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We find that the emission from laterally coupled quantum dots is strongly polarized along the coupled direction [1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar 1$$\end{document}1¯0], and its polarization anisotropy can be shaped by changing the orientation of the polarized excitation. When the nonresonant excitation is linearly polarized perpendicular to the coupled direction [110], excitons (X1 and X2) and local biexcitons (X1X1 and X2X2) from the two separate quantum dots (QD1 and QD2) show emission anisotropy with a small degree of polarization (10%). On the other hand, when the excitation polarization is parallel to the coupled direction [1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar 1$$\end{document}1¯0], the polarization anisotropy of excitons, local biexcitons, and coupled biexcitons (X1X2) is enhanced with a degree of polarization of 74%. We also observed a consistent anisotropy in the time-resolved photoluminescence. The decay rate of the polarized photoluminescence intensity along the coupled direction is relatively high, but the anisotropic decay rate can be modified by changing the orientation of the polarized excitation. An energy difference is also observed between the polarized emission spectra parallel and perpendicular to the coupled direction, and it increases by up to three times by changing the excitation polarization orientation from [110] to [1\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar 1$$\end{document}1¯0]. These results suggest that the dipole–dipole interaction across the two separate quantum dots is mediated and that the anisotropic wavefunctions of the excitons and biexcitons are shaped by the excitation polarization., Quantum dots: coupled control New information storage technologies could use polarized light to control the optical properties of coupled quantum dots. The idea of combining quantum dots – nanoscale semiconductor crystals – into coupled pairs has attracted great attention due to the increased number of exotic quantum states that can be realized for storing data. Robert Taylor at the University of Oxford, UK, with co-workers in China and Korea, directed polarized laser light onto coupled gallium arsenide quantum dots to induce photoluminescence. They found that the emitted light showed very different characteristics depending on whether the excitation was parallel or at right angles to the line formed by the two dots. This ‘optical shaping’ reflects different arrangements of excitons – bound states of electrons and holes – in the coupled dots, and could open new avenues for data storage and thermoelectric energy harvesting.

We found that optical Aharonov–Bohm oscillations in a single GaAs/GaAlAs quantum ring can be controlled by excitation intensity. With a weak excitation intensity of 1.2 kW cm–2, the optical Aharonov–Bohm oscillation period of biexcitons was observed to be half that of excitons in accordance with the period expected for a two-exciton Wigner molecule. When the excitation intensity is increased by an order of magnitude (12 kW cm–2), a gradual deviation of the Wigner molecule condition occurs with decreased oscillation periods and diamagnetic coefficients for both excitons and biexcitons along with a spectral shift. These results suggest that the effective orbit radii and rim widths of electrons and holes in a single quantum ring can be modified by light intensity via photoexcited carriers, which are possibly trapped at interface defects resulting in a local electric field.

We have considered optical nonlinearities of coupled quantum dots theoretically, where an exciton dipole-dipole interaction is mediated between the adjacent large and small quantum dots. For increasing a pump pulse area in resonance with the large quantum dot exciton the induced nonlinear refractive index of the small quantum dot exciton has been obtained. As the exciton dipole-dipole interaction depends on the relative orientation of two exciton dipoles, the optical nonlinearities for the directions parallel and perpendicular to the coupling axis of the two quantum dots are compared. The directional imbalance of optical nonlinearities in coupled quantum dots can be utilized for a polarization phase modulator by controlling a pump pulse area and propagation length.

The optical Aharonov-Bohm effect in a single quantum ring is associated with disorder effects. In the presence of structure anisotropy, localisation, internal electric field, and impurity scattering, optical Aharonov-Bohm oscillations of an electron-hole pair become modulated. Additionally, provided that a strongly correlated exciton pair is formed in a single quantum ring similar to the Wigner molecule, novel oscillations can be observed for increasing magnetic field. In this case, the biexciton emission energy changes abruptly at transition magnetic fields with a fractional oscillation period compared to that of the exciton, the so-called fractional optical Aharonov-Bohm oscillations.

Optical anisotropy of self-assembled elliptical InP quantum dots has been investigated in terms of the polarization dependence of excitons. Although large size inhomogeneity is present, two kinds of characteristic quantum dots, which are classified into large and small quantum dots, were found in terms of the polarization anisotropy. We have confirmed that the large quantum dots are more pronounced in the polarization anisotropy, where the degree of linear polarization for the large quantum dots is significantly larger (∼60%) than that for the small ones (∼36%). The effective shape of quantum dots is also estimated by using the size dependence of oscillator strength, which is in agreement with the AFM image. We also suggest that the anisotropy of exciton oscillator strength can be modified via the dipole–dipole interaction between nearest exciton dipoles.

Generally confinement size is considered to determine the dimensionality of nanostructures. While the exciton Bohr radius is used as a criterion to define either weak or strong confinement in optical experiments, the binding energy of confined excitons is difficult to measure experimentally. One alternative is to use the temperature dependence of the radiative recombination time, which has been employed previously in quantum wells and quantum wires. A one-dimensional loop structure is often assumed to model quantum rings, but this approximation ceases to be valid when the rim width becomes comparable to the ring radius. We have evaluated the density of states in a single quantum ring by measuring the temperature dependence of the radiative recombination of excitons, where the photoluminescence decay time as a function of temperature was calibrated by using the low temperature integrated intensity and linewidth. We conclude that the quasi-continuous finely-spaced levels arising from the rotation energy give rise to a quasi-one-dimensional density of states, as long as the confined exciton is allowed to rotate around the opening of the anisotropic ring structure, which has a finite rim width.

We find that the exciton dipole–dipole interaction in a single laterally coupled GaAs/AlGaAs quantum dot structure can be controlled by the linear polarization of a nonresonant optical excitation. When the excitation intensity is increased with the linearly polarized light parallel to the lateral coupling direction [11̅0], excitons (X1 and X2) and local biexcitons (X1X1 and X2X2) of the two separate quantum dots (QD1 and QD2) show a redshift along with coupled biexcitons (X1X2), while neither coupled biexcitons nor a redshift are observed when the polarization of the exciting beam is perpendicular to the coupling direction. The polarization dependence and the redshift are attributed to an optical nonlinearity in the exciton Förster resonant energy transfer interaction, whereby exciton population transfer between the two quantum dots also becomes significant with increasing excitation intensity. We have further distinguished coupled biexcitons from local biexcitons by their large diamagnetic coefficient.

We have performed a variable stripe length method at 5 K to measure the optical gain of CdS nanowires. When the excited carrier density is lower than the Mott density, we found that various inelastic scatterings of excitons and LO-phonons are involved. As a consequence of inelastic exciton-exciton scattering, the excitons scattered down to the low polariton branch cause P-emission lasing, and the excited excitons also result in a population inversion through intra-relaxation. When LO-phonon scatterings are involved, a broad optical gain spectrum is observed resulting from exciton-LO and P-LO scatterings.

Polarization modulation in CdSe quantum dots is presented in the framework of the optically-induced nonlinearities of spin dichroism. The real and the imaginary parts of the phase retardation, which are induced from the nonlinear refractive index of the spin-degenerate fine states, are shown spectrally. The polarization modulation is also described in terms of the Jones vector, where the spectral dependences of the amplitude and phase are shown. The optically-induced elliptical polarization of a Gaussian pulse in a feasible condition of multi-quantum dot-layers is predicted precisely in terms of the rotational and the elliptical angles for increasing pulse area up to π, which is also mapped on a the normalized Poincare sphere.

International audience; The Aharonov-Bohm effect in ring structures in the presence of electronic correlation and disorder is an open issue. We report novel oscillations of a strongly correlated exciton pair, similar to a Wigner molecule, in a single nanoquantum ring, where the emission energy changes abruptly at the transition magnetic field with a fractional oscillation period compared to that of the exciton, a so-called fractional optical Aharonov-Bohm oscillation. We have also observed modulated optical Aharonov-Bohm oscillations of an electron–hole pair and an anticrossing of the photoluminescence spectrum at the transition magnetic field, which are associated with disorder effects such as localization, built-in electric field, and impurities.

Interferometric correlation spectroscopy is performed to investigate the decoherence dynamics of localized excited states in a single GaAs/AlGaAs quantum ring. For increasing temperature and excitation, we observe that the decay of interferometric envelope and the Fourier-transformed spectrum remain exponential and Lorentzian, respectively. The linewidth broadening for temperature and excitation intensity can be attributed to enhanced acoustic phonon scattering and Auger scattering in the fast modulation limit through the barrier traps, where carriers are captured and escape randomly. Additionally, we suggest an internal scattering between the different vertical confinement states for the significantly short coherence time (

We report all-optical polarization switching based on resonant excitation of spin-polarized e-h pairs in CdSe nanocrystal quantum dots at room temperature. Pump-probe experiments have been performed using the same and the opposite configurations of circular polarizations. Comparing the co- and the cross-circular polarization cases, it is quite evident that selectively-excited spins are flipped to a balanced state within 1 ps and that during this time, the two spin states are equally populated. The all-optical switching results are found to be in close agreement with pump-probe experimental results. The optical switching technique proposed in the present investigation should be extremely useful for future ultrafast terabit communication systems.

The optical modal gain spectrum for a ZnO thin film was measured by using a variable stripe length method. Using the dierential equation for amplified spontaneous emission in a stripe geometry, we obtained stripe-length-dependent gain spectra, which enabled us to investigate the gain saturation in long stripe lengths. When the stripe was shorter than the saturation length, our analysis was in a good agreement with the gain obtained by commonly-used analysis [1]. For intermediate density ( nMott). Both gains saturate with increasing stripe length, but the electron-hole plasma becomes relatively dominant for long stripe lengths ( 1000 µm) while the exciton-exciton scattering is relatively suppressed. This result suggests that exciton-exciton scattering gain saturation is more vulnerable to increase in the stripe length increase since the excitonic stimulated emission occurs on a coherence scale of the localized area.

We analyzed time-resolved and magneto-micro-PL of a single droplet quantum dot in terms of polarization anisotropy, decay time, and diamagnetic coefficient in order to support the presence of localized state in a large droplet quantum dot.

The Temperature dependence of the exciton radiative decay time in ZnO nanorods has been investigated, which is associated with the density of states for the intra-relaxation of thermally excited excitons. The photoluminescence decay time was calibrated by using the photoluminescence intensity in order to obtain the radiative decay time. In the absence of an external magnetic field, we have confirmed that the radiative decay time increased with temperature in a similar manner to that seen in bulk material (similar to T-1.5). Under an external magnetic field of 6T parallel to the c-axis, we found that the power coefficient of the radiative decay time with temperature decreased (similar to T-1.3) when compared to that in the absence of a magnetic field. This result can be attributed to an enhancement of the effective mass perpendicular to the magnetic field and a redshift of the center-of-mass exciton as a consequence of perturbation effects in the weak-field regime. (C) 2014 Optical Society of America

The dynamics of carriers in GaN epilayers is investigated by using femtosecond pump-probe spectroscopy. After the residual chirp on the continuum probe is removed, the normalized difference spectra (NDS) for different probe energies are synchronized, recovering the full time resolution of our laser pulse. Our Monte-Carlo simulation agrees well with the unchirped NDS spectrum, which shows the development of the carrier distribution at early times, where phonon satellites are seen, together with a strong non-thermal electron distribution in the region of the LO-phonon energy arising from the remark-ably strong electron-LO phonon interaction. By employing a new technique which involves the integration of the normalized NDS multiplied by the corresponding energy, a measure of the mean energy of the carriers in non-thermal states is obtained. By comparing the time-dependent energy loss with the theoretical energy loss rate, we estimate the effective temperature of the phonon modes as well as the population of phonons.

Dynamics and state filling of the resonantly excited ground state (1se‐1S3∕2h) in a strongly confined CdSe quantum dot were investigated by degenerate pump-probe measurements. With increasing the electron-hole (e-h) pairs per dot the state filling and bleaching were observed with ∼2.7 e-h pairs. While radiative recombination was dominant with a small number of the e-h pairs (⪡1), the Auger-type recombination became significant near the bleaching number of e-h pairs (∼2.7). This result suggests that, for resonant excitation, either cold electron or hole of the ground state is scattered into the higher-energy states of a ZnS shell via Auger recombination process (∼28ps).

Transient gain spectra were measured for an In0.02Ga0.98N / In0.16Ga0.84N multiple quantum well by using the variable-stripe-length method (VSLM) in combination with the ultrafast optical Kerr-gate (OKG) technique. Gain dynamics were measured for a range of excitation lengths from short (50 mu m) to long (350 mu m) stripes with the sample under femtosecond photoexcitation. Analysis of the temporal behaviour of gain and chemical potential suggests that stimulated emission originates from a photoexcited electron-hole plasma at early times; at later times, localized states dominate as the electron-hole plasma becomes exhausted. Gain reduction at early times is attributable to coupling of the electron-hole plasma with photons along the stripe, whilst localized states are less susceptible to gain saturation.