Showing papers by "Le Si Dang published in 2008"

GaN/AlN short-period superlattices for intersubband optoelectronics: A systematic study of their epitaxial growth, design, and performance

Abstract: We have studied the effect of growth and design parameters on the performance of Si-doped GaN/AlN multiquantum-well (MQW) structures for intersubband optoelectronics in the near infrared. The samples under study display infrared absorption in the 1.3–1.9 μm wavelength range, originating from the photoexcitation of electrons from the first to the second electronic level in the QWs. A commonly observed feature is the presence of multiple peaks in both intersubband absorption and interband emission spectra, which are attributed to monolayer thickness fluctuations in the quantum wells. These thickness fluctuations are induced by dislocations and eventually by cracks or metal accumulation during growth. The best optical performance is attained in samples synthesized with a moderate Ga excess during the growth of both the GaN QWs and the AlN barriers without growth interruptions. The optical properties are degraded at high growth temperatures (>720 °C) due to the thermal activation of the AlN etching of GaN. Fr...

Formation of an Exciton Polariton Condensate: Thermodynamic versus Kinetic Regimes

Abstract: 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. At positive detuning, the distribution becomes thermal and the threshold is governed by the thermodynamic parameters of the system. Both regimes are a manifestation of polariton lasing, whereas only the latter is related to Bose-Einstein condensation defined as an equilibrium phase transition.

Intrinsic decoherence mechanisms in the microcavity polariton condensate.

Abstract: 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 (� 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. Interaction limited coherence times of this type have been predicted for atomic BECs, but are yet to be observed experimentally.

Second-Order Time Correlations within a Polariton Bose-Einstein Condensate in a CdTe Microcavity

Abstract: 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. Such behavior clearly differentiates polariton Bose condensation from photon lasing.

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

Abstract: 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. In the case of strong disorder, several non-phase-locked condensates can be evidenced. The transition from a synchronized phase to a desynchronized phase is addressed by sampling the cavity disorder.

Optical and microstructural studies of atomically flat ultrathin In-rich InGaN/GaN multiple quantum wells

Abstract: Optical and microstructural properties of atomically flat ultrathin In-rich (UTIR) InGaN∕GaN multiple quantum well were investigated by means of photoluminescence (PL), time-resolved PL (TRPL), and cathodoluminescence (CL) experiments. The sample exhibits efficient trapping of the photoexcited carriers into quantum wells (QWs) and the effect of internal electric field in the QWs was found negligible by excitation power-dependent PL and TRPL. These phenomena were attributed to the nature of UTIR InGaN QWs, indicating the potential of this system for application in optoelectronic devices. Variation of TRPL lifetime across the PL band and spatially resolved monochromatic CL mapping images strongly suggest that there is micrometer-scale inhomogeneity in effective band gap in UTIR InGaN∕GaN QWs, which is originated from two types of localized areas.

GaN/AlN free-standing nanowires grown by molecular beam epitaxy

Abstract: We report on the growth, structural and optical properties of GaN/AlN free standing NWs. The NW lateral growth rate was investigated using thin AlN marker layers. The lateral growth is found to be homogeneous along the NW axis. Its rate can be tuned from ∼1% to ∼10% of the axial growth rate by changing III/V flux ratio. The resulting axial and lateral heterostructures were studied by micro-photoluminescence and micro-cathodoluminescence, which revealed sharp (several meV broad) emission lines. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Phase diagram for condensation of microcavity polaritons: From theory to practice

Abstract: The first realization of a polariton condensate was recently achieved in a CdTe microcavity [Kasprzak et al., Nature (London) 443, 409 (2006)]. We compare the experimental phase boundaries, for various detunings and cryostat temperatures, with those found theoretically from a model which accounts for features of microcavity polaritons such as reduced dimensionality, internal composite structure, disorder in the quantum wells, polariton-polariton interactions, and finite lifetime.

Bose-Einstein condensation in semiconductors: myth or reality?

Abstract: Bose-Einstein condensation, predicted for a gas of non interacting bosons in 1924 by Einstein, has been demonstrated for the first time in 1995 in a dilute gas of rubidium atoms at temperatures below 10^–6 K. In this work, it is shown that Bose-Einstein condensation can be achieved at around 15 – 20 K in a solid state system by using microcavity polaritons, which are composite bosons of mass ten billion times lighter than that of rubidium atoms.