Alexander N. Zherikhin

The action of powerful laser radiation on 1-2-3 superconducting thin films and bulk materials

Abstract Laser induced decomposition, evaporation, and ablation processes in 1-2-3 superconductors are experimentally and theoretically studied. A model of laser beam action on 1-2-3 superconductors is developed. This model explains the low-energy mechanism of ablation, the processes of the high temperature superconductor film modification under laser action and the origination of the drops on the film surface deposited by a laser ablation technique. The model is in good agreement with experimental results.

Picosecond nonlinear spectroscopy of quantum‐size PbTe films

We propose a new and promising technique to study quantum‐size structures. In ultrathin monocrystalline PbTe films with thicknesses L=6, 18, and 30 nm, we have examined this technique, based on two‐photon optical excitation of the renormalized electronic structure and revealed a sharp dependence of frequencies of two‐photon resonant transitions on L. We explained this result within the framework of a model taking into account the real PbTe band structure, electron–electron and electron–phonon interactions, and interband and intra band redistribution of free carriers. We have estimated the characteristic time of interband polarization decay as T2≥300 fs and have analyzed the transformation of the nonlinear response for the case L→∞.

Nonlinear spectroscopy of Y-Ba-Cu-O nonequilibrium states excited by picosecond optical pumping

Abstract Nonequilibrium states of HTSC Y-Ba-Cu-O films with a thickness of 300 nm at the temperatures Θ =80–85 K below the phase transition point Θ c ≅91 K have been investigated by a biharmonic pumping (BP) technique. It has been found that as a result of optical excitation by picosecond laser pulses with the energy density of 10 −4 -10 −3 J/cm 2 at the wavelengths of 1.064, 0.532 and 0.620 μm, thermodynamic equilibrium is attained only after a time delay of more than 1 ns. Both for the above-specified case and for the case with the equilibrium superconductive phase at Θ c , we revealed the same specific dip in the self-diffraction efficiency as a function of the BP frequency detuning Ω in the spectral range - 10 cm −1 > Ω > − 100 cm −1 . This dip conservation has been interpreted as a manifestation of a rather slow phase transition being responsible for the pairing of carriers.

Nonuniform Bragg gratings for DWDM applications

The advent of the Dense Wavelength Division Multiplexing (DWDM) technology in Optical Fiber Networks (OFNs) has resulted in the necessity of developing advanced Optical Add/Drop Multiplexers (OADMs) on the basis of submicron Bragg gratings. The OADMs for dense multichannel OFNs with bit rates 10-40 Gbits/s per channel and channel spacing 200, 100 and 50 GHz must possess rectangular-shaped reflection/transmission spectra and linear phase characteristic within the stop/passband. These features can not be achieved using uniform Bragg gratings and therefore nonuniform gratings with space-modulated coupling coefficient and/or phase shifts π should be used. We present the recent advances in the design and fabrication of narrowband wavelength-selective filters for DWDM applications using submicron nonuniform Bragg gratings. The peculiarities of propagation, interaction and diffraction of electromagnetic waves in nonuniform grating structures are considered. Narrowband reflection filters using single-mode quartz fibers with side-polishing and relief gratings on dielectric and polymeric materials are designed and fabricated. The filters have nearly rectangular shape of the stopband with 0.4-1.6 nm width and peak reflectivity R > 99% in the 1.55 μm wavelength region. The architecture of multichannel OADMs for high-speed OFNs using planar polymer Integrated Photonic Circuits (IPCs) with biharmonic Bragg gratings, possessing phase shifts π, is discussed.

Ultrafast Thermalization Processes in PbTe Quantum-Size Films

The electronic structure and the relaxation rates for thin layers are not the same as for the bulk samples due to the quantum-size effect (QSE)1. The QSE results in quantitative (renormalization of bands) and qualitative (splitting of 3D bands into series of 2D subbands) changes of the electronic structure. Similar effects have been found in superlattices as well as in quantum wires and quantum dots2. For today, optical spectroscopy methods have been proved to be a powerful tool in experimental studying of the QSE in wide-band semiconductors. However, in narrow-band semiconductors, one needs to use lasers of the middle- or far-IR ranges. This problem can be solved by means of nonlinear spectroscopy techniques based on two-photon excitation of electronic transitions, for example, biharmonic pumping (BP)3. Using BP, one can obtain detailed information about spectra of states of electron and phonon subsystems, constants of electron-electron (e-e) and electron-phonon (e-p) interactions, relaxation times, etc. In BP, due to e-e and e-p interactions, two electromagnetic waves with different frequencies ω1, 2 and wave vectors k 1, 2 result in electron and phonon excitations at frequency Δ=ω1-ω2 with wave vector Δk=k 1-k 2. Scattering of the same waves on such a transient grating leads to generation of two new waves with frequencies ω4=2ω1, 2-ω2, 1 and wave vectors k 4=2k 1, 2-k 2, 1 It is this process — the selfdiffraction — that enables one to investigate low-frequency transitions using tunable lasers in the visible range.

Picosecond four-photon spectroscopy of kinetics of nonequilibrium state of HTSC

We propose new and promising procedure for nonlinear spectroscopy of high-Tc superconductive thin films. It enables us to determine the phase transition temperature, superconducting energy gap width, interaction constants, etc. For Y-Ba-Cu-O thin films, we present our results in testing of this procedure including investigation of relaxation kinetics after picosecond optical excitation of the film.

Laser deposition of GaAs thin films

The GaAs thin films were obtained by laser ablation technique. GaAs target and KrF excimer laser radiation ((lambda) equals 248 nm, E equals 0.3 J) were used. We excluded the drops, generated during the target sputtering, due to using special equipment. The deposition temperature was varied from 150 up to 450 degree(s)C. The x-ray analysis have showed that films have a monocrystalline structure at some temperatures. The interface thickness was tested by Auger method. These measurements showed that interface thickness is less than 5 nm. Using this laser ablation technique we have produced the p-n junction.

Four-photon picosecond spectroscopy of phase transition in high-Tc superconductive thin films

An effective procedure for nonlinear spectroscopy of high-Tc superconductive thin films is proposed. It enables to determine as their phase, as critical temperature, superconducting energy gap width, constants of interaction, etc. Results of its testing for Y-Ba-Cu-O thin films are presented, including results of investigation of relaxation kinetics of preliminary picosecond optical excitation.

Ultraviolet laser deposition of doped PbTe thin films

The possibilities and advantages of UV-laser ablation technique for deposition of doped PbTe thin films are discussed. To understand the effects of experimental parameters on layer formation the stages of laser sputtering of the target and film growth were investigated. Method of laser ionization RETOF mass spectrometry was used to obtain the energy distribution and chemical nature of evaporated particles as a function of wavelength and laser radiation power. The influence of experimental parameters on the deposited film thickness, stoichiometry and crystalline perfection was checked. The dependence of doped PbTe films photoelectrical properties on the experimental parameters.

Influence of the velocity distribution of the particles on the laser deposition of the high-temperature superconducting thin films

In this work, we investigate the velocity distribution function of neutral yttrium atoms which appeared during the laser evaporation of different targets (pure metallic yttrium, yttrium oxide Y2O3, and high Tc ceramics YBa2Cu3O7-x). We also investigate the influence of buffer gas and external electric field on the distribution function.