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Abstract: We measured the photoluminescence (PL), the optical absorption and the magneto-optical absorption of - FeSi2 single crystals which were fabricated by a chemical vapor transport method. The characteristic 0.8 eV-PL bands of - FeSi2 thin films were not observed in our single crystals. However, we found new two PL lines (0.717 and 0.737 eV) and new two absorption peaks (0.858 and 0.909 eV) at low temperatures. All the samples which were measured in this study were classified into two groups according to the absorption peak energies. The two types of samples showed different absorption energies, but the magnetic field dependencies of the energy shifts are quite the same. Furthermore, the evaluated effective g factor is about 4 for both types of samples. [Copyright &y& Elsevier]

The extremely long dephasing time of excitons in strain-compensated quantum dots at telecommunications wavelengths was measured using a polarization-dependent four-wave mixing technique. The use of a 150-layer-stacked structure enabled them to measure a four-wave mixing signal with a high signal-to-noise ratio, in spite of the fact that a high-sensitive heterodyne detection was not used. The large anisotropy of the dephasing time indicates the dominance of the radiative recombination process on dephasing. By simultaneously measuring the radiative lifetime using a pump-probe technique, they could directly estimate pure dephasing with an accuracy of better than 0.1 μeV. [ABSTRACT FROM AUTHOR]

Excitonic dephasing is investigated in InAs self-assembled quantum dots fabricated by the strain-compensation technique. The exciton ground-state emission is centered at the wavelength of 1420 nm at 5 K. Transient four-wave mixing measurements under resonant excitation clearly demonstrate a long dephasing time of 1.09 ns at 5 K, corresponding to the homogeneous broadening of 1.2 μeV. The extrapolated zero-temperature homogeneous broadening is limited only by the population lifetime of the exciton ground state. At slightly increased temperatures, the acoustic-phonon broadening becomes dominant on dephasing. [ABSTRACT FROM AUTHOR]

We report the optical anisotropy of excitons in self-assembled InAs quantum dots (QDs), the strain of which is controlled by tuning the lattice constant of the spacer layers between stacked QD layers. The strain dependence of the energy structure for excitons is investigated using the four-wave mixing technique. The fine-structure beat and the exciton-biexciton beat in four-wave mixing measurements clearly show that the fine-structure splitting of exciton states increases with increasing strain in the growth plane. The strain also causes significant optical anisotropy, which is observed by the angular-dependent measurements of the collinear polarization of excitation pulses. The strain dependence of the optical anisotropy is accurately reproduced by a model calculation in which the valence band mixing between heavy- and light-hole bands is taken into account. We demonstrate that the optical properties of excitons in QD ensembles are successfully controlled by tuning the lattice constant of the spacer layers. [ABSTRACT FROM AUTHOR]

The signals due to six-wave mixing (SWM) processes are observed for two-dimensional excitons and biexcitons. The signal intensity of the SWM process strongly depends on the pulse area of the excitation light. The pulse area is estimated by comparing the intensities of four-wave mixing (FWM) signals in two different directions. The photon echo and the transient-grating signals due to the biexcitonic state of GaSe were observed by SWM. When we change the order of the excitation pulses in our SWM experiments, the excitonic phase relaxation process can be observed in the negative delay-time region. [Copyright &y& Elsevier]

Abstract: We performed a transient four-wave mixing experiment on a strain-compensated InAs quantum dot (QD) ensemble over a wide range of excitation intensities. Under the resonant excitation of an exciton ground state, an extremely long dephasing time of 1ns was found. By increasing the areas of the excitation pulses, Rabi oscillations of excitonic polarizations were clearly observed. The corresponding Rabi frequency is three orders of magnitude higher than the measured dephasing rate. For larger pulse areas, we found that the deviation of experimental data from two-level predictions became significant. The deviations cannot be explained by taking into account, as has been suggested in other research, excitation density-dependent dephasing or Hartree–Fock-type Coulomb interactions between excitons. [Copyright &y& Elsevier]

Abstract: We demonstrate the decoherence control of excitons on a layered compound semiconductor GaSe by using successive femtosecond pulses. The intermediate pulses act as π pulses, which reverse the time evolution of non-Markovian dynamics. By changing the incident timing of the π pulses, we determine the effective pulse interval condition for suppressing the decoherence of excitons by optical pulse irradiation. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: We report the dynamical properties of the exciton orientation in GaAs thin films using the orientational grating (OG) technique. From the results of excitation-power dependence of OG signal, we confirmed that the OG signal comes from the optical nonlinearity of weakly confined excitons. In addition, the OG-decay time decreases with an increase of excitation power due to exciton–exciton interaction, and the shortest decay time is below 1ps. Our results may imply the potential application of optical nonlinearity of weakly confined exciton to ultrafast switching devices operating at 1Tbit/s. [Copyright &y& Elsevier]