Exciton condensation in coupled quantum wells

Bound electron-hole pairs—excitons—are Bose particles with small mass. Exciton Bose-Einstein condensation is expected to occur at a few degrees Kelvin—a temperature many orders of magnitude higher than for atoms. Experimentally, an exciton temperature well below 1 K is achieved in coupled quantum well (CQW) semiconductor nanostructures. In this contribution, we review briefly experiments that signal exciton condensation in CQWs: a strong enhancement of the indirect exciton mobility consistent with the onset of exciton superfluidity, a strong enhancement of the radiative decay rate of the indirect excitons consistent with exciton condensate superradiance, strong fluctuations of the indirect exciton emission consistent with critical fluctuations near the phase transition, and a strong enhancement of the exciton scattering rate with increasing concentration of the indirect excitons revealing bosonic stimulation of exciton scattering. Novel experiments with exciton condensation in potential traps, pattern formation in exciton system and macroscopically ordered exciton state will also be reviewed briefly.     

Edge excitons in a 2D topological insulator in a magnetic field

Exciton edge states and the microwave edge exciton absorption of a 2D topological insulator subject to the in-plane magnetic field are studied. The magnetic field forms a narrow gap in electron edge states that allows the existence of edge exciton. The exciton binding energy is found to be much smaller than the energy of a 1D Coulomb state. Phototransitions exist on the exciton states with even numbers, while odd exciton states are dark.     

Instabilities and Self-Organization Phenomena in High Density Exciton Systems

The influence of final value of exciton lifetime on exciton condensed phase formation is investigated in deterministic and stochastic approach. It is shown that in some region of pumping, the periodical distribution of exciton density takes place. The numerical solution for periodical distribution of exciton density is obtained for different values of pumping. There are critical values of exciton lifetime for appearance of exciton condensed phase. The distribution density function becomes narrower with increasing pumping.     

Excitonic optical absorption in semiconductors under intense terahertz radiation

The excitonic optical absorption of GaAs bulk semiconductors under intense terahertz （THz） radiation is investigated numerically. The method of solving initial-value problems, combined with the perfect matched layer technique, is used to calculate the optical susceptibility. In the presence of a driving THz field, in addition to the usual exciton peaks, 2p replica of the dark 2p exciton and even-THz-photon-sidebands of the main exciton resonance emerge in the continuum above the band edge and below the main exciton resonance. Moreover, to understand the shift of the position of the main exciton peak under intense THz radiation, it is necessary to take into consideration both the dynamical Franz-Keldysh effect and ac Stark effect simultaneously. For moderate frequency fields, the main exciton peak decreases and broadens due to the field-induced ionization of the excitons with THz field increasing. However, for high frequency THz fields, the characteristics of the exciton recur even under very strong THz fields, which accords with the recent experimental results qualitatively.     

Exciton dispersion law in diamond-like semiconductors

It is shown that in covalent diamond-like semiconductors the minimum of the exciton energy is situated at non-zero exciton momentum if the heavy hole mass is large enough. The depth of the minimum may be of the order of the exciton binding energy. The minimal exciton energy is calculated for several semiconductors with the variational procedure.     

Coherence length of cold exciton gases in coupled quantum wells

A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells. A strong enhancement of the exciton coherence length is observed at temperatures below a few Kelvin. The increase of the coherence length is correlated with the macroscopic spatial ordering of excitons. The coherence length at the lowest temperature corresponds to a very narrow spread of the exciton momentum distribution, much smaller than that for a classical exciton gas.     

Magnetic-field-induced nonparabolicity of exciton dispersion in semiconductors with a nondegenerate valence band

The nonparabolicity of exciton dispersion due to the mixing of the ground and excited states of an exciton in an external magnetic field perpendicular to the direction of its motion is considered. A model describing this effect is proposed and the nonparabolicity for an exciton in a CdTe crystal is calculated. The magnetic-field induced exciton nonparabolicity is compared with the effect caused by the nonparabolicity of the electron energy dispersion in the conduction band.     

Exciton states in wurtzite InGaN/GaN quantum wells: Strong built-in electric field and interface optical-phonon effects

Considering the strong built-in electric field (BEF) effects and large exciton–phonon interactions, we investigate the exciton states confined in an InGaN/GaN single quantum well (QW) by using the Lee–Low–Pines variational method. We find that the exciton state modification caused by the exciton–phonon interactions is remarkable. The exciton energy shift due to exciton–phonon interactions increases monotonically if the well width increases. With increasing the In fraction, the exciton energy shift firstly increases to a maximum, then decreases. The BEF has a significant influence on the exciton states in a QW with large well width. The physical reasons have been analyzed in detail. Good agreement for the zero-phonon peak energies and the Huang–Rhys factor has been obtained between our numerical results and the corresponding experimental measurements.     

Revised fine splitting of excitons in diamond

We study low-strain synthetic high pressure, high temperature diamonds by cathodoluminescence and observe novel fine structure in the free exciton and the boron-bound exciton emission. The basic spectral structure is a doublet with DeltaE approximately 11 meV common to both exciton spectra. This resolves the previously found inequivalence of free exciton ( approximately 7 meV) and bound exciton ( approximately 12 meV) fine splitting. It is argued that for a spin-orbit interaction Delta(0) much smaller than the excitonic binding ( E(X) approximately 80 meV) and the excitonic localization ( E(loc) approximately 51 meV) at the boron acceptor, the orbital momentum and the spin of the particles constituting the electron-hole pair are recoupled to form spin singlet and triplet exciton states as the elementary excitations.     

Exciton theory in amorphous materials

In the present paper a second quantization formalism is used to derive a general expression for the exciton hamiltonian in disordered materials. It is shown how exciton creation and annihilation operators, acting as bosons, can be introduced. The exciton hamiltonian is diagonalized by an appropriate transformation for the exciton operators. We furthermore introduced the concept of the electronic polarization due to an exciton. The polarization coefficients are calculated by a simple variational technique. An appropriate configurational average of the results is taken, in order to eliminate the position vectors of the different constituent atoms.     

Quantum effects in the interaction of the exciton with a leaky quasi-mode cavity

We have studied quantum effects in the interaction of the exciton with a leaky quasi-mode cavity field. When the exciton is initially prepared in a superposition state which exhibits holes in its photon-number distribution, whereas the cavity field initially is in the vacuum state, it is found that there exists an energy exchange between the exciton and the cavity field. The exciton and the cavity field may exhibit sub-Poissonian distributions and quadrature squeezings. It is shown that there does not exist a violation of the Cauchy-Schwartz inequality, which means that the correlation between the exciton and the cavity field is classical. Received 25 November 2000 and Received in final form 1st January 2001     

Analytical Ground-State of an Exciton Trapped in a Circular Parabolic Quantum Dot in the Presence of a Magnetic Field

The quasi-exact properties of an exciton are investigated theoretically in the presence of an external magnetic field using the effective-mass approach in GaAs parabolic quantum dot. The energy spectrum is obtained analytically as a function of the dot radius, interaction strength and magnetic field. It is established that, a steady bound state of an exciton in the ground state exists under the effect of a strong magnetic field; also I noticed that the exciton binding energy decreases by increasing both the radius of the dot and the magnetic field strength and the reduction becomes pronounced for larger dots. As expected, it has been found that the exciton total energy decreases with increasing the size of the dot and it enhances by increasing the magnetic field. It appears that the exciton total energy strongly depends on the magnetic field for dots with big size. The magnetic field effect on the exciton size also has been studied. It is shown that the increase in the magnetic field leads to a reduction in the exciton size; due to magnetic field confinement, while the size of an exciton reach its bulk limit as the dot size increases. Moreover, it is shown that, if the dot radius is sufficiently large the oscillator strength saturates and it becomes insensitive to the magnetic field while the increase in the magnetic field gradually weakened the oscillator strength. I have calculated the ground-state distribution for both the electron and the hole. It is found that the localization of the electron/hole increases in the presence of a magnetic field. Moreover, the ground-state optical-absorption intensity is investigated. Finally, the dependence of the lowest five states of an exciton on both the dot radius and the magnetic field are discussed.

     

Excitons perturbed by self-trapped excitons in alkali iodine crystals

A strong transient optical absorption band on the exciton shoulder has been observed in pure alkali iodine crystals following pulsed electron irradiation. The ultraviolet transition is ascribed to creation of a second exciton in the vicinity of a self-trapped exciton. The perturbation caused by the proximity of the self-trapped exciton lowers the energy needed to form the second exciton by 0.14 eV for KI and 0.12 eV for RbI.     

Exciton-photon interaction in low-dimensional semiconductor microcavities

The structure of the photon states and dispersion of cavity polaritons in semiconductor microcavities with two-dimensional optical confinement (photon wires), fabricated from planar Bragg structures with a quantum well in the active layer, are investigated by measuring the angular dependence of the photoluminescence spectra. The size quantization of light due to the wavelength-commensurate lateral dimension of the cavity causes additional photon modes to appear. The dispersion of polaritons in photon wires is found to agree qualitatively with the prediction for wires having an ideal quantum well, for which the spectrum is formed by pairwise interaction between exciton and photon modes of like spatial symmetry. The weak influence of the exciton symmetry-breaking random potential in the quantum well indicates a mechanism of polariton production through light-induced collective exciton states. This phenomenon is possible because the light wavelength is large in comparison with the exciton radius and the dephasing time of the collective exciton state is long. Zh. éksp. Teor. Fiz. 114, 1329–1345 (October 1998)     

Coherent optical writing and reading of the exciton spin state in single quantum dots

We demonstrate a one-to-one correspondence between the polarization state of a light pulse tuned to neutral exciton resonances of single semiconductor quantum dots and the spin state of the exciton that it photogenerates. This is accomplished using two variably polarized and independently tuned picosecond laser pulses. The first "writes" the spin state of the resonantly excited exciton. The second is tuned to biexcitonic resonances, and its absorption is used to "read" the exciton spin state. The absorption of the second pulse depends on its polarization relative to the exciton spin direction. Changes in the exciton spin result in corresponding changes in the intensity of the photoluminescence from the biexciton lines which we monitor, obtaining thus a one-to-one mapping between any point on the Poincaré sphere of the light polarization to a point on the Bloch sphere of the exciton spin.     

Dynamic analysis of the parametric-resonance effect in the exciton region of a spectrum

The pump-probe method of studying of the optical properties of a semiconductor in the exciton region of a spectrum is considered theoretically taking into account the exciton–photon and elastic exciton–exciton interactions. It is shown that the concentration of excitons, and the susceptibility of the medium are mainly determined by the detuning from the exciton resonance and the magnitude of the pump field. The values of the parameters corresponding to the observed parametric resonance are obtained and the dynamic analysis of the found solutions is carried out.     

Exciton absorption of light in a smooth random field

A study is made of interband absorption of light in a smooth random field that varies slowly in space. When the exciton energy in the random field is much less than the exciton binding energy, the influence of the random field on the discrete spectrum of the exciton is expressed through the appearance of an exponential “tail” to every exciton line. But if the exciton energy in the random field is much greater than the exciton binding energy, the exciton effects are unimportant because the random field breaks up the excitons.     

Optically induced detrapping of triplet excitons trapped at β-chloronaphthalene in naphthalene single crystal

Flourescence induced by photo-excitation of a triplet exciton localized at a β-chloronaphthalene molecule in a naphthalene crystal has been measured. The flourescence is shown to arise from a collision of a localized triplet exciton with a triplet exciton detrapped by the excitation. The detrapping probability is obtained as a function of the excitation energy.     

Exciton energy relaxation on acoustic phonons in double-quantum-well structures

The paper analyzes the rate of energy relaxation involving acoustic phonon emission between exciton states in a double quantum well. A theoretical study is made of the part played by two mechanisms, one of which is a one-step transition with emission of an acoustic phonon and the other is a two-step transition, which includes elastic exciton scattering from interface nonuniformities followed by energy relaxation within an exciton subband. The rate of the two-step transition in real double quantum wells is shown to be higher than that of the one-step transition. As follows from calculations, the fast energy relaxation between exciton states is determined by the elastic scattering of phonons from the interface.     

Interferometric spectroscopy for excitons in InP single quantum dots

Dynamical dephasing processes of an exciton and a charged exciton in single InP quantum dots were studied by using interferometric spectroscopy. Interferometric spectroscopy enabled us to observe with high sensitivity the dephasing of exciton or other exciton complexes in single quantum dots. In order to observe the dephasing of the exciton or exciton complexes, emitted single-photons generated from single InP quantum dots were detected through the Michelson interferometer. The contrast of the interferometric signal due to the exciton and the charged exciton shows non-exponential decay under band-to-band excitation for the GaInP matrix. The band-to-band excitation generates carriers trapped in the matrix and the trapped carriers modulate the energy of the quantum dots because of the quantum-confined Stark effect. Therefore the non-exponential decays are caused by energy fluctuation due to the trap carriers in the long timescale.