Laboratory of Laser Spectroscopy - 2003.
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Nanooptics Atom Optics Atom Nanooptics
 

 

The Laboratory of Laser Spectroscopy was set up in the summer of 1970. Its first chief, from the very beginning and till 2002, was Dr. V.S. Letokhov. The present head of the Laboratory is Dr. V.I. Balykin who took over from V.S. Letokhov in 2002. The first research workers at the Laboratory were O.N. Kompanets and O.A. Tumanov, who transferred, together with V.S. Letokhov, from the Physics Institute the Russian Academy of Sciences. At present there are 15 investigators working at the Laboratory.

The first experiments (1971–1972) were associated with the development of Doppler-free laser spectroscopy techniques in 1971–1974 (O.N. Kompanets, B.D. Pavlik), and methods for resonance conversion of

IR laser radiation by way of excitation of vibrational-rotational transitions in molecules (O.A. Tumanov, V.I. Balykin, E.A. Ryabov, and others).

One of the main methods to selectively affect atoms by laser radiation is their resonant multiple-step ionization that makes it possible both to separate atoms of different species, isotopes included, and to detect them [1]. Resonant stepwise ionization was for the first time realized in 1971 in the experiment with Rb atoms [2], [3] performed jointly with the Sector of Spectroscopy of Excited States (R.V. Ambartzumian). Since that time and till the present various modifications of this method using various stepwise ionization schemes (Fig. 01) have been developed to solve numerous problems, such as detection of single atoms, detection of extremely rare isotopes, detection of short-lived nuclei in accelerators, separation of isotopes, nuclear isobars, and nuclear isomers, production of beams of pure photoions, and so on.

 

Fig. 01

Laser resonant photoionization spectrometer for ultra-sensitive analysis of traces of elements (right – illustrating stepwise ionization via Rydberg and autoionization states).

 

Figure 02 illustrates three resonant-ionization spectroscopy approaches using ionization of atoms in (a) a beam, ( b ) a quasi-enclosed hot cavity, and ( c ) an accelerated atomic beam. The approach illustrated in Fig. 02 a was used in the experiment conducted in 1977 on the resonant-ionization detection of single Na atoms in a beam [4]. By this time the method has become a powerful ultra-sensitive tool in the spectroscopy of atoms and molecules. Every two years since 1982 International Conferences have been held on this method. The Institute of Spectroscopy is the leading scientific center on its further development and utilization.

One of the fields where the use of the resonant-ionization spectroscopy technique has proved a success is ultra-sensitive spectral analysis. This was demonstrated in the experiments on detecting noble and platinum-group elements in the ocean [5a], conducted jointly with the Central Scientific Research Institute for Geological Exploration of the Ministry of Geology, and a rhodium anomaly at the Tertiary boundary (65 million years old) [56], performed jointly with the Institute of Geochemistry of the Russian Academy of Sciences, and in the experiment (Fig. 02 b ) on measuring the Rydberg states and ionization potential of the Fr atom in a sample containing a mere 105 atoms.

Fig. 02

Three approaches of laser resonant-ionization spectroscopy. Shown on the right are some radioactive isotopes studied by each of the three methods.

 

The idea of combining the resonant-ionization spectroscopy technique with the electrostatic acceleration of the atoms of interest (in the form of ions) in a beam was suggested in [7] and used successfully (the group headed by Dr. Yu.A. Kudryavtsev) to solve an extremely difficult problem, namely, to optically detect very rare isotopes against the background of the abundant isotopes with a relative abundance of over 1010. Figure 03 presents the results of the first experiment on the laser detection of the3He atom [8].

 

Fig. 03

Detecting the rare isotope 3Íå in a beam of accelerated He atoms. Presented is the ion signal as a function of the laser frequency at the second stage of resonant excitation of the metastable atoms 4Íå è 3Íå at various relative concentrations.

 

This method is being developed today at several laboratories in the USA , Germany , and other countries for detecting other extremely rare isotopes.

Resonant-ionization spectroscopy was successfully used (the group headed by Dr. V.I. Mishin) in experiments on the laser spectroscopy of atoms with short-lived nuclei, aimed at measuring the characteristics (moments and mean charge radius variations) of the latter. These experiments were carried out using the accelerator facilities of Leningrad Institute of Nuclear Physics (LIYaF) and Conseil Europeen pour la Recherche Nucleaire (CERN). In the course of these experiments, they for the first time realized the optical separation of nuclear isomers. Figure 04 presents the results of the first experiment of this kind [9] with the isomeric nuclei 141 Sm and 164 Tm. This method was later used as a basis for creating (in cooperation with LIYaF) a Z -selective laser source of photoions (i.e., a laser isobar separator) for experiments in nuclear physics in Gatchina. The method was next implemented successfully with the ISOLDE facility at CERN [10]. It is now being used in many experiments in nuclear physics in Japan , Canada , and other countries. Figure 05 presents a schematic diagram of the laser ion source built at CERN in collaboration with the Institute of Spectroscopy for a wide class of experiments by the A+ -, a -, b -, g -, and n -spectroscopy techniques.

Along with the photoionization method for detecting single atoms, at the Laboratory has been successfully developing, since 1977, the fluorescence detection technique (the group headed by Dr. V.I. Balykin). These investigators realized and studied the detection of single sodium atoms by the fluorescence technique. They suggested and implemented a universal method to improve selectivity in detecting single atoms by their fluorescence, based on the multiple-photoelectron registration of the signal generated by the atom of interest. This method was demonstrated to be capable of detection selectivity better than that limited by such a fundamental factor as the overlapping of the absorption spectral lines of atoms.

In 1976 they started developing ideas of controlling (cooling, trapping) the motion of atoms by means of laser radiation. After the first theoretical works [12] there followed the first successful experiments on the retarding and cooling of atoms in a beam [13], on their focusing [15] and channeling [16a], and finally on the reflection of an atomic beam from a light-field gradient [166] (Fig. 06].

Fig. 04

Spectrum of resonant photoionization (4 f 13 6 s 2 7 F 7/2 -> 4 f 12 5 d 6 s 2 (6,7/2) 7/2 transition at 589.6 nm) of the 164 Tm isotope ( a ) in the ground and isomeric state of the nucleus and ( b ) in the ground state of the nucleus.

 

Fig. 05

Schematic diagram of the laser photoselective ion source developed by the Institute of Spectroscopy in collaboration with Mainz University for separating nuclear isotopes, isomers, and isobars at CERN (Geneva).

 

 

Fig. 06

Illustrating the methods implemented for the first time at the Institute of Spectroscopy to control the motion of atoms in a beam by means of laser radiation.

Figure 07 presents the results of the world-first experiment [13b] on the laser cooling of an atomic beam. The temperature of a beam of Na atoms was reduced from its initial value of 600 K to 1.5 K (the group headed by Dr. V.I. Balykin). In the case of longitudinal cooling, the transverse velocity of the atoms in the beam increases as a result of the fluctuation rise of the atomic momentum upon absorption and re-emission of laser photons. At a certain stage of longitudinal cooling, the axial velocity of the atoms becomes commensurable with their transverse velocity, and so to cool the atoms further requires their transverse cooling. In 1984 such a transverse cooling was realized at the Laboratory and this helped them to attain the next record-low atomic temperature of 0.003 K [14]. This value is close to what is known as the quantum (Doppler) cooling limit for atoms: a two-level atom in a laser field cannot be cooled to a temperature below T min = h y / k B ( h is Planck's constant, 2y is the homogeneous absorption line width of the atoms, and k B is the Boltzmann constant).

Fig. 07

Laser cooling to 1.5 Ê (deceleration and monochromatization) of a beam of Na atoms by a counter-propagating laser beam ( À - experiment, Â - theoretical calculation).

 

The expression for the quantum limit of the laser cooling of atoms was first obtained by Dr. V.G. Minogin and co-workers as far back as 1977 [12a]. In the same work they calculated a three-dimensional laser cooling scheme. The implementation of this scheme proved fairly difficult. These authors suggested irradiating the atoms with six laser beams directed in such a way that two counter beams propagated along each of the axes of coordinates. In the intersection region of these beams must be formed a centrally symmetric light field. A laser field of this configuration was used in 1985 by a group of researchers at Bell Laboratories in the USA . In this experiment, they attained a temperature of 240 μK – the next record-low temperature value. This temperature value coincides with the quantum limit hγ/kB .

All these experiments on the cooling of atoms made it possible to reduce the energy of neutral atoms to such values as enabled their spatial localization by means of electric, magnetic, and laser fields. This opened up new experimental possibilities of drastically reducing the temperature of already “cold atoms, and a new stage of investigations into a deeper laser cooling thus commenced. The chronology of these works is presented in Fig. 08.

 

Fig. 08

Progress made in the laser cooling of atoms (key experiments), starting with the works performed at the Institute of Spectroscopy in 1981.

 

As a result of studies into the action of laser light pressure forces on the translational motions of atoms, a new physical discipline – atom optics – originated in the mid-eighties. Atom optics is a kind of optics of material particles (along with electron optics, ion optics, and neutron optics) and deals with the problems of formation of flows of neutral atoms, their control, and application [17–21].

One of the promising atom optics applications is atomic nanolithography, i.e., the production of structures about 10 nm in size. An important step in atomic lithography is the production of atomic ensembles (beams) with a high phase density. Figure 09 illustrates the progress made in the attainment of maximal fluxes of cold atoms obtained by laser cooling techniques at the Institute of Spectroscopy and other scientific centers.

Fig. 09

Illustrating the progress made in the attainment of maximal flows of cold atoms obtained by laser cooling techniques at various scientific centers. [1] M. Schiffer et al. Phys. Rev. A, 61, 013405; [2] K. Dieckmann et al. Phys. Rev. A 58 , 3891; [3] ZhETF 2003 (to be published); [4] Z. T. Lu et al. PRL 77 , 3331; [5] A. Witte et al. J.O.S.A. B, 9, 1030; [6] M. D. Hoogerland et al. Aust. J. Phys., 49 , 567; [7] F. Lison et al. Phys. Rev. A, 61 , 013405.

 

The Laboratory of Laser Spectroscopy was the first in the world to start experimenting on the laser control of atomic motion. Nowadays tens of laboratories all over the world are being engaged in this most active avenue of inquiry in atomic and laser physics.

Worthy of notice among other early works is the experiment [22] conducted jointly with the Laboratory of Atomic Spectroscopy on the search for amplification in the soft X-ray region of the spectrum on multiply charged ions in a laser plasma. The authors suggested a scheme for pumping neon-like multiply charged ions. It was exactly by this scheme and on the transition predicted that the first X-ray laser was operated at Lawrence Livermore Laboratory (USA).

One should also notice the ideas concerning new types of exotic lasers, such as lasers in stellar atmospheres and space lasers with incoherent feedback due to resonant scattering (Dr. V.S. Letokhov, 1972–1974), and lasers operating on g -transitions with laser separation of isomeric nuclei (Dr. V.S. Letokhov 1971–1973).

Retrospectively one can say that the main stem of work at the Laboratory of Laser Spectroscopy for 33 years was theoretical and experimental search for new approaches to the achievement of ultimate performance characteristics of optical spectroscopy using laser light, namely:

•  ultrahigh spectral resolution (nonlinear spectroscopy techniques, methods for cooling and trapping of atoms, multiple-photoelectron registration);

•  ultrahigh time resolution (pico- and femtosecond spectroscopy techniques);

•  ultrahigh sensitivity in detecting atoms and molecules (up to single atoms and molecules);

•  ultimately high selectivity in detecting atoms and molecules, specifically ultra-rare isotopes against the background of abundant isotopes;

•  ultimately high spatial resolution (much better than the diffraction limit), up to the visualization of atoms, molecules, and absorption centers.

While problems (1) through (4) have largely been solved, so that the methods developed are already finding successful application, problem (5) is at the stage of exploration and demonstration of the appropriate techniques. This represents further lines of investigation at the Laboratory for the years immediately ahead. In essence, while solving this problem we are entering into the domain of what is known as nanooptics . Nanooptics is a new research area in physics, based on observation and utilization of new effects of interaction between laser radiation and materials (atoms, molecules, surface, etc.) on a spatial scale much smaller than the wavelength of light. It was a customary belief that such a formulation is meaningless, for laser light cannot be localized (e.g., focused) within a space measuring less than the wavelength of light across. In actual fact, when using sub-micron-sized structures and even nanostructures, light can be confined within smaller volumes, as is the case with microwave radiation. However, as distinct from microwaves, in the optical case to role played by the atomic-molecular (discrete) structure of the material interacting with such a localized light becomes essential. Hence stem two new possibilities:

first, the use of highly localized laser radiation makes it possible to analyze the structure of the material with a nanometer-high spatial resolution and

secondly, the optical response of the material to localized light differs substantially from that to propagating light field.

The studies of these capabilities of atomic and quantum nanooptics are the prime task of the Laboratory of Laser Spectroscopy. These capabilities have at least two potentially important practical applications: (1) the laser spectromicroscopy of materials with a nanometer-high spatial resolution and chemical selectivity and (2) optical memory of ultrahigh density (of the order of 100 GB per square centimeter).

The main research areas the Laboratory is being presently engaged in are as follows.

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I. Nanooptics

 

(1) Laser resonant photoelectron microscopy with a spatial resolution of 3-30 nm [23]. The development of a microscope using excitation by laser pulses, femtosecond pulses included, that makes it possible to investigate and select optical fiber probe tips with a single excitable resonant center for subsequent use as a nanosensor of molecular structures, surface, and so on [24]. The same microscope is also used in experimental studies into the possibility of creating a WROM with an ultrahigh data recording and reading density [25, 26].

(2) Laser scanning near-field microscopy with a transverse resolution around 30 nm and a longitudinal resolution better than 1 nm. As distinct from the existing near-field microscopy versions, the microscope developed exactly measures the distance between the nanotip and the surface under investigation [26]. This enables one to study the processes of resonant interaction between molecules (resonant energy transfer [23, 24]) as a function of the distance between them. This in turn forms the basis for the realization of the above laser microscope with a nanometer-high spatial resolution and spectral (chemical) selectivity. This type of microscope will open up entirely new instrumental possibilities for nanoscience and nanotechnology.

(3) Variation of the optical characteristics of materials (atoms, molecules) on a nanometer scale, i.e., in the vicinity of and within nanostructures. Subject to theoretical studies are variations of resonance frequencies, radiative transition probability in atoms [31], and dipole-dipole interaction of molecules [32], which allows one to induce nanolocal modifications in the properties of materials for numerous applications (microscopy [27, 28], optical memory, and so on).

II. Atom Optics

This is a new type of optics (along with photon, electron, and neutron optics) that has originated in recent years as a result of investigations into the action of electromagnetic radiation on the motion of atoms. The physical problems under study [17–21, 36] include the search for the potentials of interaction between atoms and electromagnetic fields that provide for controllable actions on the motion of atoms, such as the diffraction of atomic waves, reflection, focusing, and localization of atoms, and also the enhancement of the phase density of atomic ensembles. The development of the methods of atom optics opens up the way to lithography with atomic-scale resolution.

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III. Atom Nanooptics

It is obvious from general physical considerations that the use of spatially localized potentials is more preferable for the construction of atom optical elements. The possibility is being studied of using laser nanofields for atom optics–atom nanooptics purposes [37].

Another avenue of investigation is the explanation of abnormal spectral effects occurring in the vicinity of h Carinae , the most massive star in our Galaxy, which is being carried out, since 2002, by Prof. V.S. Letokhov in collaboration with Prof. S. Johansson from Lund University ( Sweden ) [32]. These investigations have resulted in the discovery of the astrophysical laser [33, 34].

Two doctoral and more than thirty Ph.D. dissertations were written and defended at the Laboratory over the past years.

Nine monographs were published on the basis of the results of investigations performed at the Laboratory.

Monographs of the Laboratory of Laser Spectroscopy

  1. (a) V.S. Letokhov, V.P.Chebotayev. Principles of Nonlinear Laser Spectroscopy (Nauka, Moscow ), 1975; (b) V.S. Letokhov, V.P.Chebotayev. Nonlinear Laser Spectroscopy ( Springer-Verlag , Berlin ), 1977.
  2. V.S. Letokhov. Laserspektroskopie ( Akademic-Verlag , Berlin ), 1977.
  3. ( à ) V.S. Letokhov. Nonlinear Selective Photoprocesses in Atoms and Molecules (Fizmatgiz, Moscow ), 1983. (b) V.S. Letokhov. Nonlinear Laser Chemistry with Multiple-Photon Excitation ( Springer-Verlag , Berlin ), 1983.
  4. à ) V.P. Zharov, V.S. Letokhov. Laser Optoacoustical Spectroscopy (Nauka, Moscow ), 1984; (b) V.P Zharov, V.S. Letokhov. Laser Optoacoustical Spectroscopy ( Springer-Verlag , Berlin ), 1986.
  5. ( à ) Laser Analytical Spectroscopy, ed. by V.S. Letokhov (Nauka, Moscow ), 1986; (b) Laser Analytical Spectrochemistry, ed. by V.S. Letokhov (Adam Hilger, Bristol ), 1986.
  6. ( à ) V.G. Minogin, V.S. Letokhov. Laser Radiation Pressure on Atoms (Nauka, Moscow ), 1986; (b) V.G. Minogin, V.S. Letokhov. Laser Light Pressure on Atoms (Gordon and Breach, New York ), 1986.
  7. ( à ) V.S. Letokhov. Laser Photoionization Spectroscopy (Nauka, Moscow ), 1987; (b) V.S. Letokhov. Laser Photoionization Spectroscopy (Academic Press, Orlando), 1987.
  8. V.S. Letokhov and V.P. Chebotaev. Ultrahigh-Resolution Nonlinear Laser Spectroscopy (Nauka, Moscow ), 1990.

  9. V.I. Balykin, V.S. Letokhov. Atom Optics with Laser Light. (Harwood Acad. Publ., Chur).

References

  1. V.S. Letokhov. A Method of Photoionization of Gases by Laser Radiation. Author's Certificate No. 784679, Priority Date 30.03.1970. Bull. Izobretenii, 1982, No. 8, p. 308.
  2. R.V. Ambartzumian, V.P. Kalinin, V.S. Letokhov. Two-Step Selective Photoionization of Rubidium Atoms by Laser Radiation. Pis'ma v ZhETF, 13 , 305 (1971).
  3. R.V. Ambartzumian and V.S. Letokhov. Selective Two-Step (STS) Photoionization of Atoms and Photodissociation of Molecules by Laser Radiation. Appl. Optics, 11 , No. 2, 354 (1972).
  4. G.I. Bekov, V.S. Letokhov, V.I. MIshin. Laser Photoionization Detection of single sodium atoms via Rydberg States. Pis'ma v ZhETF, 27 , 52 (1972).
  5. G.I. Bekov, V.S. Letokhov et al . (a) Ruthenium in the Ocean. Nature, 312 , 748 (1984); (b) Rhodium distribution at the Cretaceous/Tertiary Boundary Analyzed by Ultrasensitive Laser Photoionizaiton. Nature, 332 , 146 (1986).
  6. S.V. Andreev, V.S. Letokhov, V.I. Mishin. Laser Resonance Photoionization Spectroscopy of Rydberg Levels in Fr. Phys. Rev. Lett. 59 , 1274 (1987).
  7. ( à ) Yu.A. Kudriavtsev and V.S. Letokhov. Method of Highly Selective Detection of Rare Radioactive Isotopes through Multistep Photoionization of Accelerated Atoms. Appl. Phys. B29 , 219 (1982); (b) S.A .Aseyev, Yu.A. Kudriavtsev, V.S. Letokhov and V.V. Petrunin. Laser Detection of the Rare Isotope He at Concentrations as Low as 10 –9 . Optics Letters 16 , 514 (1991).
  8. V.I. Balykin, V.S. Letokhov, V.I. Mishin, V.A. Semchishen. Laser Fluorescence Detection of Single Atoms. Pis'ma v ZhETF 26 , 492 (1977).
  9. V.I. Mishin et al . Resonance Photoionization Spectroscopy and Laser Separation of 141 Sm and 164 Tm Nuclear Isomers. Opt. Comm. 61 , 383 (1987).
  10. (a) G.P. Alkhazov et al . Nucl. Instr. and Methods in Phys. Res., A306 , 402 (1991); (b) V.I. Mishin, V.N. Fedoseyev et al . Instr. and Methods in Phys. Res. B73 , 550 (1993).
  11. V.I. Balykin et al. (a) Laser Fluorescence Detection of Single Atoms. Pis'ma v ZhETF, 26 , 492 (1977); ZhETF, 77 , 2221 (1979); (b) Multiphotoelectron Fluorescence Selective Detection of Single Atoms by Laser Radiation. Appl. Phys. 22 , 245 (1980).
  12. (a) V.S. Letokhov, V.G. Minogin and B.D. Pavlik. Cooling and Trapping of Atoms and Molecules by Resonant Laser-Field. Optics Comm. 19 , 72 (1976); ÆÝÒÔ , 72 1328 (1977); (b) V.S. Letokhov and B.D. Pavlik. Spectral Line n arrowing in g as by a toms t rapped in a standing light wave. Appl. Phys. 9 , 229 (1976).
  13. (a) V.I. Balykin, V.S. Letokhov, V.I. Mishin. Cooling of N à Atoms by Resonant Laser Radiation. Pis'ma v ZhETF, 29 , 614 (1979); ZhETF, 78 1376 (1980); (b) S.V. Andreev, V.I. Balykin, V.S. Letokhov, V.G. Minogin. Radiative Deceleration to 1.5 Ê and Monochromatization of a Beam of Na Atoms by a Counter-Propagating Laser Beam. Pis'ma v ZhETF, 34 , 463 (1981); ZhETF, 82 , 1429 (1982).
  14. V.I. Balykin et al . Radiative Collimation of an Atomic Beam by Way of Two-Dimensional Cooling by Laser Radiation. Pis'ma v ZhETF 40 , 251 (1984); JOSA, B2 , 1776 (1985).
  15. V.I. Balykin et al. Focusing of an Atomic Beam by the Dissipative Light Pressure Force of Laser Radiation. Pis'ma v ZhETF ÆÝÒÔ 43 , 172 (1986); V.I. Balykin et al. Deep Focusing of an Atomic Beam. Pis'ma v ZhETF 59 , 219 (1994).
  16. ( à ) V.I. Balykin et al . Channeling of Atoms in a Standing Spherical Wave. Opt. Letters 13 , 958 (1988); (b) V.I. Balykin et al. Reflection of a Light Beam from a Light-Field Gradient. Pis'ma v ZhETF, 45 , 282 (1987); Phys. Rev. Lett. 60 , 2137 (1988);
  17. V.I. Balykin, V.S. Letokhov. Phys. Today 4 , 23 (1989).
  18. V.I. Balykin, V.S. Letokhov. “ Atom Optics with Laser Light” , Laser Science and Technology, Vol. 18, Harwood Academic Publ. , Australia et al . (1985).
  19. V.I. Balykin. Advances in Atomic, Molecular and Optical Physics 41 , 181 (1999).
  20. V.I. Balykin, V.G. Minogin, V.S. Letokhov. Rep. Prog. Phys. 63 , 1429 (2000).
  21. V. Minogin et al . Phys. Reports (to be published).
  22. A.N. Zherikhin et al. On Amplification in the Far Vacuum Ultraviolet Region on Transitions in Multiply Charged Ions. Kvantovaya Elektronika 3 , 152 (1976).
  23. ( à ) V.S. Letokhov. The Use of Laser Radiation in Autoelectron and Autoion Microscopy for Observation of Biomolecules. Kvantovaya Elektronika 2 , 930 (1975); (b) V.N. Konopsky, S.K. Sekatskii, V.S. Letokhov. Laser Resonance Photoelectron Microscopy with a Subwavelength-High Spatial Resolution: the First Observation of Single Color Centers of Surface. Optics Comm. 13 , 251 (1966).
  24. S.K. Sekatskii, V.S. Letokhov. Scanning Optical Microscopy with a Nanometer-High Spatial Resolution Based on Resonant Excitation of Fluorescence from a Single Excited Center. Pis'ma v ZhETF 63 , 311 (1966).
  25. V.S. Letokhov, S.K. Sekatskii. On the Possibility of Ultra-High Density WROM Based on the Use of Laser and Corpuscular Beams. Optics Comm. 147 , N1-3, 19-25 (1998).
  26. D.A. Lapshin, V.N. Reshetov, S.K. Sekatskii, V.S. Letokhov. Contact Scanning Optical Near-Field Microscopy. Pis'ma v ZhETF 67 , N4, 245–250 (1998).
  27. S.K. Sekatskii, G.T. Shubeita, M. Chergui, G. Dietler, B.N. Mironov, D.A. Lapshin, V.S. Letokhov. Towards the Fluorescence Resonance Energy Transfer (FRET) Scanning Near-Field Optical Microscopy: Investigation of Nanolocal FRET Processes and FRET Probe Microscope. Zh. Eksp. Teor. Fiz. 117 , 885-894 (2000); J. Exp. Teor. Phys. 90 , 769-777 (2000).
  28. G.T. Shubeita, S.K. Sekatskii, G. Dietler, V.S. Letokhov. Local Fluorescent Probes for the Fluorescence Resonance Energy Transfer Scanning Near-Field Optical Microscopy. Appl. Phys. Lett. 80 , 2625-2627 (2002).
  29. V.V. Klimov, V.S. Letokhov. Quadrupole Radiation of an Atom in the Vicinity of a Dielectric Microsphere. Phys. Rev. A54 , 4408 (1996).
  30. V.V. Klimov, V.S. Letokhov. Resonance Energy Exchange at a Nanoscale Curved Interface. Chem. Phys. Lett. 285 , 313-320 (1998).
  31. V.V. Klimov, M. Ducloy, V.S. Letokhov. A Model of an Apertureless Near-Field Scanning Microscope with a Prolate Nanospheroid as a Tip and Excited Molecule as an Object. Chem. Phys. Lett. 358 , 192–198 (2002).
  32. S. Johansson, V.S. Letokhov. Mysterious UV Lines of FeII from h Car Blobs. In: Eta Carinae and Other Mysterious Stars. T. Gull, S. Johansson, K. Davidson (Eds.), ASP Conf. Series 242 , 297-308 (2001).
  33. S. Johansson and V.S. Letokhov. Laser Action in Space: 1 μm Lines of FeII in Gaseous Condensations in the Vicinity of η Car. Pis'ma v ZhETF 75 , 591–594 (2002);
  34. S. Johansson and V.S. Letokhov. Radiative Cycling of FeII with Stimulated Emission from Atoms and Ions in an Astrophysical Plasma. Phys. Rev. Lett. 90 , No. 1, 011101-1-4 (2003).
  35. V.V. Klimov, V.S. Letokhov. Opt. Commun. 121 , 130 (1995).
  36. V.I. Balykin, V.V. Klimov, V.S. Letokhov. Pis'ma v ZhETF 59 , 219 (1994).
  37. V.I. Balykin, V.S. Letokhov, V.V. Klimov. Pis'ma v ZhETF 78 ( 1 ), 11–15 (2003); [JETP Lett. 78 ( 01 ), 8–12 (2003)].

    38.