
Alexander A. Kaplyanskii
	home page	*Papers* 2000, 1999, 1998, 1997, 1996, 1995, 1994, 1993, 1992, 1991, 1990, 1989, 1988, 1987, 1986, 1985, 1984, 1983, 1982, 1981, 1980, 1979, 1978, 1977, 1976, 1975, older
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2000

S.A.Basun, S.P.Feofilov, A.A.Kaplyanskii, U.Happek, J.Choi, K.W.Jang, R.S.Meltzer
Photoionization spectral hole burning and its erasure in Li₂Ge₇O₁₅ : Cr³⁺: Effects of site-dependent energy-level shifts within the band gap
Physical Review B 61 (19), 12848-12853 (2000): We report on investigations of the dependence of ionization spectral hole burning efficiencies on the location of impurity ion energy levels relative to both the host valence band and the conduction band. An ideal system is Cr-doped Li2Ge7O15, where the Cr3+ ions occupy three distinct noncentrosymmetric Ge4+ sites, which differ from one another by the location of their charge-compensating ions. Most important for our investigation is the large (similar to eV) and site-specific shift of the Cr3+ energy levels relative to the host valence and conduction bands. As a result, only one of the three sites shows persistent spectral hole burning. It is shown that this result can be understood from an analysis of the differences of the energy-level locations of the ground and excited states of the three Cr3+ centers relative to the host bands. Two of the centers have their energy levels sufficiently close to the host conduction band such that two-step ionization, and thus persistent spectral hole burning, can occur. However, for one of the centers the ground state lies so close to the valence band that an erasure process involving promotion of an electron from the valence band to the ionized center leads to the restoration of the initial charge state. This erasure process, clearly demonstrated in this paper, has to be considered when evaluating the hole burning potential of doped insulators, especially those with relatively narrow band gaps.

Y.A.Vlasov, V.N.Astratov, A.V.Baryshev, A.A.Kaplyanskii, O.Z.Karimov, M.F.Limonov
Manifestation of intrinsic defects in optical properties of self-organized opal photonic crystals
Physical Review E 61 (5), 5784-5793, Part B (2000): Self-organized synthetic opals possessing a face centered cubic (fcc) lattice are promising fur fabrication of a three-dimensional photonic crystal with a full photonic band gap in the visible. The fundamental limiting factor of this method is the large concentration of lattice defects and, especially, planar stacking faults, which are intrinsic to self-assembling growth of colloidal crystal. We have studied the influence of various types of defects on photonic band structure of synthetic opals by means of optical transmission, reflection and diffraction along different crystallographic directions. We found that in carefully chosen samples the stacking probability cu can be as high as 0.8-0.9 revealing the strong preference of fee packing sequence over the hexagonal close-packed (hcp). It is shown that scattering on plane stacking faults located perpendicular to the direction of growth results in a strong anisotropy of diffraction pattern as well as in appearance of a pronounced doublet structure in transmission and reflection spectra taken from the directions other than the direction of growth. This doublet is a direct manifestation of the coexistence of two crystallographic phases-pure fee and strongly faulted. As a result the inhomogeneously broadened stop-bands overlap over a considerable amount of phase space. The latter, however, does not mean the depletion of the photonic density of states since large disordering results in filling of the partial gaps with both localized and extended states.

A.A.Kaplyanskii, S.A.Basun, U.Happek, R.S.Meltzer, M.Raukas.
Mechanisms of optical erasure of ionization spectral holes in doped insulating crystals.
Journal of Luminescence 87-89, 983-985 (2000).: We review recently discovered processes that can lead to the erasure of persistent spectral holes burnt via two-step ionization (photochemical hole burning) in doped insulating crystals. These hole erasure processes are based on photoinduced recharging of ions. Two mechanisms are described here: acceptor transitions, which occur in the system Li2Ge7O15 : Cr3+, and electron tunneling between impurity ions in different charge states, as observed in CaS : Eu.

G.M.Salley, S.A.Basun, A.A.Kaplyanskii, R.S.Meltzer, K.Polgar, U.Happek.
Chromium centers in stoichiometric LiNbO₃.
Journal of Luminescence 87-89, 1133-1135 (2000).: The nature of Cr3+ center in LiNbO3 has been the subject of intense discussions in recent years. Partly, contradictory assignments of the centers resulted from intrinsic defects in the so-called congruent samples. More recently, stoichiometric samples, produced via the VTE process, have become available and show a largely reduced number of Cr sites. In this paper we present new spectroscopic data on a stoichiometric LiNbO3 :Cr3+ Sample produced using the HTTSSG process. This sample shows an even further reduced number of lines in the so-called R-line region. In addition, the study of this sample sheds new light on the origin of near infrared emission lines first discovered in LiNbO3 : Cr : Mg, which might be due to Cr3+-Nb4+ pairs. All emission lines show an unprecedented small line width, which allows high-resolution studies on this fascinating system.


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1999

S.P.Feofilov, A.A.Kaplyanskii, A.B.Kulinkin, A.B.Kutsenko, T.N.Vasilevskaya, R.I.Zakharchenya.
Optical spectroscopy of RE³⁺ ions in sol-gel prepared alpha-Al₂O₃.
Radiation Effects and Defects in Solids 151 (1-4), 131-135 (1999).: The monolithic polycrystalline small-grain corundum alpha-Al₂O₃ doped with trivalent rare earth ions RE³⁺ (Eu³⁺, Er³⁺, Pr³⁺) was produced for the first time using the sol-gel technology. The fluorescence and absorption spectra of these materials were studied which exhibit the sets of narrow lines belonging to f-f transitions between RE³⁺ states. It was shown that Eu³⁺ and Er³⁺ ions form in corundum lattice the single dominating type of regular centers. The energy scheme of Stark sublevels for ground and excited states of dominating Eu³⁺ and Er³⁺ centers in corundum was determined.

M. van der Voort, A.V.Akimov, O.L.Muskens, J.I.Dijkhuis, N.A.Feoktistov, A.A.Kaplyanskii, A.B.Pevtsov.
Phonon dynamics in amorphous and nanocrystalline silicon.
Journal of Luminescence 83-84, 161-165 (1999).: We present the results of experiments on the decay of nonequilibrium phonons created by pulsed laser excitation in an amorphous silicon film (a-Si:H) with and without a high volume fraction of 5 nm crystalline Si clusters (nc-Si). In one type of experiments, 29 cm(-1) phonons are detected in the ruby substrate via phonon-induced luminescence and appear to decay on a much longer time scale (much greater than 100 ns) in the film with nc-Si than in the film without nc-Si (similar to 40 ns). In pump-probe Raman experiments, we observe a marked increase in the decay time for high-energy TA (similar to 150 cm(-1)) phonons in the film with nc-Si. We discuss the results in terms of a model in which phonons propagate in loosely coupled nanoscopic regions in the a-nc-Si material.

J.K.Krebs, S.P.Feofilov, A.A.Kaplyanskii, R.I.Zakharchenya, U.Happek.
Non-radiative relaxation of Yb³⁺ in highly porous gamma-Al₂O₃.
Journal of Luminescence 83-84, 209-213 (1999).: Sol-gel grown aluminum oxide is a highly porous material composed of a network of nano-crystalline gamma-Al2O3 particles. The sol-gel process allows the incorporation of trivalent rare-earth ions into the crystal structure. These ions can be used to study, via optical techniques, the structure and dynamics of the nanoparticles in this material. Here we report on optical studies of Yb3+-doped gamma-Al2O3. We find non-exponential decay curves for the F-2(5/2) to F-2(7/2) Yb3+ fluorescence and a strong temperature dependence of the decay rate between 4 and 300 K. We show that this behavior is not caused by confinement but is indicative of non-radiative relaxation channels. The non-radiative relaxation channel is provided by OH molecules at the surface of the gamma-Al2O3 nanoparticles, demonstrating the importance of surface effects in nanocrystalline materials.

A.A.Kaplyanskii, S.A.Basun, U.Happek, R.S.Meltzer.
Photoionization spectral hole-burning and inversion symmetry of sites in doped insulators.
Journal of Luminescence 83-84, 335-341 (1999).: The linear Stark shift of optical frequencies in impurity centers lacking inversion site symmetry results in various external electric field effects observed in persistent spectral hole-burning (PSHB) phenomena. This paper reviews some new effects of inversionless symmetry of centers in internal electric fields observed in PSHB of doped insulators with photoionization hole burning. (1) In multisite R-line ((4)A(2)-E-2) spectra of Cr-3 (+) ions, substituting Ge4 + in the inversionless position in the crystal lattice of Li2Ge7O15 (LGO), the two-photon self-gated photoionization PSHB was observed in the R-spectra of one site only. This drastic site dependence of PSHB demonstrates extremely strong ( similar to eV) relative shifts of the position within the host energy gap of the (4)A(2), E-2 levels of Cr-3 (+) ions, located at complex centers. These centers differ in the nature and the position of the nearby charge-compensating defect, where the Coulomb field produces different large linear Stark shifts of the electronic states of Cr-3 (+). (2) Photon-gated PSHB in f-d spectra of Eu-2 (+) multisites in CaS crystal was observed with unusually broad hole wings in the case of lines belonging to perturbed inversionless sites. This efficient off-resonant PSHB is due to strong linear Stark shifts of the f-d transition frequencies of Eu-2 (+) ions in time-varying Coulomb fields resulting from photoionization and recharging of Eu-2 (+) -EU3 + ions during the hole-burning process in optically excited crystals.

G.M.Salley, S.A.Basun, G.F.Imbusch, A.A.Kaplyanskii, S.Kapphan, R.S.Meltzer, U.Happek.
Chromium centers in LiNbO₃ revisited.
Journal of Luminescence 83-84, 423-427 (1999).: In this paper we present novel spectroscopic data clarifying the nature of the dominant Cr3+ centers in LiNbO3. In the past, the Cr3+ centers in LiNbO3 have been the subject of intense debate, which was partly caused by the intrinsic Li+ deficit present in the available congruent samples, which leads to highly complex emission spectra. Recently, stoichiometric samples have become available which show a greatly reduced number of emission lines of defect-related high-field sites in the so-called R-line region. We have carried out extensive absorption, emission, resonant photoexcitation, and time-resolved measurements between 2 K and room temperature to unravel the nature of the dominant Cr3+ sites in LiNbO3. From these studies we conclude that the dominant luminescent center is a Cr3+ ion at a low-field site in both stoichiometric and congruent samples.

S.A.Basun, G.M.Salley, A.A.Kaplyanskii, H.G.Gallagher, K.Polgar, L.Lu, U.Happek.
A novel luminescent center in LiNbO₃ : Cr : Mg crystals.
Journal of Luminescence 83-84, 435-439 (1999).: We report on the observation of a novel luminescent center in LiNbO3 : Cr : Mg. This center is characterized by a set of narrow emission lines located in the 770 - 850 nm region which occur in addition to the well-known broad T-4(2) emission of the major low-field Cr3+ site. The set of narrow emission lines can be readily observed in samples containing more than 4% Mg. All lines exhibit the same relaxation time and are strongly rc polarized. Substitution of Mg by Zn produces the same set of lines, indicating that the Mg ions are not directly incorporated in the novel center, but play an essential charge compensating role. Based on our absorption, excitation, luminescence, and fluorescent line-narrowing experiments, we associate the novel center with an exchange coupled Cr-center.

M. van der Voort, G.D.J.Smit, A.V.Akimov, J.I.Dijkhuis, N.A.Feoktistov, A.A.Kaplyanskii, A.B.Pevtsov.
Decay of nonequilibrium phonons in nanocrystalline silicon.
Physica B 263, 473-475 (1999).: We present results of pulsed Raman experiments at 1.8 K on the lifetimes of phonons in silicon nanocrystals embedded in amorphous silicon (a-nc-Si:H). In the spectral region of the TO vibrations in the nanocrystallites, the observed dynamics is different both from that found in a-Si:H and c-Si. The TO-phonon decay appears to become slower with decreasing phonon frequency and crystallite size. That smaller nanoparticles have a slower anharmonic decay may be explained by the effect of phonon confinement. Furthermore, for TA-like vibrations in a-nc-Si:H we measured a much longer decay time (similar to 50 ns) than for the same vibrations in a-Si:H.

S.P.Feofilov, A.A.Kaplyanskii, A.B.Kulinkin, R.I.Zakharchenya.
Optical studies of terahertz phonons dynamics in small-grain polycrystalline corundum.
Physica B 263, 695-697 (1999).: The dynamics of terahertz acoustic phonons generated by optical pumping in ceramic-like alpha-Al2O3 with grain size similar to 100 nm produced with the help of sol-gel technology was studied with the technique of optical detection of phonons by observation of probe Cr3+ and Mn4+ ions fluorescence. The dynamics of phonon distribution is very slow ( similar to ms) and drastically differs from that in regular alpha-Al2O3 ceramics with micron grain size and is similar to that observed earlier in oxide glasses. The results are discussed in the framework of studies of phonons in different structured and spatially restricted Al2O3 materials with different ratios between phonon wavelength lambda and crystallite size a. It is shown that the acoustic mismatch model which describes phonon scattering in regular ceramics is not valid for small-grain sol-gel produced ceramics-like material.


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1998

A.A.Kaplyanskii, A.B.Kulinkin, A.B.Kutsenko, S.P.Feofilov, R.I.Zakharchenya, T.N.Vasilevskaya.
Optical spectra of triply-charged rare-earth ions in polycrystalline corundum.
Physics of the Solid State 40 (8), 1310-1316 (1998).: Solid samples of polycrystalline corundum alpha-Al2O3 activated by triply-charged rare-earth ions RE3+ (R=Eu3+, Er3+, Pr3+) were synthesized by the sol-gel technology. Characteristic narrow-line optical absorption and luminescence spectra produced by intraconfigurational 4f-4f transitions in RE3+ ions have been measured. RE3+ ions have been established to form one dominant type of optical centers in the corundum matrix, and the energy diagram of Eu3+ and Er3+ Stark levels in corundum has been determined.

A.A.Kaplyanskii, S.A.Basun, M.Raukas, U.Happek, J.Rennie, J.C.Vial, W.M.Yen, R.S.Meltzer.
Non-resonant hole burning in CaS : Eu by photoinduced recharging of Eu ions.
Journal of Luminescence 76-77, 292-294 (1998).: Persistent spectral hole burning has been observed on the zero-phonon lines of the main and several perturbed sites of Eu2+ in CaS:Eu single crystals. Hole burning occurs by two-step photoionization, can be strongly gated with IR irradiation, and takes place by excited-slate absorption from the metastable 4f(6)5d excited state to the conduction band followed by electron transport to Eu3+ centers which are the dominant traps. Unusually broad similar to 100 GHz wings were observed in hole spectrum for perturbed Eu2+ sites. This is explained by a new off-resonant hole-burning mechanism due to the linear Stark effect of inversionless Eu2+ sites in time-varying internal Coulomb fields produced by dynamical recharging of Eu ions in the course of hole-burning process.


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1997

V.N.Astratov, Y.A.Vlasov, O.Z.Karimov, A.A.Kaplyanskii, Y.G.Musikhin, N.A.Bert, V.N.Bogomolov, A.V.Prokofiev.
Photonic band structure of 3D ordered silica matrices.
Superlattices and Microstructures 22 (3), 393-397 (1997).: The symmetry and orientation of a photonic lattice of 3D ordered porous silica matrices (synthetic opals) are determined by analysis of transmission electron micrographs. By optical transmission measurements the photonic band edges are mapped out at points L and K of the Brillouin zone. It is shown that modulating the refractive index of opal pores by filling them with liquids results in variation of the light attenuation length within the forbidden gap. To increase the refractive index contrast of the lattice, the pores of opals were filled with CdS. A dramatic decrease in attenuation length is observed for opal/CdS. The parameters of a photonic lattice with a 'pseudo-gap', that is, with a prominent depletion of the photonic density of states, are estimated.

A.V.Akimov, A.A.Kaplyanskii, D.A.Mazurenko, E.S.Moskalenko, A.L.Zhmodikov, L.J.Challis, T.S.Cheng, C.T.Foxon.
Exciton tunnelling induced by nonequilibrium phonons in slightly asymmetric double quantum wells.
physica status solidi (b) 204 (1), 400-403 (1997).: Studies have been made of the effect of nonequilibrium phonons on the population of coupled exciton states in GaAs/(AlGa)As double quantum wells (DQWs) with very similar widths. Comparison of luminescence spectra from the DQWs measured in the presence of nonequilibrium phonons with the spectra obtained in thermal equilibrium shows that the changes in exciton population induced hy phonons could either be due to phonon-assisted tunnelling or to exciton heating.

S.A.Basun, M.Raukas, U.Happek, A.A.Kaplyanskii, J.C.Vial, J.Rennie, W.M.Yen, R.S.Meltzer.
Off-resonant spectral hole burning in CaS:Eu by time-varying Coulomb fields.
Physical Review B 56 (20), 12992-12997 (1997).: Persistent spectral hole burning has been observed on the zero-phonon lines of the main site and several perturbed sites of Eu2+ in CaS:Eu single crystals. Hole burning occurs by two-step photoionization, can be strongly gated with IR irradiation, and takes place by excited-state absorption from the metastable 4f(b)5d excited state to the conduction band followed by electron transport to Eu3+ centers which are the dominant traps. A complex hole structure consisting of a narrow feature (200 MHz), and broader features (approximate to 5 and approximate to 100 GHz) is observed. A mechanism is described for the occurrence of these unusually broad features. Time-varying internal electric fields which occur during the hole burning due to photoionization and trapping can lead to burning of holes at frequencies non-resonant with that of the laser. In addition, a mechanism for hole erasure, tunneling between Eu ions, is demonstrated. This mechanism is identified from the frequency dependence of the hole erasure which follows the Eu2+ absorption, and the linear dependence of the photoconductivity and hole erasure efficiency on irradiation power, both of which indicate erasure in a single-photon process.

A.V.Akimov, E.S.Moskalenko, A.L.Zhmodikov, D.A.Mazurenko, A.A.Kaplyanskii, L.J.Challis, T.S.Cheng, C.T.Foxon.
Luminescence of excitons in slightly asymmetric double quantum wells.
Physics of the Solid State 39 (4), 649-653 (1997).: We report the first studies of exciton luminescence spectra from asymmetric double quantum wells (DQWs) of very similar width. The DQWs were of GaAs/AlGaAs and the differences in widths of the coupled wells were one or two monolayers. The coupled direct and indirect exciton states anticross with a resonance splitting of 1.33 meV. An additional luminescence line appearing at low temperatures is identified as a localized indirect exciton.

S.P.Feofilov, A.A.Kaplyanskii, R.I.Zakharchenya.
Luminescence and laser spectroscopy of highly porous gamma-Al₂O₃ doped with 3d- and 4f-ions: Effects of spatial phonon confinement.
Journal of Luminescence 72-74, 41-42 (1997).: Inhomogeneously broadened luminescence spectra of Cr3+, Eu3+ and Pr3+ ions in sol-gel-grown monolithic highly porous Y-Al2O3 were observed. In FLN and HB experiments, the interaction of ions with the size-resonant terahertz acoustic vibrations of crystalline nanoparticles which form the porous network was studied. The incorporation of Eu3+ and Pr3+ into alpha-Al2O3 (corundum) lattice is demonstrated in annealed (T > 1200 degrees C) sol-gel-prepared samples.

V.N.Astratov, Y.A.Vlasov, V.N.Bogomolov, A.A.Kaplyanskii, O.Z.Karimov, D.A.Kurdjukov, A.V.Prokofiev.
Influence of refractive index contrast on photonic band gap in 3D periodic SiO₂ matrices filled with a semiconductor.
Compound Semiconductors 1996,
Institute of Physics Conference Series (155), 73-76 (1997).: By spectral transmission measurements the optical stop-bands in 3D periodic SiO2 matrices (synthetic opals) are studied as a function of the refractive index contrast. In order to vary the refractive index contrast the pores of opals were filled with various liquids and with semiconductor microcrystals. In the latter case a dramatic decrease of Bragg-defined light attenuation length within the stopband was observed indicating strong perturbation of photonic states. The parameters of photonic lattice of the synthetic opals required to achieve a prominent depletion of the photonic density of states (pseudogap) are estimated.

Y.A.Vlasov, V.N.Astratov, O.Z.Karimov, A.A.Kaplyanskii, V.N.Bogomolov, A.V.Prokofiev.
Existence of a photonic pseudogap for visible light in synthetic opals.
Physical Review B 55 (20), 13357-13360 (1997).: Synthetic opals, composed of submicron silica spheres close packed in a three-dimensional fee lattice, are shown to display photonic stop bands at optical frequencies. We have investigated the light attenuation within the stop band as a function of refractive index contrast. Based on measurements of the Bragg attenuation length and on theoretical considerations, we show that a prominent depletion of the photonic density of states (pseudogap) can be achieved in opals by adjusting the volume packing fraction of the spheres and increasing the refractive index of the pores. To approach the pseudogap criterion the pores of opal were impregnated with CdS nanocrystals. We find a dramatic decrease of the attenuation length in opal-CdS, which indicates the strong perturbation of photonic states.

S.P.Feofilov, A.A.Kaplyanskii, A.B.Kutsenko, T.N.Vasilevskaya, R.I.Zakharchenya.
Sol-gel technology grown monolithic highly porous aluminum oxide with chromium and rare earth ions and size-resonant terahertz acoustic vibrations of nanocrystalline particles.
Proc. of the 13th Internat. Conf. on Defects in Insulating Materials (ICDIM 96),
Materials Science Forum 239-241, 687-690 (1997).: Monolithic highly porous aluminum oxide synthesized by sol-gel technology is built of crystalline gamma-Al2O3 nanometer sized particles. In the present work highly porous gamma-Al2O3 is doped with 3d (Cr3+) and 4f (Eu3+, Pr3+) ions. Observation of narrow-band optical spectra of these ions in gamma-Al2O3 enabled us to exploit the idea of optical probe ions for the studies of general properties of doped porous insulators. In different spectroscopic experiments the lowest-frequency size-quantized acoustic vibrations of nanocrystalline particles were revealed. A very long decay time of these vibrations was observed. In the experiments with thermal treatment of doped gamma-Al2O3 resulting in the transition from gamma-phase to alpha-Al2O3 (corundum) the possibility of incorporation of rare earth ions into the corundum lattice was demonstrated.


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1996

D.A.Mazurenko, A.V.Akimov, E.S.Moskalenko, A.L.Zhmodikov, A.A.Kaplyanskii.
Tunnelling of direct and indirect excitons in slightly asymmetric double quantum wells.
Acta Physica Polonica A 90 (5), 895-898 (1996).: We report the first studies of luminescence spectra from asymmetric double quantum wells of GaAs with widths around 100 Angstrom differing by only one (2.8 Angstrom) or two monolayers. Studies of the position and intensity of the direct and indirect exciton lines suggest the existence of acoustic phonon-assisted tunnelling between exciton states separated by a few meV. At temperatures 10-20 K the electron tunnelling process is fast enough to maintain thermal equilibrium between these direct and indirect excitons which are connected with holes in the same quantum well.

V.N.Astratov, Y.A.Vlasov, O.Z.Karimov, A.A.Kaplyanskii, Y.G.Musikhin, N.A.Bert, V.N.Bogomolov, A.V.Prokofiev.
Photonic band gaps in 3D ordered fcc silica matrices.
Physics Letters 222 (5), 349-353 (1996).: The photonic band structure of 3D ordered silica matrices was studied by optical transmission measurements. A decrease of the light attenuation length within the forbidden gap was observed after filling of the opal pores with CdS microcrystals. The parameters of a photonic lattice required for a depletion of the photonic density of states were estimated.

A.A.Kaplyanskii, M.B.Melnikov, S.P.Feofilov.
Distribution of nonequilibrium tetrahertz acoustic phonons in the dense crystalline ceramics based on alpha-Al₂O₃.
Fizika Tverdogo Tela 38 (5), 1434-1445 (1996).

S.A.Basun, T.Danger, A.A.Kaplyanskii, D.S.McClure, K.Petermann, W.C.Wong.
Optical and photoelectrical studies of charge-transfer processes in YAlO₃:Ti crystals.
Physical Review B 54 (9), 6141-6149 (1996).: The donor and acceptor charge-transfer transitions in YAlO3:Ti3+/Ti4+ were investigated through absorption, emission, excitation, one-photon and two-photon photoconductivities, excited-state absorption, and thermoluminescence. The results are contrasted to similar studies previously made on Al2O3:Ti3+/Ti4+. The dominant Ti3+ population has a photoionization threshold at 37 000 cm(-1). The lowest energy Ti4+ charge transfer transition has its zero-phonon energy near to 33 900 cm(-1). The sum of these two quantities agrees with the reported band-gap energy of YAlO3. The position of the Ti3+/Ti4+ level in the band gap of YAlO3 is determined by these measurements. Stimulated emission from the (2)E State of Ti3+ is limited by photoionization from this state.

S.P.Feofilov, A.A.Kaplyanskii, R.I.Zakharchenya, Y.Sun, K.W.Jang, R.S.Meltzer.
Spectral hole burning in Eu³⁺-doped highly porous gamma-aluminum oxide.
Physical Review B 54 (6), R3690-R3693 (1996).: Transient and persistent spectral holes were burned in the F-7(0)-D-5(0) transition of Eu3+ ions in macromonolithic transparent porous nanocrystalline gamma-Al2O3 produced by sol-gel technology. An unusual temperature dependence of the transient hole width was observed that differs markedly from that in either crystalline or disordered systems. This temperature dependence is explained in terms of Raman processes involving size-resonant vibrations of the nanoparticles comprising the structure. Possible mechanisms of hole burning are suggested.

A.V.Akimov, E.S.Moskalenko, L.J.Challis, A.A.Kaplyanskii.
Interaction of phonons with 2D exciton gas.
Physica B 220, 9-12 (1996).: We give an overview of experiments in which luminescence detection of nonequilibrium phonons in semiconductors is used to study the interaction of acoustic phonons with two-dimensional excitons. Questions which are specific for the 2D exciton-phonon interaction are discussed: momentum conservation in the exciton-phonon interaction; role of exciton-exciton collisions; drag of 2D excitons by phonons; 2D excitons as a phonon spectrometer.

E.S.Moskalenko, A.V.Akimov, A.A.Kaplyanskii, A.L.Zhmodikov, L.J.Challis, T.S.Cheng, C.T.Foxon.
Effect of nonequilibrium acoustic phonons on exciton states in interrupted grown GaAs/Al_0.33Ga_0.67As quantum wells.
Physica B 220, 59-61 (1996).: We present the first studies of the effect of heat pulses on the exciton luminescence from GaAs/Al0.33Ga0.67As quantum wells (QW) prepared using growth interrupts to reduce the interface step density. We have observed the redistribution of excitons between the main thinner part and islands one monolayer thick of the QW induced by the heat pulses. The effect strongly depends on the relative position of heater and exciton gas and is discussed in terms of the drag of free excitons produced by ''phonon wind''.

P.A. van Walree, A.F.M.Arts, H.W. de Wijn, A.A.Kaplyanskii.
Nonequilibrium phonons in Cr³⁺-doped germanate glass.
Physica B 220, 757-759 (1996).: The optically induced phonon distribution in Cr3+-doped germanate glass is investigated after pulsed laser excitation. Via the anti-Stokes spectrum a nonequilibrium phonon distribution is observed, which is characterized by a marked overpopulation at higher frequencies, and persists over several milliseconds.

S.P.Feofilov, A.A.Kaplyanskii, M.B.Melnikov.
Transport of terahertz nonequilibrium acoustic phonons in dense corundum ceramics.
Physica B 220, 773-774 (1996).: The propagation of nonequilibrium terahertz acoustic phonons in dense alpha-Al2O3 ceramics with grain size similar to 10 mu m was studied. Phonons were generated by the heat pulse technique and optical detection of 29 cm(-1) (0.874 THz) phonons via the Cr3+ impurity ions fluorescence was used. The diffusive propagation of phonons from heater to optical detector was observed with mean free path of phonons (l) over bar = 150 mu m which is significantly larger than the grain size. This value is in agreement with the result of computer calculation of (l) over bar taking into account reflection and refraction at interfaces of disoriented alpha-Al2O3 single-crystalline grains in the framework of acoustic mismatch model.


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Alexander A. Kaplyanskii

2000

1999

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1996