Optical net gain measurement on AlGaN/GaN multi-quantum wells.

We report net gain measurements at room temperature in AlGaN/GaN 10-period multi-quantum well layers emitting at 367 nm, using the variable stripe length method. The separate confinement heterostructure was designed targeting electron-beam pumped lasing at 10 kV. The highest net gain value was 131 cm, obtained at the maximum pumping power density of the experimental setup (743 kW/cm).

Room-Temperature Doping of CsPbBr Nanocrystals with Aluminum.

B-site doping is an emerging strategy for tuning the emission wavelength of cesium lead halide ABX nanocrystals. We present a simple method for the postsynthetic doping of CsPbBr nanocrystals with aluminum at room temperature by exposing them to a solution of AlBr in dibromomethane. Despite the much smaller ionic radius of Al compared to that of Pb, nominal doping levels in a range from 8.

AlGaN/GaN asymmetric graded-index separate confinement heterostructures designed for electron-beam pumped UV lasers.

We present a study of undoped AlGaN/GaN separate confinement heterostructures designed to operate as electron beam pumped ultraviolet lasers. We discuss the effect of spontaneous and piezoelectric polarization on carrier diffusion, comparing the results of cathodoluminescence with electronic simulations of the band structure and Monte Carlo calculations of the electron trajectories. Carrier collection is significantly improved using an asymmetric graded-index separate confinement heterostructure (GRINSCH).

Light Controlled Optical Aharonov-Bohm Oscillations in a Single Quantum Ring.

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, 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.

Quasi-one-dimensional density of states in a single quantum ring.

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.

Exciton Dipole-Dipole Interaction in a Single Coupled-Quantum-Dot Structure via Polarized Excitation.

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 (X and X) and local biexcitons (XX and XX) of the two separate quantum dots (QD and QD) show a redshift along with coupled biexcitons (XX), 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.

Determination of the Optimal Shell Thickness for Self-Catalyzed GaAs/AlGaAs Core-Shell Nanowires on Silicon.

We present a set of experimental results showing a combination of various effects, that is, surface recombination velocity, surface charge traps, strain, and structural defects, that govern the carrier dynamics of self-catalyzed GaAs/AlGaAs core-shell nanowires (NWs) grown on a Si(111) substrate by molecular beam epitaxy. Time-resolved photoluminescence of NW ensemble and spatially resolved cathodoluminescence of single NWs reveal that emission intensity, decay time, and carrier diffusion length of the GaAs NW core strongly depend on the AlGaAs shell thickness but in a nonmonotonic fashion. Although 7 nm AlGaAs shell can efficiently suppress the surface recombination velocity of the GaAs NW core, the influence of the surface charge traps and the strain between the core and the shell that redshift the luminescence of the GaAs NW core remain observable in the whole range of the shell thickness.

Observation of a Biexciton Wigner Molecule by Fractional Optical Aharonov-Bohm Oscillations in a Single Quantum Ring.

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.

Self-aligned deterministic coupling of single quantum emitter to nanofocused plasmonic modes.

The quantum plasmonics field has emerged and been growing increasingly, including study of single emitter-light coupling using plasmonic system and scalable quantum plasmonic circuit. This offers opportunity for the quantum control of light with compact device footprint. However, coupling of a single emitter to highly localized plasmonic mode with nanoscale precision remains an important challenge.

Exciton scattering mechanism in a single semiconducting MgZnO nanorod.

Excitonic phenomena, such as excitonic absorption and emission, have been used in many photonic and optoelectronic semiconductor device applications. As the sizes of these nanoscale materials have approached to exciton diffusion lengths in semiconductors, a fundamental understanding of exciton transport in semiconductors has become imperative. We present exciton transport in a single MgZnO nanorod in the spatiotemporal regime with several nanometer-scale spatial resolution and several tens of picosecond temporal resolution.

Carrier depletion and exciton diffusion in a single ZnO nanowire.

Carrier depletion and transport in a single ZnO nanowire Schottky device have been investigated at 5 K, using cathodoluminescence measurements. An exciton diffusion length of 200 nm has been determined along the nanowire axis. The depletion width is found to increase linearly with the reverse bias.

Submicrometre resolved optical characterization of green nanowire-based light emitting diodes.

The electroluminescent properties of InGaN/GaN nanowire-based light emitting diodes (LEDs) are studied at different resolution scales. Axial one-dimensional heterostructures were grown by plasma-assisted molecular beam epitaxy (PAMBE) directly on a silicon (111) substrate and consist of the following sequentially deposited layers: n-type GaN, three undoped InGaN/GaN quantum wells, p-type AlGaN electron blocking layer and p-type GaN. From the macroscopic point of view, the devices emit light in the green spectral range (around 550 nm) under electrical injection.

A single-step electron beam lithography of buried nanostructures using cathodoluminescence imaging and low temperature.

We report a new electron beam lithography process using the cathodoluminescence properties of semiconductors to visualize nanostructures buried underneath the resist and to subsequently write the pattern associated with these nanostructures. This single-step process could be used, for example, to make electrical contacts to nanowires (as illustrated in this work) or to design a photonic crystal resonator centered on a single quantum dot. Fabrication speed and positioning accuracy are significantly increased as compared to the standard process since no alignment marks and the mapping step of the nanostructures with respect to these marks are needed.

Dynamics of long-range ordering in an exciton-polariton condensate.

We report on time-resolved measurements of the first order spatial coherence in an exciton-polariton Bose-Einstein condensate. Long-range spatial coherence is found to set in right at the onset of stimulated scattering, on a picosecond time scale. The coherence reaches its maximum value after the population and decays slowly, staying up to a few hundred picoseconds.

Zinc oxide nanorod based photonic devices: recent progress in growth, light emitting diodes and lasers.

Zinc oxide (ZnO), with its excellent luminescent properties and the ease of growth of its nanostructures, holds promise for the development of photonic devices. The recent advances in growth of ZnO nanorods are discussed. Results from both low temperature and high temperature growth approaches are presented.

Spatial inhomogeneity of donor bound exciton emission from ZnO nanostructures grown on Si.

We report low temperature cathodoluminescence spectroscopy measurements of the band edge emission from ZnO nanostructures grown by vapour phase transport on Si. A range of donor bound exciton emission lines are found and the Al-related emission at 3.3605 eV in particular shows a marked inhomogeneity in its distribution throughout the sample.

Formation of an exciton polariton condensate: thermodynamic versus kinetic regimes.

We measure the polariton distribution function and the condensation threshold versus the photon-exciton detuning and the lattice temperature in a CdTe microcavity under nonresonant pumping. The results are reproduced by simulations using semiclassical Boltzmann equations. At negative detuning we find a kinetic condensation regime: the distribution is not thermal and the threshold is governed by the relaxation kinetics.

Intrinsic decoherence mechanisms in the microcavity polariton condensate.

The fundamental mechanisms which control the phase coherence of the polariton Bose-Einstein condensate (BEC) are determined. It is shown that the combination of number fluctuations and interactions leads to decoherence with a characteristic Gaussian decay of the first-order correlation function. This line shape, and the long decay times ( approximately 150 ps) of both first- and second-order correlation functions, are explained quantitatively by a quantum-optical model which takes into account interactions, fluctuations, and gain and loss in the system.

Synchronized and desynchronized phases of exciton-polariton condensates in the presence of disorder.

Condensation of exciton polaritons in semiconductor microcavities takes place despite in-plane disorder. Below the critical density, the inhomogeneity of the disorder limits the spatial extension of the ground state. Above the critical density, in the presence of weak disorder, this limitation is spontaneously overcome by the nonlinear interaction, resulting in an extended synchronized phase.

Second-order time correlations within a polariton Bose-Einstein condensate in a CdTe microcavity.

Second-order time correlations of polaritons have been measured across the condensation threshold in a CdTe microcavity. The onset of Bose-Einstein condensation is marked by the disappearance of photon bunching, demonstrating the transition from a thermal-like state to a coherent state. Coherence is, however, degraded with increasing polariton density, most probably as a result of self-interaction within the condensate and scatterings with noncondensed excitons and polaritons.

Spatial fluctuations of optical emission from single ZnO/MgZnO nanowire quantum wells.

MgZnO/ZnO quantum wells on top of ZnO nanowires were grown by pulsed laser deposition. Ensembles of spatially fluctuating and narrow cathodoluminescence peaks with single widths down to 1 meV were found at the spectral position of the quantum well emission at 4 K. In addition, the number of these narrow QW peaks increases with increasing excitation power in micro-photoluminescence, thus pointing to quantum-dot-like emission centers.

Bose-Einstein condensation of exciton polaritons.

Phase transitions to quantum condensed phases--such as Bose-Einstein condensation (BEC), superfluidity, and superconductivity--have long fascinated scientists, as they bring pure quantum effects to a macroscopic scale. BEC has, for example, famously been demonstrated in dilute atom gas of rubidium atoms at temperatures below 200 nanokelvin. Much effort has been devoted to finding a solid-state system in which BEC can take place.

Spontaneous coherent phase transition of polaritons in CdTe microcavities.

We report on evidence for polariton condensation out of a reservoir of incoherent polaritons. Polariton population and first-order coherence are investigated by spectroscopic imaging of the far-field emission of a CdTe-based microcavity under nonresonant pumping. With increasing pumping power, stimulated emission with thresholdlike behavior and spectral narrowing is observed in the strong exciton-photon coupling regime.

Polariton parametric amplifier pump dynamics in the coherent regime.

We studied the pump coherent dynamics in a II-VI microcavity parametric amplifier, using angle-resolved four-wave mixing. The polariton parametric amplification is found to result in a strong quenching and saturation of the pump coherence lifetime above the threshold. For the polariton scattering processes that remain below the amplification threshold, we find an angle-dependent collision broadening associated with the efficiency of the polariton scattering towards the excitonic reservoir.