Time-resolved optical characterization of InAs/InGaAs quantum dots emitting at 1.3 mu m

We present the time-resolved optical characterization of InAs/InGaAs self-assembledquantum dots emitting at 1.3 μm at room temperature. The photoluminescence decay time varies from 1.2 (5 K) to 1.8 ns (293 K). Evidence of thermalization among dots is seen in both continuous-wave and time-resolved spectra around 150 K. A short rise time of 10±2 ps is measured, indicating a fast capture and relaxation of carriers inside the dots

Optical properties of (AlxGa1-x)(0.52)In0.48P at the crossover from a direct-gap to an indirect-gap semiconductor

The optical properties and the dynamics of excitons and the electron-hole plasma have been studied in disordered (AlxGa1−x)0.52In0.48P near to the direct-to-indirect band gap crossover. In particular we have investigated three epitaxial layers grown by solid-source molecular beam epitaxy with varying Al content x. Two of them have compositions in the immediate vicinity of the crossover point, the other is assigned to the indirect-gap regime. Both direct and indirect recombination processes contribute to the photon emission from the material. Since the relative importance of the different recombination processes depends strongly on temperature, excitation intensity, and excitation pulse duration, the processes can be identified by changing these parameters. As a result, we can determine the relative alignment of the conduction band minima and the distribution of the electrons among them. At high excitation levels the two crossover samples show stimulated emission at a photon energy of ∼2.29 eV, i.e., in the green spectral range. Using the variable stripe length method, we find an optical gain of up to ∼600 cm−1 at excitation levels of ∼350 kW/cm2.Stimulated emission involves direct recombination. This conclusion is reached from the experiments and from line-shape modeling, including a self-consistent treatment of populations and renormalization of the conduction band minima

Radiative corrections to the excitonic molecule state in GaAs microcavities

The optical properties of excitonic molecules (XXs) in GaAs-based quantum well microcavities (MCs) are studied, both theoretically and experimentally. We show that the radiative corrections to the XX state, the Lamb shift $\Delta^{\rm MC}_{\rm XX}$ and radiative width $\Gamma^{\rm MC}_{\rm XX}$, are large, about $10-30$ of the molecule binding energy $\epsilon_{\rm XX}$, and definitely cannot be neglected. The optics of excitonic molecules is dominated by the in-plane resonant dissociation of the molecules into outgoing 1$\lambda$-mode and 0$\lambda$-mode cavity polaritons. The later decay channel, ``excitonic molecule $\to$ 0$\lambda$-mode polariton + 0$\lambda$-mode polariton'', deals with the short-wavelength MC polaritons invisible in standard optical experiments, i.e., refers to ``hidden'' optics of microcavities. By using transient four-wave mixing and pump-probe spectroscopies, we infer that the radiative width, associated with excitonic molecules of the binding energy $\epsilon_{\rm XX} \simeq 0.9-1.1$ meV, is $\Gamma^{\rm MC}_{\rm XX} \simeq 0.2-0.3$ meV in the microcavities and $\Gamma^{\rm QW}_{\rm XX} \simeq 0.1$ meV in a reference GaAs single quantum well (QW). We show that for our high-quality quasi-two-dimensional nanostructures the $T_2 = 2 T_1$ limit, relevant to the XX states, holds at temperatures below 10 K, and that the bipolariton model of excitonic molecules explains quantitatively and self-consistently the measured XX radiative widths. We also find and characterize two critical points in the dependence of the radiative corrections against the microcavity detuning, and propose to use the critical points for high-precision measurements of the molecule bindingenergy and microcavity Rabi splitting.Comment: 16 pages, 11 figures, accepted for publication in Phys. Rev.

Electron microscopic and optical investigations of the indium distribution GaAs capped InxGa1-xAs islands

Results from a structural and optical analysis of buried InxGa1-xAs islands carried out after the process of GaAs overgrowth are presented. It is found that during the growth process, the indium concentration profile changes and the thickness of the wetting layer emanating from a Stranski-Krastanow growth mode grows significantly. Quantum dots are formed due to strong gradients in the indium concentration, which is demonstrated by photoluminescence and excitation spectroscopy of the buried InxGa1-xAs islands. (C) 1997 American Institute of Physics

Looking at Localized Excitons in Quantum Structures: A Theoretical Description

We present a theory of scanning local optical spectroscopy in quantum structures taking into account structural disorder. The calculated spatially resolved spectra show the individual spectral lines from the exciton states localized by the disordered potential as well as the quasicontinua spectra at positions close to the potential barriers in agreement with the experimental findings

Anti-Kaon Induced Reactions on the Nucleon

Using a previously established effective Lagrangian model we describe anti-kaon induced reactions on the nucleon. The dominantly contributing channels in the cm-energy region from threshold up to 1.72 GeV are included (K N, \pi \Sigma, \pi \Lambda). We solve the Bethe-Salpeter equation in an unitary $K$-matrix approximation.Comment: 21 pages, 13 figures, minor typos corrected, accepted for publication in Phys. Rev.

Exciton dephasing via phonon interactions in InAs quantum dots: dependence on quantum confinement

We report systematic measurements of the dephasing of the excitonic ground-state transition in a series of InGaAs?GaAs quantum dots having different quantum confinement potentials. Using a highly sensitive four-wave mixing technique, we measure the polarization decay in the temperature range from 5 to 120 K on nine samples having the energy distance from the dot ground-state transition to the wetting layer continuum (confinement energy) tuned from 332 to 69 meV by thermal annealing. The width and the weight of the zero-phonon line in the homogeneous line shape are inferred from the measured polarization decay and are discussed within the framework of recent theoretical models of the exciton-acoustic phonon interaction in quantum dots. The weight of the zero-phonon line is found to decrease with increasing lattice temperature and confinement energy, consistently with theoretical predictions by the independent Boson model. The temperature-dependent width of the zero-phonon line is well reproduced by a thermally activated behavior having two constant activation energies of 6 and 28 meV, independent of confinement energy. Only the coefficient to the 6-meV activation energy shows a systematic increase with increasing confinement energy. These findings rule out that the process of one-phonon absorption from the excitonic ground state into higher energy states is the underlying dephasing mechanism

Localization-enhanced biexciton binding in semiconductors

The influence of excitonic localization on the binding energy of biexcitons is investigated for quasi-three-dimensional and quasi-two-dimensional AlxGa1−xAs structures. An increase of the biexciton binding energy is observed for localization energies comparable to or larger than the free biexciton binding energy. A simple analytical model for localization in the weak confinement regime ascribes the increase to a quenching of the additional kinetic energy of the exciton-exciton motion in the biexciton

The Casimir Problem of Spherical Dielectrics: Numerical Evaluation for General Permittivities

The Casimir mutual free energy F for a system of two dielectric concentric nonmagnetic spherical bodies is calculated, at arbitrary temperatures. The present paper is a continuation of an earlier investigation [Phys. Rev. E {\bf 63}, 051101 (2001)], in which F was evaluated in full only for the case of ideal metals (refractive index n=infinity). Here, analogous results are presented for dielectrics, for some chosen values of n. Our basic calculational method stems from quantum statistical mechanics. The Debye expansions for the Riccati-Bessel functions when carried out to a high order are found to be very useful in practice (thereby overflow/underflow problems are easily avoided), and also to give accurate results even for the lowest values of l down to l=1. Another virtue of the Debye expansions is that the limiting case of metals becomes quite amenable to an analytical treatment in spherical geometry. We first discuss the zero-frequency TE mode problem from a mathematical viewpoint and then, as a physical input, invoke the actual dispersion relations. The result of our analysis, based upon the adoption of the Drude dispersion relation at low frequencies, is that the zero-frequency TE mode does not contribute for a real metal. Accordingly, F turns out in this case to be only one half of the conventional value at high temperatures. The applicability of the Drude model in this context has however been questioned recently, and we do not aim at a complete discussion of this issue here. Existing experiments are low-temperature experiments, and are so far not accurate enough to distinguish between the different predictions. We also calculate explicitly the contribution from the zero-frequency mode for a dielectric. For a dielectric, this zero-frequency problem is absent.Comment: 23 pages, LaTeX, 7 ps figures; expanded discussion, especially in Sec. 5. To appear in Phys. Rev.

Relaxation and dephasing of multiexcitons in semiconductor quantum dots

We measure the dephasing time of ground-state excitonic transitions in InGaAs quantum dots under electrical injection in the temperature range from 10 to 70 K. Electrical injection into the barrier region results in a pure dephasing of the excitonic transitions. Once the injected carriers fill the electronic ground state, the biexciton to exciton transition is probed and a correlation of the exciton and biexciton phonon scattering mechanisms is found. Additional filling of the excited states creates multiexcitons that show a fast dephasing due to population relaxation