Optical spectra of a quantum dot in a microcavity in the nonlinear regime

The optical emission spectrum of a quantum dot in strong coupling with the single mode of a microcavity is obtained in the nonlinear regime. We study how exciton-exciton interactions alter the emission spectrum of the system, bringing the linear Rabi doublet into a multiplet structure that is strongly dependent on the cavity-exciton energy detuning. We emphasise how nonlinearity can be used to evidence the genuine quantum nature of the coupling by producing satellites peaks of the Rabi doublet that originate from the quantized energy levels of the interactions.Comment: Low quality figures. To be published in Phys. Rev. B. 78 (2008

A Cooper pair light emitting diode

We demonstrate Cooper-pair's drastic enhancement effect on band-to-band radiative recombination in a semiconductor. Electron Cooper pairs injected from a superconducting electrode into an active layer by the proximity effect recombine with holes injected from a p-type electrode and dramatically accelerate the photon generation rates of a light emitting diode in the optical-fiber communication band. Cooper pairs are the condensation of electrons at a spin-singlet quantum state and this condensation leads to the observed enhancement of the electric-dipole transitions. Our results indicate the possibility to open up new interdisciplinary fields between superconductivity and optoelectronics.Comment: 5 pages (4 figures

On Critical Velocities in Exciton Superfluidity

The presence of exciton phonon interactions is shown to play a key role in the exciton superfluidity. We apply the Landau criterion for an exciton-phonon condensate moving uniformly at zero temperature. It turns out that there are essentially two critical velocities in the theory. Within the range of these velocities the condensate can exist only as a bright soliton. The excitation spectrum and differential equations for the wave function of this condensate are derived.Comment: 7 pages, Latex; to be published in Phys.Rev.Lett (1997

Collective oscillations driven by correlation in the nonlinear optical regime

We present an analytical and numerical study of the coherent exciton polarization including exciton-exciton correlation. The time evolution after excitation with ultrashort optical pulses can be divided into a slowly varying polarization component and novel ultrafast collective modes. The frequency and damping of the collective modes are determined by the high-frequency properties of the retarded two-exciton correlation function, which includes Coulomb effects beyond the mean-field approximation. The overall time evolution depends on the low-frequency spectral behavior. The collective mode, well separated from the slower coherent density evolution, manifests itself in the coherent emission of a resonantly excited excitonic system, as demonstrated numerically.Comment: 4 pages, 4 figures, accepted for publication in Physical Review Letter

Anisotropic Hubbard model on a triangular lattice -- spin dynamics in Ho Mn O_3

The recent neutron-scattering data for spin-wave dispersion in $\rm Ho Mn O_3$ are well described by an anisotropic Hubbard model on a triangular lattice with a planar (XY) spin anisotropy. Best fit indicates that magnetic excitations in $\rm Ho Mn O_3$ correspond to the strong-coupling limit $U/t > \sim 15$, with planar exchange energy $J=4t^2/U \simeq 2.5$meV and planar anisotropy $\Delta U \simeq 0.35$meV.Comment: 4 pages, 3 figure

Manifestation of exciton Bose condensation in induced two-phonon emission and Raman scattering

The unusual two-photon emission by Bose-condensed excitons caused by simultaneous recombination of two excitons with opposite momenta leaving the occupation numbers of excitonic states with momenta $p nonequal to 0$ unchanged (below coherent two-exciton recombination) is investigated. Raman scattering accompanied by the analogous two-exciton recombination (or creation) is also analyzed. The excess momentum equal to the change of the electromagnetic field momentum in these processes can be transferred to phonons or impurities. The processes under consideration take place if there is Bose condensation in the interacting exciton system, and, therefore, can be used as a new method to reveal exciton Bose condensation. If the frequency of the incident light $\omega< 2\Omega$ ($\Omega$ is the frequency corresponding to the recombination of an exciton with p=0), the coherent two-exciton recombination with the excess momentum elastically transferred to impurities leads to the appearance of the spectral line $2\Omega-\omega$ corresponding to the induced two-photon emission. In this case the anti-Stokes line on frequency $\omega+2\Omega$ also appears in the Raman spectrum. If $\omega>2\Omega$, there are both Stokes and anti-Stokes lines on frequencies $\omega\pm2\Omega$ in the Raman spectrum. The induced two-photon emission is impossible in this case. The spectral lines mentioned above have phonon replicas on frequencies $|\omega\pm (2\Omega-n\omega^s_0)|$ corresponding to the transmission of the excess momentum (partially or as a whole) to optical phonons of frequency $\omega^s_0$ ($n$ is an integer number).Comment: 21 pages, 2 Postscript figures. Submitted to Phys. Rev. B (1998

Bosons in a Lattice: Exciton-Phonon Condensate in Cu2O

We explore a nonlinear field model to describe the interplay between the ability of excitons to be Bose-condensed and their interaction with other modes of a crystal. We apply our consideration to the long-living para-excitons in Cu2O. Taking into account the exciton-phonon interaction and introducing a coherent phonon part of the moving condensate, we derive the dynamic equations for the exciton-phonon condensate. These equations can support localized solutions, and we discuss the conditions for the moving inhomogeneous condensate to appear in the crystal. We calculate the condensate wave function and energy, and a collective excitation spectrum in the semiclassical approximation; the inside-excitations were found to follow the asymptotic behavior of the macroscopic wave function exactly. The stability conditions of the moving condensate are analyzed by use of Landau arguments, and Landau critical parameters appear in the theory. Finally, we apply our model to describe the recently observed interference and strong nonlinear interaction between two coherent exciton-phonon packets in Cu2O.Comment: 34 pages, LaTeX, four figures (.ps) are incorporated by epsf. Submitted to Phys. Rev.

Free induction signal from biexcitons and bound excitons

A theory of the free induction signal from biexcitons and bound excitons is presented. The simultaneous existence of the exciton continuum and a bound state is shown to result in a new type of time dependence of the free induction. The optically detected signal increases in time and oscillates with increasing amplitude until damped by radiative or dephasing processes. Radiative decay is anomalously fast and can result in strong picosecond pulses. The expanding area of a coherent exciton polarization (inflating antenna), produced by the exciting pulse, is the underlying physical mechanism. The developed formalism can be applied to different biexciton transients.Comment: RevTeX, 20 p. + 2 ps fig. To appear in Phys. Rev. B1

Theory of exciton-exciton correlation in nonlinear optical response

We present a systematic theory of Coulomb interaction effects in the nonlinear optical processes in semiconductors using a perturbation series in the exciting laser field. The third-order dynamical response consists of phase-space filling correction, mean-field exciton-exciton interaction, and two-exciton correlation effects expressed as a force-force correlation function. The theory provides a unified description of effects of bound and unbound biexcitons, including memory-effects beyond the Markovian approximation. Approximations for the correlation function are presented.Comment: RevTex, 35 pages, 10 PostScript figs, shorter version submitted to Physical Review

Excitonic Strings in one dimensional organic compounds

Important questions concern the existence of excitonic strings in organic compounds and their signatures in the photophysics of these systems. A model in terms of Hard Core Bosons is proposed to study this problem in one dimension. Mainly the cases with two and three particles are studied for finite and infinite lattices, where analytical results are accessible. It is shown that if bi-excitonic states exist, three-excitonic and even, n-excitonic strings, at least in a certain range of parameters, will exist. Moreover, the behaviour of the transitions from one exciton to the biexciton is fully clarified. The results are in agreement with exact finite cluster diagonalizations of several model Hamiltonians.Comment: 36 pages, 4 eps figs. to appear in Phys. Rev.