P. Elyutin

Second-harmonic generation in metal and semiconductor low-dimensional structures

Abstract In this article previous and recent results of our nonlinear optical studies of low-dimensional structures are surveyed. Size effect in the optical second-harmonic generation (SHG) from Ag nanocrystals in island films and CdSe quantum dots in glass matrices as well as thermo- and DC-electric-field-induced effects in SHG from SiSiO2 multiple quantum wells (MQWs) on Si substrate are studied experimentally and theoretically. Essential (by several orders of magnitude) enhancement of quadratic optical response upon decrease in the particle size is observed for both Ag nanocrystals and CdSe quantum dots. Regular oscillations in the second-harmonic intensity measured as functions of temperature and applied DC electric field are observed for MQWs. It is shown that the size effect for metal nanocrystals can result from the fluctuations of the particle size breaking the local inversion symmetry. Another mechanism studied (based on concepts of dynamic chaos) describes the observed size effects for both metal and semiconductor particles. The dependence of the second-harmonic intensity on the SiO2 layer thickness in MQWs is described with taking into account the retardation of the second-harmonic radiation in MQWs, whereas the thermoinduced effect is interpreted as resulting from the optical interference in the MQWs substrate having thickness dependent on temperature (due to the thermal expansion). A possible approach to explaining the observed electroinduced effect in MQWs is also discussed.

Three-photon squeezing: exploding solutions and possible experiments

Abstract In the frame of quantum optics the properties of a parametric amplifier of the third subharmonic, described by the Hamiltonian H 3 = i κ(a +3 −a 3 ) 3 , are considered. The average output energy turns into infinity (explodes) at certain critical values of para meters even in the absence of feedback. The photon number distribution stays smooth at the explosion moment. The processes of squeezing of the quantum noise and of the photon bunching are discussed. The value of the parameter κ is estimated for a travelling wave amplifier.

The new mechanism of surface-enhanced second harmonic generation in small metallic particles

Abstract A surface enhancement factor growth over 4 orders of magnitude is observed for the second harmonic generation from metallic particles of 10 A radius. The effect can be explained by a specific mechanism, originating from the irregular distortions of the shapes of the particles.

Interaction of a single-photon wave packet with an excited atom

The interaction of a single-photon wave packet with an initially excited two-level atom in free space is studied in semiclassical and quantum approaches. It is shown that the final state of the field does not contain doubly occupied modes. The process of the atoms transition to the ground state may be accelerated, decelerated or even reversed by the incoming photon, depending on parameters. The spectrum of emitted radiation is close to the sum of the spectrum of the incoming single-photon wave packet and the natural line shape, with small and complicated deviations.

The partition functions and thermodynamic properties of small clusters of rare gas atoms

The partition functions Z(T) for the clusters Arn, Krn, and Xen (n=2, 3, and 4) were calculated with the smoothed density of energy levels ρ(E). The latter was determined in the semiclassical approximation by Monte Carlo integration over the phase space and corrected by the rotational asymptotic for the lowest levels and by the trajectory separation method for the bound states above the dissociation threshold. In the temperature range of 5<T<150 K that is of crucial interest for the cluster formation studies in the supersonic jets, the results have an estimated accuracy of about 5%. The structure of the phase space of tetramer (n=4) clusters and their conformational transition dynamics were studied. The possibility of a link between such transitions and clusters melting is discussed.

A percolational criterion for the Anderson transition in structurally disordered systems

The condition for the Anderson transition in systems where the atoms are completely randomly distributed in space with concentration c is obtained by assuming the coincidence of the onset of extended states and of the infinite cluster in a special percolation model. This model consists of the random system of sites with the probability of connection between sites depending on the distance between them and on their local surroundings. The thresholds of percolation in this model, determined in the three-dimensional case by considering the dependence of the average numbers of sites connected to a given site on the configuration of unbroken connections and by direct computer simulation are in good agreement. They yield for a parameter M=c-1/3 alpha (where alpha -1 is a characteristic length of the exponentially decreasing transfer integral) the critical value Mc=5.8+or-0.4. In two dimensions the model predicts a transition for Mc=3.9+or-0.2.

On the kinetics of bimolecular quenching of incoherent excitons

The kinetics of bimolecular quenching of incoherent Frenkel excitons is investigated in the one-dimensional case. The equation for the distribution of distances between neighbouring excitons is derived and solved analytically in the limit of large times. It is found that the time dependence of the exciton concentration c has the form c approximately t-12/. The analytical expressions are confirmed by the results of direct computer simulations.

Adiabaticity and random wave propagation

We examine the assumption of adiabaticity that underlies mode-coupling analysis of stochastic phenomena such as roughness and inhomogeneity in passive photonics devices such as couplers, gratings, and simple waveguides. We show that although the adiabaticity condition is formally violated by such phenomena, only the low spatial frequencies actually contribute significantly to such phenomena, thus preserving the validity of the analysis.

Off-resonant suppression of stochasticity in the generalized Pullen-Edmonds system

Abstract The influence of a small difference between the frequencies of the normal modes of a two-dimensional nonlinear oscillator on the onset of stochasticity is studied. The energy half-width of the stochastic layer in the vicinity of the separatrix is calculated using Chirikovs approximation. The nonmonotonic dependence of the half-width on the deviation from resonance and the eventual vanishing of the stochastic layer are found.

Localisation in the Anderson model with correlated site energies

The problem of localisation is considered for the Anderson model with diagonal disorder and where energy values of neighbouring sites are correlated. The percolational criterion of Thouless is used to determine the delocalisation condition. Estimates for the critical disorder are obtained by straightforward calculations for the Cayley tree and with the help of continuous percolation theory for common lattices. It is shown that the condition for the Anderson transition is mainly determined by short-range order in these systems.