Alexander A. Kaplyanskii

  
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1995

A.A.Kaplyanskii,  S.A.Basun,  S.P.Feofilov.
New spectroscopic effects of ferroelectric phase transition in Li2Ge7O15 crystals doped with 3d3-ions.
Radiation Effects and Defects in Solids 135 (1-4), 567-570 (1995).
In Li2Ge7O15(LGO) crystals doped with Cr3+ and Mn4+ ions the transformation of luminescence spectra
in the region of R-lines at ferroelectric phase transition D-2h --> C-2v in LGO was used to determine the
point group symmetry and microstructure of 3d(3)-centers. In experiments on pseudo-Stark splitting of
R-lines in external electric field, the phase transition induced polarization of 3d(3)-ions was observed, the
induced dipole moments being directed perpendicular to ferroelectric axis of C-2v phase.
Feofilov SP, Kaplyanskii AA, Zakharchenya RI
Optical generation of nonequilibrium terahertz resonant vibrational excitations in highly porous aluminium oxide
Journal of Luminescence 66-67 (1-6), 349-357 (1995).
The dynamics of terahertz phonons generated at liquid helium temperature by laser excitation in sol-gel
technology grown transparent highly porous crystalline gamma-Al2O3 samples, was studied through the
phonon-induced anti-Stokes FLN spectra of Cr3+ impurity ions ((2)E-(4)A(2) lines). The generated phonon
spectrum contains essentially a nonequilibrium narrow peak around omega approximate to 0.6 THz (20
cm(-1)) with high occupation numbers (n) over bar(omega) and decay time similar to 1 ms besides and a
slowly cooling (similar to tens of ms) thermalized Planckian distribution. The frequency (20 cm(-1)) of
these long-living nonthermalized vibrations is consistent with the lowest acoustic, size-resonant surface
mode of gamma-Al2O3 spherical nanoparticles with 80 Angstrom diameter, which form the network of the
sample. Discussed are the nature of the long lifetime and the possible role of these surface modes in the
spectral dynamics of optically generated phonons.
Basun SA, Bianchi U, Bursian VE, Kaplyanskii AA, Kleemann W, Sochava LS, Vikhnin VS
Charge transfer processes and dynamic equilibrium of electronic state occupancies in optically excited SrTiO3:Cr crystals
Journal of Luminescence 66-67 (1-6), 526-530 (1995).
Spectral and kinetic measurements of (2)E --> (4)A(2) photoluminescence (PL) in SrTiO3 crystals
containing small amounts of Cr were performed and a strong effect of IR illumination on the PL was
observed. The dominant photoionization mechanism of Cr-3 + PL excitation was established. The locations
in the SrTiO3 band gap of Cr3+ donor levels (1.85 eV below the bottom of the conduction band) and of
Cr4+ acceptor levels (1.4 eV above the top of the valence band) were found. A new mechanism of optical
erasing of burnt spectral holes of the R lines in SrTiO3:Cr was determined.

Optical spectroscopy of opal matrices with CdS embedded in its pores: Quantum confinement and
                                photonic band gap effects.
 Astratov VN, Bogomolov VN, Kaplyanskii AA, Prokofiev AV, Samoilovich LA, Samoilovich SM, Vlasov
                                          YA
    NUOVO CIMENTO DELLA SOCIETA ITALIANA DI FISICA D-CONDENSED MATTER
     ATOMIC MOLECULAR AND CHEMICAL PHYSICS FLUIDS PLASMAS BIOPHYSICS
                           17 (11-12): 1349-1354 NOV-DEC 1995
The spectra of transmission and reflection of synthetic opal which has 3-dimensional periodic structure
were measured at different orientations of incident beam relative to the sample facets. It is shown that opal
behaves as ''semi-metallic'' photonic band gap (PBG) material in the vicinity of photon energy 2.3 eV. The
synthesis of CdS microcrystals embedded in the pores of opal was made for the first time in an attempt to
form a system of quantum dots. Optical spectra (reflection and transmission, photoluminescence and Raman
scattering) were studied. The results demonstrate good crystallinity of microcrystals embedded in opal
matrix and exhibit well-pronounced quantum confinement effects in fundamental edge absorption spectra.
The spectral overlap of the PBG of opal with electronic band gap of many of II-VI semiconductors seems to
make opal/semiconductor system a promising media for experimental studies of such PBG-related effects as
inhibition of spontaneous emission, microcavity polariton, etc.

Optical study of long-lived size quantized vibrational excitations in porous aluminum oxide
                        Kaplyanskii AA, Feofilov SP, Zakharchenya RI
                             OPTIKA I SPEKTROSKOPIYA
                                79 (5): 709-717 NOV 1995

PHONON HEATING OF 2-DIMENSIONAL EXCITON GASES IN GAAS/ALGAAS
                                  QUANTUM-WELLS
  MOSKALENKO ES, ZHMODIKOV AL, AKIMOV AV, KAPLYANSKII AA, CHALLIS LJ, CHENG T,
                                      HUGHES OH
                                ANNALEN DER PHYSIK
                                   4 (2): 127-135 1995
We report the first measurements of the interaction of non-equilibrium phonons with two-dimensional
exciton gases (2 DExGs). The rise in the effective temperature of the 2 DExG produced by the phonons
depends on the width of the quantum well and the exciton sheet density and hence on the ratio tau-1
(ex-ph)/tau-1 (ex-ex). The dependence of the effective temperature rise on this ratio is attributed to the
non-equilibrium frequency distribution of the phonons incident on the 2 DExG.
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1994

DIPOLE CENTERS IN LI2GE7O15 CRYSTALS ACTIVATED BY (3D)(3)-IONS -
     MICROSTRUCTURE AND SPECTROSCOPIC EFFECTS OF INNER AND EXTERNAL
                                 ELECTRICAL FIELDS
                       BASUN SA, KAPLYANSKII AA, FEOFILOV SP
                               FIZIKA TVERDOGO TELA
                               36 (11): 3429-3449 NOV 1994

HEATING OF 2-DIMENSIONAL EXCITON GAS GAAS/ALGAAS QUANTUM HOLES BY
                             NONEQUILIBRIUM PHONONS
  MOSKALENKO ES, AKIMOV AV, ZHMODIKOV AL, KAPLYANSKII AA, CHENG T, HUGHES O,
                                      CHALLIS LD
                               FIZIKA TVERDOGO TELA
                               36 (10): 3140-3150 OCT 1994

VIRTUAL PHASE-TRANSITION IN HG2I2 CRYSTALS
                  KAPLYANSKII AA, KNORR K, MARKOV YF, TURAEV AS
                               FIZIKA TVERDOGO TELA
                                36 (9): 2744-2751 SEP 1994

PHOTOELECTRIC PROPERTIES OF LI2GE7O15CR CRYSTALS AND CURRENT-PUMP
      INTENSITY CHARACTERISTIC OF STEADY-STATE PHOTOCURRENT IN DOPED
                                    DIELECTRICS
       BASUN SA, KAPLYANSKII AA, FEOFILOV SP, MELNIKOV OK, SEVASTYANOV BK
                               FIZIKA TVERDOGO TELA
                               36 (5): 1451-1460 MAY 1994

OPTICAL-DETECTION OF FIR GENERATED MONOCHROMATIC PHONONS IN CR3+
                                DOPED BORATE GLASS
                  FEOFILOV SP, FLINN GP, DENNIS WM, KAPLYANSKII AA
                            JOURNAL OF LUMINESCENCE
                                 60-1: 165-169 APR 1994
Anti-Stokes emission following optical excitation within the inhomogeneously broadened R lines (2E
double-line arrow pointing left and right 4A2) of Cr3+ doped borate glass has previously been used as a
detector of high frequency non-equilibrium acoustic phonons. We report the optical detection by anti-Stokes
emission of similar phonon populations generated by the absorption of far infra-red radiation (FIR) at 43
cm-1 in this same material at 1.8 and 8.5 K. This technique for the generation of phonons is similar to that of
defect-induced one phonon absorption (DIOPA) as previously used in crystalline materials. Here, the
disordered structure eliminates the necessity of a defect to activate the FIR absorption. These results are
discussed in relation to the general properties of phonons in glassy materials.

PHOTOIONIZATION, TRAPPING AND RECOMBINATION PROCESSES IN
                    OPTICALLY-EXCITED LI2GE7O16-CR CRYSTALS
                       BASUN SA, KAPLYANSKII AA, FEOFILOV SP
                            JOURNAL OF LUMINESCENCE
                                58 (1-6): 281-283 JAN 1994
In chromium doped Li2Ge7O15 (LGO) crystals the stationary photocurrent dependence on the visible
optical pumping intensity and the effect of additional IR illumination on the photocurrent drastically depend
on the content of Cr ions in different charge states. A model of photoionization, trapping and recombination
processes in optically excited LGO:Cr crystals is proposed.
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1993

IN MEMORY OF KORNFELD,M.I. (1908-1993)
   ALFEROV ZI, ALEKSANDROV AP, ALEKSANDROV AD, ALEKSANDROV EB, ZHURKOV SN,
 ZAKHARCHENYA BP, KAPLYANSKII AA, OSIPYAN YA, ABAEV MI, GRIGOREVA NP, KOROVIN
                  LI, LEMANOV VV, MIRLIN DN, RESHINA II, SOCHAVA LS
                               FIZIKA TVERDOGO TELA
                               35 (8): 2285-2287 AUG 1993

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1992

FERROELECTRIC PHASE TRANSITION-INDUCED PSEUDO-STARK SPLITTING IN
                              LI2GE7O15CR3+ CRYSTALS
                       BASUN SA, KAPLYANSKII AA, FEOFILOV SP
                               FIZIKA TVERDOGO TELA
                               34 (11): 3377-3386 NOV 1992

PSEUDOSTARK SPLITTING INDUCED BY A PHASE-TRANSITION IN THE
      OPTICAL-SPECTRA OF FERROELECTRICS WITH DIPOLE IMPURITY CENTERS
                       BASUN SA, KAPLYANSKII AA, FEOFILOV SP
                                    JETP LETTERS
                                56 (1): 50-55 JUL 10 1992
A splitting of R lines has been observed in the course of the D2h --> C2v phase transition in
Li2Ge7O15:Cr3+ crystals. This splitting indicates that the ensemble of Cr3+ centers of the paraelectric
phase splits into two physically nonequivalent ensembles in the course of the transition to the ferroelectric
phase. This effect stems from the triclinic local symmetry of the Cr3+ centers, which have a dipole moment.
The splitting of the R lines is a "pseudo-Stark" splitting in the odd internal crystal field induced in the
ferroelectric phase by a polar order parameter.

OPTICAL STUDIES OF HIGH-FREQUENCY NONEQUILIBRIUM PHONONS IN
                              NONCRYSTALLINE SOLIDS
    KAPLYANSKII AA, AKIMOV AV, BASUN SA, FEOFILOV SP, MOSKALENKO ES, KOCKA J,
                                      STUCHLIK J
                            JOURNAL OF LUMINESCENCE
                                 53 (1-6): 7-14 JUL 1992
The first results of optical (fluorescent) technique application to the study of nonequilibrium terahertz range
Phonons in noncrystalline solids are reported. 1) The Propagation of heat pulses through thin amorphous
semiconductor films was studied by phonon detection via phonon-induced effects in exciton luminescence of
a crystalline c-Si substrate. The effective frequency down-conversion of THz phonons in a-Si:H films was
observed depending on H content. In a-Ge films a time delay in THz phonon pulses propagation was
detected which is due to multiple phonon scattering in the films. 2) In Cr3+-doped bulk oxide glasses the
relaxation of optically generated THz phonons was studied by observation of phonon-induced anti-Stokes
FLN R-spectra. It was found that fully trapped phonon distribution in the excited glass volume does not
thermalize even in a millisecond time scale. The possible role of configurational "softness" and TLS in the
observed specific behaviour of THz phonons in amorphous and glassy materials is discussed.

DYNAMIC ELECTRON AND PHONON PROCESSES IN THE EXCITED-STATE OF LGO-CR-3+
                                      CRYSTALS
 BASUN SA, FEOFILOV SP, KAPLYANSKII AA, BYKOV AB, SEVASTYANOV BK, SHARONOV MY
                            JOURNAL OF LUMINESCENCE
                                 53 (1-6): 24-27 JUL 1992
In pseudo-Stark effect studies on the R, R' lines of LGO:Cr3+ the C1 point symmetry of both types of Cr3+
centers in the low-temperature ferroelectric (C2v) phase as well as evidence for local compensation of
Cr3+ (Ge4+) extra charge in LGO lattices were established. In direct photoelectric studies it was found that
Cr3+ photoionization takes place in a two-quantum process via the intermediate 2E excited state. With the
help of fluorescent phonon detection the diffusive regime of THz acoustic phonon propagation in LGO at T
= 2 K was observed; the anharmonic phonon decay times were measured for frequencies 55 cm-1 (150 ns)
and 63 cm-1 (75 ns).

PHOTOELECTRIC STUDIES OF 2-STEP PHOTOIONIZATION OF TI-3+ IONS IN OXIDE
                                      CRYSTALS
      BASUN SA, FEOFILOV SP, KAPLYANSKII AA, SEVASTYANOV BK, SHARONOV MY,
                                    STAROSTINA LS
                            JOURNAL OF LUMINESCENCE
                                 53 (1-6): 28-31 JUL 1992
In photoconductivity studies in Al2O3:Ti3+ and Y3Al5O12:Ti3+ (YAG:Ti3+) a nearly quadratic
dependence of Photocurrent on excitation intensity is experimentally observed. Spectral and kinetics
measurements show that the Ti3+ Photoionization under visible and UV excitation in the t --> e transition
region takes place as a result of a two-step process via the intermediate e state. The experimentally
estimated energy between this e level of Ti3+ and the conduction band bottom is in Al2O3 larger and in
YAG sma er than the zero-phonon e-t line (ZPL) energy. This determines the essential difference in
persistent hole-burning in the ZPL of Al2O3:Ti3+ and YAG:Ti3+ [1] and in the efficiency of e level optical
pumping.

NONEQUILIBRIUM PHONONS IN SINGLE-CRYSTAL RUBY FIBERS
                   BASUN SA, FEOFILOV SP, KAPLYANSKII AA, YEN WM
                            JOURNAL OF LUMINESCENCE
                                 53 (1-6): 32-34 JUL 1992
The dynamical behavior of 29 cm-1 phonons in single crystal fibers of Al2O2:Cr3+ (ruby) at 2 K has been
investigated. The fiber diameters are of the order of the phonon mean free path and transport is dominated
by surface reflections and scattering and by the acoustic mismatch at the crystal interfaces.
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1991

THE SCATTERING OF TERAHERTZ PHONONS IN AMORPHOUS-SILICON AND
                                     GERMANIUM
          AKIMOV AV, KAPLYANSKII AA, KOCKA I, MOSKALENKO ES, STUCHLIK I
               ZHURNAL EKSPERIMENTALNOI I TEORETICHESKOI FIZIKI
                               100 (4): 1340-1352 OCT 1991
The transmission of terahertz phonons through thin amorphous films a-Si:H and a-Ge on the crystal c-Si
backing at T = 1, 7 K is studied experimentally by means of non-equilibrium phonon optical detection, using
the phonon effect on the excitonic luminescence c-Si. It is found that the effective high frequency-to-low
frequency phonon conversion takes place owing to inelastic phonon scattering which the probability
increases with the growth of the hydrogen content. The strong phonon scattering appears also in observing
the time delay of a phonon pulse with the phonon transmission through both a-Ge and a-Si:H films. The
nature of the elastic and inelastic terahertz phonon scattering in a-Si:H and a-Ge, the possible role of
two-level systems, and soft potentials caused by the presence of impurity H atoms in a-Si:H are discussed.

TRANSPORT AND DYNAMICS OF NONEQUILIBRIUM PHONONS IN SINGLE-CRYSTAL
                                     RUBY FIBERS
                   BASUN SA, FEOFILOV SP, KAPLYANSKII AA, YEN WM
                             PHYSICAL REVIEW LETTERS
                             67 (22): 3110-3112 NOV 25 1991
The transport of 29-cm-1 phonons in single-crystal fibers of Al2O3:Cr3+ (ruby) has been investigated at 2
K. The fiber diameters are of the order of the mean free path, so that transport is dominated by diffuse
scattering and by acoustic mismatch at the lateral crystal interfaces. Depending on experimental conditions,
we have observed ballistic and diffusive phonon transport. We determine for the first time the probability of
frequency nonconserving surface scattering for high-frequency phonons.

PHONON WIND ON EXCITONS IN SILICON
                    AKIMOV AV, KAPLYANSKII AA, MOSKALENKO ES
                                      PHYSICA B
                               169 (1-4): 382-387 FEB 1991
We observed experimentally the strong effect of nonequilibrium phonons on the intensity of the
luminescence of free and bound excitons, created by optical illumination of a Si surface immersed in liquid
helium (T = 1.8 K). The polarity of the phonon-induced luminescence signal strongly depends on the
perfection of the surface. Two factors were included in the theoretical explanation of this effect: (1) the
phonon-induced dissociation of bound excitons, and (2) the drag of free excitons by phonon flux
(phonon-wind effect) which shifts the exciton cloud to the surface, where exciton recombination is strong.
This effect is used to study spectral features of phonon propagation in Si and a-Si:H films.

ELECTRICAL INSTABILITY AND DYNAMIC PROCESSES IN THE EXCITED-STATE OF RUBY
                                   KAPLYANSKII AA
                           JOURNAL OF LUMINESCENCE
                              48-9: 1-10, Part 1 JAN-FEB 1991
Intense optical excitation of Cr3+ ions in concentrated ruby to the metastable 2E-state results in a
spontaneous transition to a new stationary spatially nonuniform state with polar domains of strong electric
field +/- E(s) (\E(s)\ approximately 5 x 10(5) V/cm). Phenomenologically this electrical instability of
optically excited ruby is due to the peculiar N-shaped field dependence of the photocurrent j(E), with
negative absolute electrical conductivity at E < E(s). The photocurrent is due to photostimulated charge
exchange between Cr3+ ions excited to 2E and Cr4+ and/or Cr2+ ions which serve as charge carriers in
ruby.

Polar site-selective excitation of Cr3+ ions, which participate in charge exchange with the help of resonant
nonradiative 2E-energy transfer from the bulk of the excited Cr3+ ions, plays a decisive role in the
microscopic mechanism of the anomalous photoelectric phenomena in ruby.


FAST RESONANT NONRADIATIVE ENERGY-TRANSFER IN ALEXANDRITE
                       BASUN SA, FEOFILOV SP, KAPLYANSKII AA
                            JOURNAL OF LUMINESCENCE
                            48-9: 166-170, Part 1 JAN-FEB 1991
We measured the R-line fluorescence (2E --> 4A2) under site-selective dye laser excitation via
pseudo-Stark R-line components in a modulated external electric field, and hence the kinetics of 2E-energy
transfer among the Cr3+ ions in mirror (C(s)) sites in alexandrite at T = 2 K were studied. Fast
(microsecond range) resonant nonradiative 2E-energy transfer was observed. This shows that during the
metastable 2E-state lifetime tau-R = 2.4 ms the electronic 2E-excitations can perform many jumps over the
Cr3+ (C(s)) ion ensemble. This results correlates directly with the results of four-wave mixing experiments
[1,2], indicating the existence of long-range spatial diffusion of 2E excitation among the Cr3+ (C(s)) ions.
 
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1980, 1979, 1978, 1977, 1976, 
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