Excitonic Molecules Research Articles

Excitonic molecules in type-II superlattices

Excitonic molecules in GaAs/AlAs type-II superlattices are numerically investigated. In spite of large difference of electronic structures between type-II and type-I superlattices, variational calculations show that the configuration of particles is similar to that in type-I superlattices. This is because the layer width is smaller than the extent of excitonic wavefunctions in the direction parallel to the layers in the present superlattices.

Экситонные комплексы в InGaAs/GaAs квантовых точках

Экситонные комплексы в InGaAs/GaAs квантовых точках

Biexcitonic binding energies in the transition regime from three- to two-dimensional semiconductors

We study the dependence of the binding energies of excitonic molecules on the confinement in semiconductor quantum wells. A set of symmetrically strained (GaIn)As/Ga(PAs) quantum wells with different well depths and equal well widths is investigated with transient degenerate four-wave mixing. The ratio of biexcitonic to excitonic binding energy increases with stronger confinement. This experimental result is discussed in detail in the framework of recent theoretical predictions and experimental results.

Coherent transient in photoluminescence of excitonic molecules in GaAs quantum wells

Time-resolved photoluminescence (PL) of excitonic molecules in GaAs quantum wells (QW's) reveals an initial transient characterized by a finite rise and an extremely fast nonexponential decay [H. Wang, J. Shah, and T. C. Damen et al., Solid State Commun. 98, 807 (1996)]. The transient is attributed to coherent quantum evolution towards the molecule ground state of two optically correlated excitons, ${\ensuremath{\sigma}}^{+}$ and ${\ensuremath{\sigma}}^{\ensuremath{-}},$ which undergo the Coulombic attraction. In the present paper, we develop a theory of the transient PL and fit the experimental data. The radiative decay of a quasi-two-dimensional (2D) excitonic molecule is analyzed with the giant oscillator strength model adapted to QW's and with the bipolariton model. Within the 2D bipolariton model, the main ``hidden'' channel of the radiative decay of an excitonic molecule is the resonant dissociation into two outgoing interface (surface) polaritons rather than the observable decay into the bulk radiative modes. We conclude that quasi-2D molecules dissociate already within the initial transient of PL, while the following exponential decay of the molecule-mediated PL is due to the escape of secondary interface polaritons into the bulk modes. According to the 2D bipolariton model, the sequence ``two incoming $\mathrm{photons}\ensuremath{\rightarrow}\mathrm{two}$ virtual $\mathrm{excitons}\ensuremath{\rightarrow}2\mathrm{D}$ $\mathrm{molecule}\ensuremath{\rightarrow}\mathrm{two}$ outgoing interface polaritons'' is a completely coherent process. The fit of the time-resolved molecule-mediated PL provides us with numerical estimates of the exciton-photon coupling in GaAs QW's.

Binding energy and oscillator strength of excitonic molecules in type-II quantum wells

The binding energy and oscillator strength of excitonic molecules in type-II quantum wells are theoretically investigated by a variational method with the effective-mass approximation. We have studied two cases for an arrangement of electrons and holes in an excitonic molecule: One is that two holes in the molecule are confined within the same well, adjoined to the well for two electrons; the other is that each hole is confined within different wells, which sandwich the well for two electrons. We found that (i) the binding energy of the molecule depends on the electron-hole mass ratio and well widths, (ii) the bound state of the molecules exists for the limiting range of the well width, and (iii) in the case where each hole in the molecule is confined within a different well the binding energy of the molecule for the heavy-hole limit amounts to 10--30 % of the exciton binding energy for the well width comparable to the exciton Bohr radius. From consideration of the oscillator strength, it is found that, with appropriate parameters, the processes due to both the optical conversion of an exciton into an excitonic molecule and the molecule formation by a two-photon absorption have a similar order of magnitude for an absorption coefficient to that in the single exciton formation. A comparison with the experiment is also briefly discussed.

Optical nonlinearities and lasing in II–VI multiple quantum wells

We investigated optical nonlinearities and stimulated emission in Zn 1− x Cd x Se/ZnSe multiple quantum well structures by using pump and probe transmission and high intensity photoluminescence (PL) measurements with or without magnetic field. We found that the origin of stimulated emission in the investigated samples depends on the exciton stability which can be increased by localization due to compositional and structural parameters. Exciton localization is directly demonstrated by time resolved PL measurements. A complex dynamics between excitons and free-carriers governs the physical mechanisms responsible for stimulated emission. This has been elucidated in detail by a many body theoretical model which has been used to analyze the experimental data based on an equilibrium phase diagram between excitons and free-carriers. Evidence of biexciton recombination is also given, and the stability of such excitonic molecules is investigated in modulation-doped quantum well structures.

Gain spectroscopy of excitonic molecules and its dynamics in a ZnSe single quantum well.

We demonstrate spectroscopically how optical gain is formed in an interacting system of excitons and biexcitons in a single ZnSe quantum well. The dynamics of gain formation are studied on a subpicosecond time scale in pump-probe experiments, revealing the influence of coherent effects and the impact of exciton-exciton and exciton-biexciton interactions. Spectrally resolved, polarization-dependent degenerate four-wave-mixing experiments further demonstrate how the excitonic many-body phenomena profoundly influence the nonlinear optical response in this II-VI semiconductor heterostructure. \textcopyright{} 1996 The American Physical Society.

Theory of one-dimensional quantum gap solitons.

We establish a quantum theory of gap solitons in a one-dimensional periodic dielectric structure [distributed Bragg reflector (DBR)] of a Kerr nonlinearity. The electromagnetic field in the structure is quantized within the two-band approximation. The basic idea for this quantization is that incident photons with frequencies in the band gap of the DBR are scattered into the upper ``conduction'' and lower ``valence'' bands by the nonlinearity. The effective Hamiltonian of quantum gap solitons is derived within the effective-mass regime. The eigenstates of the Hamiltonian are constructed exactly by Bethe's ansatz method. We find that in certain band gaps of the DBR these eigenstates can be bound, consisting of one or more photon pairs from the conduction and valence bands. These bound multiphoton states are optical analogs of exciton molecules [optical multiexcitons (OMEs)]. Their existence should be manifested by the discrete spectrum of band-gap transmission as a function of the transmitted photon number and by the multiexponential falloff of intensity-intensity correlations on a 0.1-mm scale. Quantum gap solitons are shown to arise as superpositions of OMEs whenever the nonlinear binding exceeds phonon scattering. \textcopyright{} 1996 The American Physical Society.

Ions of excitonic molecule in β - ZnP 2 crystals

Broad T emission band appears in the low-temperature spectra of β - ZnP 2 crystals between bands of the free excitons and biexcitons. This band is shown to be due to free ions of excitonic molecule (trions). The dependence of this band intensity I T on the excitation intensity I exc ( I T ∝ I exc 3 2 ) proves trion origin of T band. The binding energy of trion has been determined as 4.1 meV. Correlation of experimental intensities of the bands of trions and excitons is shown to be in good agreement with theoretical estimations. The behaviour of emission spectrum in external electric field has been studied. The intensity of T band decreases with increase of the applied voltage, proving the destroying of trions in electric field.

Interparticle interaction in spin-aligned and spin-degenerate exciton systems and magnetoplasmas in II-VI quantum wells.

Magnetoluminescence studies of highly excited undoped CdTe/${\mathrm{Cd}}_{0.88}$${\mathrm{Mn}}_{0.12}$Te and ${\mathrm{Cd}}_{0.97}$${\mathrm{Mn}}_{0.03}$Te/${\mathrm{Cd}}_{0.75}$${\mathrm{Mg}}_{0.25}$Te quantum wells (QW's) have been carried out in magnetic fields up to 14 T at 4.2 K. Qualitatively different excitonic systems, spin aligned in (Cd,Mn)Te QW and spin depolarized in CdTe QW, have been generated. The interaction of spin-aligned excitons are found to be weakly repulsive. In contrast, an additional well pronounced emission line M has been found to appear at approximately 5 meV below the excitonic line in the spectra of the spin-degenerate system. This line is assigned to the emission of excitonic molecules consisting of excitons with different spins. Finally, we have shown that the renormalization of Landau-level transitions in dense electron-hole magnetoplasmas with several occupied Landau levels depends on their spin state, it is markedly weaker in the spin-polarized plasmas. \textcopyright{} 1996 The American Physical Society.

Radiative transitions in an excitonic molecule

The ratio of intensities of inverse hydrogenlike series (IHS) luminescence bands which are due to two-electron transitions in an excitonic molecule (EM) have been calculated. The rates of two-photon and two-electron spontaneous transitions in the excitonic molecule have been estimated. Their ratio has been obtained as 0.13. The correlation between theoretical estimations and experimental results obtained from the luminescence spectra of β - ZnP 2 crystals has been analyzed.

Transient optical emission from excitonic molecules in GaAs quantum wells: Coherent quantum evolution in momentum space

Resonantly-excited photoluminescence (PL) of excitonic molecules with femtosecond resolution reveals a surprising initial transient characterized by a finite rise and an extremely fast non-exponential decay. We show that this initial transient stems from coherent quantum evolution of relative momentum of two excitons in the molecule, in sharp contrast to radiative dynamics of excitons. The initial transient also reflects a type of quantum beats that differ from the usual quantum beats in transient optical measurements.

Stochastic treatment of the dynamics of excitons and excitonic molecules in CuCl nanocrystals.

Dynamics of excitons and excitonic molecules in CuCl nanocrystals were investigated by means of both picosecond time-resolved luminescence and femtosecond transient absorption. Stochastic as well as rate-equation treatments are made to explain the temporal changes of both luminescence and absorption. In the stochastic treatment, a Monte Carlo simulation is made for ${10}^{5}$ nanocrystals with the initial condition that the exciton number distribution follows the Poisson distribution. The stochastic treatment satisfactorily explains all the experimental results, while the rate-equation treatment does not. Success of the stochastic treatment shows the unique property of the dynamics of excitons and excitonic molecules in quantum dots arising from discreteness of the exciton number. \textcopyright{} 1996 The American Physical Society.

Biexcitonic liquid in monoclinic zinc diphosphide crystals

The dielectric gas-dielectric liquid phase transition in the system of excitonic molecules in β - ZnP 2 crystals is considered. The influence of crystal anisotropy is under discussion. The features of β - ZnP 2 photoluminescence (PL) spectra under intermediate excitation intensities ( I exc ∼ 10 3 - - 10 5 W/ cm 2) at liquid helium temperatures are considered. In previous works optical manifestations of the dielectric biexcitonic liquid have been observed. An experimental study of the phase diagram of this liquid is performed. Biexcitonic liquid critical parameters are estimated.

Excitonic molecules and stimulated emission in a ZnSe single quantum well.

Spectroscopy on a single ZnSe quantum well characterized by a sharp exciton resonance (\ensuremath{\Delta}E=0.6 meV) shows how a gas of interacting excitonic molecules induces blue-green laser emission in an optically pumped heterostructure at cryogenic temperatures. Spectrally resolved four-wave-mixing experiments yield a molecular binding energy of 5 meV. However, the lasing occurs in an electron-hole density regime, where strong many-exciton interaction effects are evident. \textcopyright{} 1996 The American Physical Society.

Effects of high excitation on localized excitons in cubic ZnCdS lattice matched to GaAs

Effects of high excitation on localized excitons have been studied in cubic Zn0.42Cd0.58S lattice-matched to GaAs. The time-integrated photoluminescence (PL) spectrum at 2 K under weak excitation conditions (0.025 μJ/cm2) was composed of a single emission peaking at 2.863 eV and its LO-phonon replica. The linewidth of the main peak was 18.5 meV. With increasing excitation energy density (Iex), the spectra broadened, especially on the lower energy side and a small shoulder appeared at about 2.85 eV for Iex ≳ 2 μJ/cm2. In order to study the detailed mechanism, the nonlinear luminescence component was studied by the population mixing technique. The positive signals were detected on the lower energy side (2.85 eV) at an Iex value in the range between 1.8 and 2.9 μJ/cm2. This nonlinear component was attributed to localized excitonic molecules.

Observation of the stochastic exciton dynamics in CuCl quantum dots

Dynamics of excitons and excitonic molecules in CuCl quantum dots are investigated by means of both time-resolved luminescence and transient absorption with use of sub-picosecond laser pulses. To explain the experimental results, a stochastic treatment by means of the Monte Carlo simulation and a rate-equation treatment are made. The former nicely explains the experimental results, while the latter does not. It is found that the exciton dynamics in CuCl quantum dots are characterized by its stochastic nature due to discreteness of the exciton number in a quantum dot.

Bipolariton in quantum wells

We develop the bipolariton concept for a quasi-2D excitonic molecule (m) in quantum wells (QWLs). The bipolariton wave equation, which includes both the exciton-exciton (x-x) attraction and thex-photon (γ) coupling, is analysed. In the 2D bipolariton model, a quasi-2Dm exists in QWLs mainly as two quasi-boundsurface polaritons and, consequently, decays mainly into two outgoing surface polaritons. In quasi-2D GaAs QWLs the polariton renormalizations give rise to a «Mexican hat» structure in them dispersion at small momenta together with a considerable increase of both them effective binding energy ɛm and the inversem radiative lifetimeГm. A van Hove singularity in the joint density of polariton states is responsible for this effect.

Two-photon generation of excitonic molecules in CuCl: An exactly solvable bipolariton model and high-precision experiments.

The conventional giant oscillator strength model of the two-photon (\ensuremath{\gamma}) generation of an excitonic molecule (m) attributes this process to the x-\ensuremath{\gamma} optical conversion (where x is the exciton) i.e, to the scheme \ensuremath{\gamma}+\ensuremath{\gamma}\ensuremath{\rightarrow}x+\ensuremath{\gamma}\ensuremath{\rightarrow}m. Recenty [A.L. Ivanov and H. Haug, Phys. Rev. B 48, 1490 (1993)], it was argued that a more adequate description can be done within the bipolariton model which follows the other scenario: \ensuremath{\gamma}+\ensuremath{\gamma}\ensuremath{\rightarrow}x+x\ensuremath{\rightarrow}m, where the Coulombic resonant coupling of the x components of the two interacting polaritons gives rise to the m formation. In the present work, we develop and analyze for the deuteron potential of an x-x interaction an exactly solvable bipolariton model of the two-\ensuremath{\gamma} m generation. This model treats an m optical creation in terms of the polariton-polariton resonant scattering and includes both the x-y polariton coupling and the x-x attraction beyond a low-order perturbation theory. The bipolariton model and the giant oscillator strength model give different descriptions of the third-order m nonlinear optical susceptibility ${\mathrm{\ensuremath{\chi}}}^{(3)}$ and of the two-\ensuremath{\gamma} m absorption. With the high-precision measurements in CuCl of the m radiative width ${\mathrm{\ensuremath{\Gamma}}}^{\mathit{m}}$, ${\mathrm{\ensuremath{\chi}}}^{(3)}$, and the two-\ensuremath{\gamma} m absorption coefficient ${\mathit{K}}^{(2)}$, we make a systematic comparison between the experiments and the two models, which allows us to unambiguously discriminate both models in favor of the bipolariton one.

Study of Excitonic Molecules in Monoclinic Zinc Diphosphide Crystals

AbstractThe photoluminescence (PL) spectra of the monoclinic modification of the semiconductor crystal ZnP2 (β‐ZnP2) are studied under high, intermediate, and low laser excitation intensities at liquid helium temperature. Under high excitation intensities the inverse hydrogenlike M‐series was observed. It is symmetrical with the hydrogen‐like excitonic series. This M‐series is due to the two‐electron transitions from the excitonic molecule (biexciton) state to the ground, excited, and ionized excitonic states. The binding energy of the biexciton in β‐ZnP2 is determined as Δbex = 14.9 meV (0.32 of exciton Rydberg). Under intermediate excitation intensities another G‐band appears in the PL spectrum of β‐ZnP2. This band is interpreted as a result of two‐photon annihilation of free excitonic molecules in the gas phase. A narrow C‐line appears in the PL spectrum under intermediate excitation intensities, too. The analysis of the temperature dependence of the C‐line intensity allows us to assume this line to be the result of two‐photon annihilation of the biexcitons condensed into a dielectric liquid with the binding energy ΔL ≈︁ 0.5 meV.