A.A. Kaplyanskii

Optical detection of terahertz phonon dynamics in disordered doped insulator systems using a new FLN-based technique

Abstract A new technique for the optical detection of nonequilibrium phonons in disordered solids is accomplished for the case of the strongly disordered crystal MgAl 2 O 4 :Cr 3+ and of Cr 3+ doped glasses with the help of site-selective laser excitation of dopant Cr 3+ ions within the inhomogeneously broadened 4 A 2 - 2 E transition (Δ v ≈150 cm -1 ) at liquid helium temperature. Nonequilibrium phonons, produced either by the heat pulse technique or in the course of electronic relaxation after optical excitation, are detected in the frequency range 5–100 cm -1 with both temporal and frequency resolution using phonon-induced anti-Stokes luminescence from the upper crystal field components of the 2 E-doublets of Cr 3+ ions excited resonantly into the lower 2 E-components at laser frequency v exc . The phonon-induced anti-Stokes luminescence kinetics at frequency v = v exc + ω reflect the kinetics of phonons with frequency ω in the excited volume.

Optical studies of high-frequency nonequilibrium phonons in noncrystalline solids

Abstract The first results of optical (flourescent) 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 film 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 Cr 3+ -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.

Interaction of phonons with 2D exciton gas

Abstract 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. Question 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.

Luminescence study of exciton drag by acoustic phonons in silicon

Abstract We have observed experimentally the strong effect of heat pulses on the intensity of the luminescence of free and bound excitons, created by optical illumination of the 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 counteracting 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 which shifts the exciton cloud to the surface, where exciton recombination is strong. Strong reflection of subterahertz phonons was observed to occur from atomically pure, as-cleaved, surfaces in Si in contact with superfluid helium; whereas, for a ‘conventional’ surface with oxide film, noticeable penetration of phonons into the He was detected.

Energy transfer and anomalous electrical properties of optically excited ruby

Abstract Studies of the spectral response of photocurrent under excitation in R-lines have revealed the mechanism of photocurrent in ruby. It consists in a motion over the Cr 3+ ion system of an “excess” charge (EC) (i.e. the Cr 2+ or Cr 4+ states), the charge transfer occuring under excitation to the 2 E state of Cr 3+ ions which are close to EC. The sign of current is determined essentially by the polar asymmetry in the spatial excitation to 2 E of the Cr 3+ ions around EC and is reversed with the excitation frequency swept across the R-lineshape. When excited in the broad U-band, the asymmetry resulting in the photocurrent flowing against the applied field orginates from resonant nonradiative transfer to EC environment of the 2 E excitations from the bulk of the Cr 3+ ions. The kinetics of this negative photocurrent agrees with known kinetics of nonradiative resonant energy transfer between Cr 3+ ions.

Effect of nonequilibrium acoustic phonons on exciton states in interrupted grown GaAs/Al0.33Ga0.67As quantum wells

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”.

FAST RESONANT NONRADIATIVE ENERGY TRANSFER IN ALEXANDRITE

We measured the R-line fluorescence ( 2 E→ 4 A 2 ) under site-selective dye laser excitation via pseudo-Stark R-line components in a modulated external electric field, and hence the kinetics of 2 E-energy transfer among the Cr 3+ ions in mirror (C s ) sites in alexandrite at T=2K were studied. Fast (microsecond range) resonant nonradiative 2 E-energy transfer was observed. This shows that during the metastable 2 E-state lifetime τ R =2.4ms the electronic 2 E-excitations can perform many jumps over the Cr 3+ (C s ) ion ensemble. This result correlates directly with the results of four-wave mixing experiments [1,2], indicating the existence of long-range spatial diffusion of 2 E excitation among the Cr 3+ (C s ) ions

Transport of terahertz nonequilibrium acoustic phonons in dense corundum ceramics

Abstract The propagation of nonequilibrium terahertz acoustic phonons in dense α-Al2O3 ceramics with grain size ∼ 10 μm 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 ¯ = 150 μm which is significantly larger than the grain size. This value is in agreement with the result of computer calculation of l ¯ taking into account reflection and refraction at interfaces of disoriented α-Al2O3 single-crystalline grains in the framework of acoustic mismatch model.

Nonequilibrium phonons in single crystal ruby fibers

Abstract 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.

Phonon wind on excitons in silicon

Abstract 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.