Toshihide Takagahara

Electron-phonon interactions in semiconductor nanocrystals

Abstract The electron—phonon interactions in semiconductor nanocrystals, especially concerning the acoustic phonon modes are derived and the size dependence of the coupling strength is clarified for typical coupling mechanisms. On the basis of these results, the commonly observed linearly temperature-dependent term of the excitonic dephasing rate and the proportionality of its magnitude to the inverse square of the nanocrystal size are attributed to the pure dephasing due to the deformation-potential coupling. The luminescence Stokes shift and the Huang-Rhys factor due to acoustic phonon modes in Si nanocrystals are discussed in conjunction with the origin of the recently observed luminescence onset energy.

Theory of Exciton Dephasing in Semiconductor Quantum Dots

We formulate a theory of exciton dephasing in semiconductor quantum dots extending the Huang-Rhys theory of F centers to include the mixing among the exciton state manifold through the exciton-acousticphonon interaction and we identify the mechanisms of pure dephasing. We can reproduce quantitatively the magnitude as well as the temperature dependence of the exciton dephasing rate observed in GaAs quantum dotlike islands. In this system it turns out that both the diagonal and off-diagonal exciton-phonon interactions are contributing to the exciton pure dephasing on the same order of magnitude. Examining the previous data of the exciton dephasing rate in GaAs islands, CuCl and CdSe nanocrystals, we point out the correlation between the temperature dependence of the dephasing rate and the strength of the quantum confinement and we explain the gross features of the temperature dependence in various materials quantum dots. Furthermore, we discuss likely mechanisms of the exciton population decay. @S0163-1829 ~99!02728-9#

Quantum wells with enhanced exciton effects and optical non-linearity

Abstract Exciton effects are studied theoretically for a quantum well of a semiconductor sandwiched by barriers with a smaller dielectric constant and a larger energy gap. The exciton binding energy increases markedly so that the radiative decay rate of the exciton and the non-linear optical susceptibility are also shown to be enhanced.

Spin relaxation dynamics in highly uniform InAs quantum dots

We have investigated carrier spin dynamics in highly uniform self-assembled InAs quantum dots. The highly uniform quantum dots allowed us to observe the spin dynamics in the ground state and that in the second state separately, without the disturbance of inhomogeneous broadening. The spin relaxation times in the ground state and the second state were measured to be 1.0 and 0.6 ns, respectively. Our measurements reveal the absence of the carrier density dependence of the spin relaxation time. The measured spin relaxation time decreases rapidly from 1.1 ns at 10 K to 200 ps at 130 K. This large change in the spin relaxation time is well explained in terms of the mechanism of acoustic phonon emission.

RADIATIVE RECOMBINATION LIFETIME OF EXCITONS IN THIN QUANTUM BOXES

Exciton radiative recombination lifetime in a thin quantum box in the intermediate spatial dimension between the two-dimension and the zero-dimension is investigated by a theoretical analysis which rigorously treats the electron-hole Coulomb interaction. The higher exciton states as well as the ground exciton state are explicitly taken into account to estimate the temperature dependence of exciton recombination lifetime. We clarify how the temperature dependence of the recombination lifetime varies with a change in the quantum confinement dimension which can be controlled by the lateral width of a thin quantum box. We also discuss the effect of the exciton localization due to structural imperfection on the radiative recombination lifetime.

Quantum dot lattice and enhanced excitonic optical nonlinearity

We propose a concept of quantum dot lattice for enhancing the excitonic optical nonlinearity, especially its figures of merit. The most important aspect of the quantum dot lattice for enhancing the optical nonlinearity is the enlarged exciton coherence length due to the cooperation of a large number of quantum dots. The optical nonlinearity of the quantum dot lattice composed of typical semiconductor materials is estimated and an enhancement in the figures of merit of several orders of magnitude over the ever known values is predicted.

Decoherence processes during optical manipulation of excitonic qubits in semiconductor quantum dots

Using photoluminescence spectroscopy, we have investigated the nature of Rabi oscillation damping during optical manipulation of excitonic qubits in self-assembled quantum dots. Rabi oscillations were recorded by varying the pulse amplitude for fixed pulse durations between 4 ps and 10 ps. Up to five periods are visible, making it possible to quantify the excitation dependent damping. We find that this damping is more pronounced for shorter pulse widths and show that its origin is the nonresonant excitation of carriers in the wetting layer, most likely involving bound-to-continuum and continuum-to-bound transitions.

Excitonic relaxation processes in quantum well structures

Abstract Excitonic relaxation processes in quantum well structures are reviewed, focusing attention on the localized and the weakly delocalized regimes. In the localized regime, the acoustic phonon-assisted exciton transfer among the localized sites due to the interface roughness plays an important role in determining the energy and phase coherence relaxation at low temperatures. The effect of magnetic field on the exciton localization is also discussed. In the weakly delocalized regime the possible mechanisms of the dephasing relaxation are the acoustic phonon-mediated intra- and inter-subband scattering, the Fano-type resonance between the light hole exciton and the heavy hole exciton continuum and the elastic scattering by the interface roughness. Under the high intensity excitation, the interactions between excitons and free carriers become increasingly important in determining the dephasing relaxation.

Pure dephasing induced by exciton–phonon interactions in narrow GaAs quantum wells

Abstract We investigate both dephasing and population relaxation of excitons localized in quantum dot like islands in narrow GaAs quantum wells by using stimulated photon echoes. A direct comparison of these two closely related decay processes reveals a pure dephasing contribution that dominates excitonic dephasing at elevated temperatures but does not involve exciton population relaxation. The pure dephasing contribution arises from coupling of excitonic states with a continuum of acoustic phonons and is enhanced by 3D quantum confinement. Both the magnitude and the temperature dependence of the pure dephasing rate can be described by a theoretical model that generalizes the Huang–Rhys theory of F-centers.

Phonon-induced exciton dephasing in quantum dot molecules

A new microscopic approach to the optical transitions in quantum dots and quantum dot molecules, which accounts for both diagonal and nondiagonal exciton-phonon interaction, is developed. The cumulant expansion of the linear polarization is generalized to a multilevel system and is applied to calculation of the full time dependence of the polarization and the absorption spectrum. In particular, the broadening of zero-phonon lines is evaluated directly and discussed in terms of real and virtual phonon-assisted transitions. The influence of Coulomb interaction, tunneling, and structural asymmetry on the exciton dephasing in quantum dot molecules is analyzed.