RADIATION OF DRESSED EXCITONS IN THE SEMICONDUCTOR MICROCAVITY

In this paper, we introduced the dressed exciton model of the semiconductor micro-cavity device. In the semiconductor micro cavity of vertical-cavity surface-emission device, the excitons first coupled with the cavity through an intra-electromagnetic field and formed the dressed excitons. Then these dressed excitons decayed into the vacuum cavity optical mode, as a multi-particle process. Through the quantum electrodynamics method, the dipole emission density and system energy decayed equation were obtained. And it was predicted that the excitons decay into a very narrow mode when the exciton-cavity coupling becomes strong enough.     

Electron-hole diagrams versus exciton diagrams: the simplest problem on interacting excitons

We study the interaction of an exciton with a distant metal, which is the simplest problem on interacting excitons: The semiconductor and metal electrons being “different” species, we do not have to worry about the tricky consequences of Pauli exclusion between identical carriers, which appear in any other problem on interacting excitons. We show how the exciton absorption, in the presence of semiconductor-metal interaction, can be derived in a very simple and transparent way from an exciton diagram procedure, provided that we use the appropriate exciton-metal interaction vertex, which contains the scattering from an exciton state to another exciton state under a Coulomb excitation. We also show that the resolution of this problem using standard electron-hole diagrams is dreadfully complicated at the lowest order in the semiconductor-metal interaction already, preventing a full calculation of the exciton-metal coupling from this usual technique. Received 26 February 2001     

Dipolar excitons, spontaneous phase coherence, and superfluid-insulator transition in bilayer quantum Hall systems at nu = 1

The spontaneous interlayer phase coherent (111) state of a bilayer quantum Hall system at filling factor nu = 1 may be viewed as a condensate of interlayer particle-hole pairs or excitons. We show that when the layers are biased in such a way that these excitons are very dilute, they may be viewed as pointlike bosons. We calculate the exciton dispersion relation and show that the exciton-exciton interaction is dominated by the dipole moment they carry. In addition to the phase coherent state, we also find a Wigner crystal/glass phase in the presence/absence of disorder which is an insulating state for the excitons. The position of the phase boundary is estimated and the transition between these two phases is discussed.     

Exciton lifetime in quantum well with vicinal high-density excitons

Radiative lifetime of an exciton in a GaAs quantum well (QW) is controlled by high-density excitons, which restrict the exciton coherence through scattering. In order to circumvent the phase space filling effect of high-density excitons, we have prepared a QW structure in such a way that a reservoir for high-density excitons is separated from the QW. The lifetime increases (up to 30%) with the exciton density in the reservoir and saturates at 1×10¹⁷/cm³. The upper bound lifetime is determined by the excitonic relative motion.     

The trion as an exciton interacting with a carrier

The X⁻ trion is essentially an electron bound to an exciton. However, due to the composite nature of the exciton, there is no way to write an exciton-electron interaction potential. We can overcome this difficulty by using a commutation technique similar to the one we introduced for excitons interacting with excitons, which allows to take exactly into account the close-to-boson character of the excitons. From it, we can obtain the X⁻ trion creation operator in terms of excitons and electrons. We can also derive the X⁻ trion ladder diagram between an exciton and an electron. These are the basic tools for future works on many-body effects involving trions.     

Primary photoexcitations and the origin of the photocurrent in rubrene single crystals

By simultaneously measuring the excitation spectra of transient luminescence and transient photoconductivity after picosecond pulsed excitation in rubrene single crystals, we show that free excitons are photoexcited starting at photon energies above 2.0 eV. We observe a competition between photoexcitation of free excitons and photoexcitation into vibronic states that subsequently decays into free carriers, while molecular excitons are instead formed predominantly through the free exciton. At photon energies below 2.25 eV, free charge carriers are created only through a long-lived intermediate state with a lifetime of up to 0.1 ms and no free carriers appear during the exciton lifetime.     

Mott transition of excitons in coupled quantum wells

In this work we study the phase diagram of indirect excitons in coupled quantum wells and show that the system undergoes a phase transition to an unbound electron-hole plasma. This transition is manifested as an abrupt change in the photoluminescence linewidth and peak energy at some critical power density and temperature. By measuring the exciton diamagnetism, we show that the transition is associated with an abrupt increase in the exciton radius. We find that the transition is stimulated by the presence of direct excitons in one of the wells and show that they serve as a catalyst of the transition.     

Strong Exciton-Plasmon Coupling and Hybridization of Organic-Inorganic Exciton-Polaritons in Plasmonic Nanocavity

We investigate strong exciton-plasmon coupling and plasmon-mediated hybridization between the Frenkel(F)and Wannier-Mott(WM)excitons of an organic-inorganic hybrid system consisting of a silver ring separated from a monolayer WS_2 by J-aggregates.The extinction spectra of the hybrid system calculated by employing the coupled oscillator model are consistent with the results simulated by the finite-difference time-domain method.The calculation results show that strong couplings among F excitons,WM excitons,and localized surface plasmon resonances(LSPRs) lead to the appearance of three plexciton branches in the extinction spectra.The weighting efficiencies of the F exciton,WM exciton and LSPR modes in three plexciton branches are used to analyze the exciton-polaritons in the system.Furthermore,the strong coupling between two different excitons and LSPRs is manipulated by tuning F or WM exciton resonances.     

Ultrafast nonlinear optical response and high density excitation effects of the stacking fault exciton in BiI3

On the exciton states localized at a two-dimensional stacking fault interface in a layered crystal BiI₃, some nonlinear optical phenomena clearly appear reflecting large transition probability. The optical Stark shift and other high density exciton effect on the energy-shift and the spectral broadening are observed with clear separation under intense laser pumping by time-resolved measurements. The optical Stark shift is analyzed based on the dressed exciton model. The ultrafast optical response faster than 3 ps for the pump-laser field is confirmed on the Stark shift. Degenerate four-wave-mixing signals show fairly long dephasing time of 40 ps in this system. The dephasing probability depends linearly on the pump-laser intensity in the same manner as that of the spectral line-broadening reflecting the relaxation process. The dephasing mechanisms are understood by the exciton scattering at high density in parallel with the spectral changes. The blue-shift due to the high density excitons are discussed on the basis of exciton-exciton interaction in connection with a phase-space filling theory in two-dimensional systems.     

High-resolution nonlinear laser spectroscopy of exciton relaxation in GaAs quantum wells

This paper describes measurements of exciton relaxation in GaAs/AlGaAs quantum well structures based on high resolution nonlinear laser spectroscopy. The nonlinear optical measurements show that low energy excitons can be localized by monolayer disorder of the quantum well interface. We show that these excitons migrate between localization sites by phonon assisted migration, leading to spectral diffusion of the excitons. The frequency domain measurements give a direct measure of the quasi-equilibrium exciton spectral redistribution due to exciton energy relaxation, and the temperature dependence of the measured migration rates confirms recent theoretical predictions. The observed line shapes are interpreted based on solutions we obtain to modified Bloch equations which include the effects of spectral diffusion.     

Charged excitons in quantum dots: novel magnetic behavior and Auger processes

We describe theoretically multiply-charged excitons interacting with a continuum of delocalized states. Such excitons exist in relatively shallow quantum dots and have been observed in recent optical experiments on InAs self-assembled dots. The interaction of an exciton and delocalized states occurs via Auger-like processes. To describe the optical spectra, we employ the Anderson-like Hamiltonian by including the interaction between the localized exciton and delocalized states of the wetting layer. In the absence of a magnetic field, the photoluminescence line shapes exhibit interference effects. When a magnetic field is applied, the photoluminescence spectrum demonstrates anticrossings with the Landau levels of the extended states. We show that the magnetic-field behavior of charged excitons is very different to that of diamagnetic excitons in three and two-dimensional systems.     

Exciton condensation driving the periodic lattice distortion of 1T-TiSe2

We address the lattice deformation of 1T-TiSe2 within the exciton condensate phase. We show that, at low temperature, condensed excitons influence the lattice through electron-phonon interaction. It is found that at zero temperature, in the exciton condensate phase of 1T-TiSe2, this exciton condensate exerts a force on the lattice generating ionic displacements comparable in amplitude to what is measured in experiment. This is thus the first quantitative estimation of the amplitude of the periodic lattice distortion observed in 1T-TiSe2 as a consequence of the exciton condensate phase.     

Dependence of resonance energy transfer on exciton dimensionality

We investigate the dependence of resonance energy transfer from Wannier-Mott excitons to an organic overlayer on exciton dimensionality. We exploit the excitonic potential disorder in a single quantum well to tune the balance between localized and free excitons by scaling the Boltzmann distribution of excitons through temperature. Theoretical calculations predict the experimentally observed temperature dependence of resonance energy transfer and allow us to quantify the contribution of localized and free excitons. We show that free excitons can undergo resonance energy transfer with an order of magnitude higher rate compared to localized excitons, emphasizing the potential of hybrid optoelectronic devices utilizing resonance energy transfer as a means to overcome charge transfer related limitations.     

Excitons dressed by a sea of excitons

We here consider an exciton i embedded in a sea of N identical excitons 0. If the excitons are taken as true bosons, a bosonic enhancement factor N is found for i=0. If the exciton composite nature is kept, this enhancement not only exists for i=0, but also for any exciton having a center of mass momentum equal to the sea exciton momentum. This physically comes from the fact that an exciton with such a momentum can be transformed into a sea exciton by Pauli scattering, i.e., carrier exchange with the sea, making this exciton i not so much different from a sea exciton. This possible scattering, directly linked to the composite nature of the excitons, is irretrievably lost when the excitons are bosonized. The underlying interest of this work is in fact the calculation of the scalar products of N-exciton states, which turns out to be quite tricky, due to possible carrier exchanges between excitons. This work actually constitutes a crucial piece of our many-body theory for interacting composite bosons, because all physical effects involving composite bosons ultimately end by the calculation of such scalar products. The skeleton diagrams we here introduce to represent them, allow to visualize many-body effects linked to carrier exchanges in an easy way. They are conceptually different from Feynman diagrams, because of the special feature of the Pauli scatterings which originate from boson statistics departure.     

Coulomb Drag of Dipole Excitons in a Hybrid Exciton–Electron System

The effect of Coulomb drag on a gas of dipole excitons in spatially separated two-dimensional quantum wells containing electron and exciton gases is studied theoretically. The Coulomb drag of excitons can be used to control exciton transport in transistor structures whose active element is a two-dimensional gas of dipole excitons. Expressions for the exciton cross conductivity as a function of temperature are obtained for the diffusion and ballistic transport regimes. For each regime, the limiting cases in terms of the ratio of the Coulomb interaction screening length to the distance between the gases are analyzed. It is shown that, at temperatures exceeding considerably the exciton-gas degeneracy temperature, the cross conductivity is independent of the temperature, while in the opposite case it vanishes exponentially.     

Lasing threshold in traps for Bose-condensation of dipolar excitons

We consider exciton recombination lasing in heterostructure traps for Bose–Einstein condensation of dipolar excitons. We show that such structures suit well for class D lasers where cavity decay strongly exceeds polarization decay. We evaluate lasing threshold taking into account specific inhomogeneous broadening of the exciton spectral line owing to Bose–Einstein condensation phenomenon under quasi-equilibrium conditions.It is found that narrowing of the exciton momentum distribution just before the condensation onset considerably lowers lasing threshold. At the same time, it is pointed out that a subsequent formation of condensate itself does not help lasing much. We conclude that it is possible to achieve lasing on polariton modes in nowadays experiments aimed on Bose–Einstein condensation of excitons.     

Magnetic-field effects on exciton–polaron in a CdSe quantum disk

We study theoretically the interaction between excitons and longitudinal optical (LO) phonons in a cylindrical disk-like semiconductor quantum dot under an applied magnetic field. Due to the intensity of the interaction in the strong coupling regime, a composite quasi-particle called exciton–polaron is formed. We focus on the effect of the disk size and an external magnetic field on the exciton–phonon interaction energy and the exciton–polaron modes. The numerical computation for a CdSe quantum disk have shown that the exciton–phonon interaction energy is very significant and is even dominant when the disk height is small, which leads to a large Rabi splitting between the exciton–polaron modes. We investigate also the effect of the temperature on the integrated photoluminescence (PL) intensity, and show that at relatively high temperature the LO phonons have a noticeable effect on it. This physical parameter also shows a great dependence on quantum disk size and on magnetic field.     

Effects of excitation spectral width on decay profile of weakly confined excitons

We report the effect due to a simultaneous excitation of several exciton states on the radiative decay profiles on the basis of the nonlocal response of weakly confined excitons in GaAs thin films. In the case of excitation of single exciton state, the transient grating signal has two decay components. The fast decay component comes from nonlocal response, and the long-lived component is attributed to free exciton decay. With an increase of excitation spectral width, the nonlocal component becomes small in comparison with the long-lived component, and disappears under irradiation of a femtosecond-pulse laser with broader spectral width. The transient grating spectra clearly indicates the contribution of the weakly confined excitons to the signal, and the exciton line width hardly changes by excitation spectral width. From these results, we concluded that the change of decay profile is attributed not to the many-body effect but to the effect of simultaneous excitation of several exciton states.     

Polaronic exciton in quantum wells wires and nanotubes

We investigate the effect of the longitudinal-optical phonon field on the binding energies of excitons in quantum wells, well-wires and nanotubes based on ionic semiconductors. We take into account the exciton-phonon interaction by using the Aldrich-Bajaj effective potential for Wannier excitons in a polarizable medium. We extend the fractional-dimensional method developed previously for neutral and negatively charged donors to calculate the exciton binding energies in these heterostructures. In this method, the exciton wave function is taken as a product of the ground state functions of the electron polaron and hole polaron with a correlation function that depends only on the electron-hole separation. Starting from the variational principle we derive a one-dimensional differential equation, which is solved numerically by using the trigonometric sweep method. We find that the potential that takes into account polaronic effects always give rise to larger exciton binding energies than those obtained using a Coulomb potential screened by a static dielectric constant. This enhancement of the binding energy is more considerable in quantum wires and nanotubes than in quantum wells. Our results for quantum wells are in a good agreement with previous variational calculations. Also, we present novel curves of the exciton binding energies as a function of the wire and nanotubes radii for different models of the confinement potential.     

The exciton luminescence in Zn(Cd)Se/ZnMgSSe quantum wells

We study exciton states in Zn(Cd)Se/ZnMgSSe quantum wells (QWs) with various degrees of diffusion blurring in the interfaces by the methods of optical spectroscopy. We show that at low temperatures the QW emission spectra are determined by free and neutral donor-bound excitons. Blurring of the heterointerfaces leads to the increase in the energy shift between the emission line maxima of free and bound excitons. We explain the nonlinear dependence of the steady-state photoluminescence intensity on the excitation-power density in terms of the neutralization of charged donors at the photoexcitation of heterostructures. We observed a complex long-time dynamics of the reflection coefficient, evoked by the charge-redistribution processes in the heterostructure, near the QW exciton resonances under the irradiation.