. Lozovik, S.P. Merkulova, V.M. Farztdinov, N.P. Ernsting. Femtosecond Spectroscopy of Relaxation Processes in Metals and High-Temperature Superconductors. ZhETF 119 , 1–12 (2001).
  23. S.V. Chekalin, V.M. Farztdinov, V.V. Golovlev, V.S. Letokhov, Yu.E. Lozovik, Yu.A. Matveets, A.G. Stepanov. Femtosecond Spectroscopy of YBa2Cu3O7-δ : Electron-Phonon Interaction Measurement and Energy Gap Observation. Phys. Rev. Lett. 67 , No. 27, 3860–3863 (1991).
  24. 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).
  25. A.L. Dobryakov, V.V. Golovlev, V.S. Letokhov, Yu.E. Lozovik, Yu.A. Matveets, A.G. Stepanov, V.M. Farztdinov, S.V. Chekalin. Femtosecond Spectroscopy of Superconductors YBa2Cu3O7-δ : Measurement of the Electron-Phonon Interaction Parameter and Energy Gap Observation. Optika i Spektroskopiya 76 , No. 6, 975–983 (1994).
  26. I.I. Vengrus, A.L. Dobryakov, S.A. Kovalenko, V.S. Letokhov, Yu.E. Lozovik, G. Marovsky, Yu.A. Matveets, V.M. Farztdinov, N.R. Ernsting. Spectral Dependence of Relaxation in YBa2Cu3O7-δ Subject to Femtosecond Laser Excitation. Pis'ma v ZhETF 66 , No. 9, 739–743 (1995).
  27. S.V. Chekalin, V.M. Farztdinov, E. Akesson , V. Sundstr o m. Relaxation of C 60 in Solutions and Films: Results of Femtosecond Studies. Pis'ma v ZhETF 58 ( 4 ), 286–290 (1993).
  28. V.M. Farztdinov, Yu.E. Lozovik, Yu.A. Matveets, A.G. Stepanov, and V.S. Letokhov. Femtosecond Dynamics of Photoinduced Darkening in C 60 Films. J. Phys. Chem. 98 , No. 13, 3290–3294 (1994)
  29. I.V. Bezel, S.V. Chekalin, Yu.A. Matveets, A.G. Stepanov, A.P. Yartsev, and V.S. Letokhov. Two-Photon Absorption of Powerful Femtosecond Pulses in C 60 Films. Chem. Phys. Lett. 218 , No. 5, 6, 475–478 (1994).
  30. A.L. Dobryakov, S.A. Kovalenko, V.S. Letokhov, Yu.E. Lozovik, G. Marovsky, Yu.A. Matveets, V.M. Farztdinov, N.R. Ernsting. Coherent Phonons in Fullerites under Femtosecond Laser Excitation. Pis'ma v ZhETF 61 , No. 12, 957–961 (1995).
  31. S.V. Chekalin. Femtosecond Spectroscopy of Intra- and Intermolecular Relaxation in Photoexcited Fullerenes and Fullerites. Femtochemistry. In: M. Chergui, ed. Ultrafast Chemical and Physical Processes in Molecular Systems, 649–653 (1995).
  32. 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 C 60 Films. Phys. Rev. B 56 ( 7 ), 4176–4185 (1997).
  33. S.V Chekalin. Phototransformation in Visible and Near-IR Femtosecond Pump-Probe Studies of C-60 Films – Comment. Appl. Phys. Lett., 71 , No. 9, 1276–1276 (1997).
  34. D. Bushe , S.A. Kovalenko, P. Masselin, Yu.A. Matveets, M.G. Novikov, V.V. Ragulsky, A.G. Stepanov, S.V. Chekalin. Wide-Band Femtosecond Laser Spectroscopy Studies of the Ultrafast Relaxation of Photoexcited Fullerenes under Various Excitation Conditions. Izv. AN. Ser. Fizicheskaya 62 , No . 2, 237 – 244 ( 1998 ) .
  35. V.O. Kompanets, N.N. Mel'nik, B. Hess, S.V. Chekalin. Photoinduced Transformations in C 60 Films Exposed to Femtosecond Laser Pulses. Pis'ma v ZhETF 68 , No. 4, 320–325 (1998).
  36. S.V. Chekalin, A.P. Yartsev, V. Sundstr o m. Femtosecond Spectroscopy of Charge Carrier Generation in C 60 Films by 100-Femtosecond Laser Pulses. ZhETF 120 , No. 4(10), 810–822 (2001).
  37. N.I. Afanasieva, Yu.E. Lozovik, Yu.A. Matveets, A.G. Stepanov, S.V. Chekalin, A.N. Shchegolikhin. Femtosecond Spectroscopy of Polydiacetylene Single Crystals . Optika i Spektroskopiya 85 ( 2 ), 808 – 812, (1997).
  38. 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. Fizicheskaya Khimiya 17 , No. 6 (1998).
  39. E.A. Vinogradov, A.L. Dobryakov, S.A. Kovalenko, Yu.E. Lozovik, Yu.A. Matveets, V.M. Farztdinov. Spectroscopy of Polaritons in a Semiconductor Cavity. Uspekhi Fizicheskikh Nauk 169 , No. 3, 347–348 (1999).
  40. D.A Dementiev, A.L. Ivanov, O.B. Serov, A.M. Smolovich, A.G. Stepanov, S.V. Chekalin. Achromatic Reconstruction of Femtosecond Laser Pulse Wavefront. JETP Lett. 65 (1997).
  41. Yu.E. Lozovik, D.V. Lisin, A.L. Ivanov, V.O. Kompanets, Yu.A. Matveets, S.V. Chekalin, S.P. Merkulova. Femtosecond Laser Pulse Nanolithography Using an STM tip. Laser Phys. 9 , No. 2, 564–569 (1999).
  42. V.O. Kompanets, Yu.A. Matveets, S.V. Chekalin. Measurement of Coherent Polarization Relaxation Times in Condensed Media by the Femtosecond Interference Spectroscopy Technique. Kvantovaya Elektronika 31 , No. 5, 393–394 (2001).
  43. S.K. Sekatskii, S.V. Chekalin, A.L. Ivanov, V.O. Kompanets, Yu.A. Matveets, A.G. Stepanov, V.S. Letokhov. Ultra-High-Spatial-Resolution Projection Photoelectron Microscopy Using Femtosecond Lasers. ZhETF 115 , No. 5, 1680–1688 (1999).
  44. S.K. Sekatskii, B.N. Mironov, V.O. Kompanets, Yu.A. Matveets, S.V. Chekalin, G. Dietler and V.S. Letokhov. Nonperturbing Observation of Optical Near Field. Appl. Phys. Lett. (2003) (in press).
  45. S.V. Chekalin, V.O. Kompanets, M.S. Kurdoglyan, A.N. Oraevsky, N.F. Starodubtsev, V. Sundstr o m, A.P. Yartsev. Femtosecond Pump-Probe Investigation of Primary Photoinduced Processes in C 60 /Sn Nanostructures. Synthetic Metals 139 , 799 (2003).