arXiv subjects

Le Si Dang

Publications and source records attributed to Le Si Dang.

15 recordsLinked to original sources

arXiv2022

Reduction of the lasing threshold in optically pumped AlGaN/GaN lasers with two-step etched facets

We report a two-step process to obtain smooth and vertical {10-10} m-plane facets in AlGaN/GaN separate confinement heterostructures designed to fabricate UV lasers emitting at 355 nm. The process consists in a dry etching by RIE-ICP combined with a crystallographic-selective wet etching process using a KOH-based solution. The anisotropy in the wet etching rates between the different crystallographic planes of the AlGaN structure, allows the fabrication of flat and parallel facets without a degradation of the multilayered ensemble. The optical performance of the lasers display a major improved when using the two-step process for the definition of the cavity, in comparison to cavities fabricated by mechanical cleaving, with the lasing threshold under optical pumping being reduced to almost half.

physics.optics↗

arXiv2021

Decorrelation of internal quantum efficiency and lasing threshold in AlGaN-based separate confinement heterostructures for UV emission

In this paper, we study the internal quantum efficiency and lasing threshold of AlGaN/GaN separate confinement heterostructures designed for ultraviolet laser emission. We discuss the effect of carrier localization and carrier diffusion on the optical performance. The implementation of graded index separate confinement heterostructures results in an improved carrier collection at the multi-quantum well, which facilitates population inversion and reduces the lasing threshold. However, this improvement is not correlated with the internal quantum efficiency of the spontaneous emission. We show that carrier localization at alloy inhomogeneities results in an enhancement of the radiative efficiency but does not reduce the laser threshold, more sensitive to the carrier injection efficiency.

physics.app-ph↗

arXiv2021

Optical net gain measurement on Al$_{0.07}$Ga$_{0.93}$N/GaN multi-quantum well

We present net gain measurements at room temperature in Al$_{0.07}$Ga$_{0.93}$N/GaN 10-period multi-quantum well emitting at 367 nm, using the variable stripe length method. Measurements were conducted at two different positions on the sample, where the net gain threshold was attained at 218 kW/cm$^{2}$ and 403 kW/cm$^{2}$. At the position with higher threshold, we observed an anomalous amplification of the photoluminescence intensity that occurs for long stripe lengths (superior to 400 $μ$m) and high pumping power (superior to 550 kW/cm$^{2}$), leading to an overestimation of the gain value. We attribute such a phenomenon to the feedback provided by the reflection from cracks, which were created during the epitaxial growth due to the strong lattice mismatch between different layers. The highest gain value without anomalous amplification was 131 cm$^{-1}$, obtained at the maximum pumping power density of the experimental setup (743 kW/cm$^{2}$). Using the intrinsic efficiency limit of the cathodoluminescence process, we estimate a lower limit for electron beam pumped laser threshold at room temperature of 390 kW/cm$^{2}$ for this multi-quantum well structure.

cond-mat.mtrl-sci↗

arXiv2021

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). The graded layers avoid potential barriers induced by polarization differences in the heterostructure and serve as strain transition buffers which reduce the mosaicity of the active region and the linewidth of spontaneous emission.

physics.optics↗

arXiv2015

Determination of the optimal shell thickness for self-catalysed GaAs/AlGaAs core-shell nanowires

We present a set of experimental results identifying various effects that govern the carrier dynamics of self-catalyzed GaAs/AlGaAs core-shell nanowires (NWs) grown by molecular beam epitaxy i.e. surface recombination velocity, surface charge traps, and structural defects. 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 cores strongly depend on AlGaAs shell thickness but in a non-monotonic fashion. Although 7 nm-AlGaAs shell can efficiently suppress the surface recombination velocity of the GaAs NW cores, the effect of the band bending caused by the surface charges remains dominant if the shell thickness is less than 50 nm; that is, the carrier diffusion length is smaller in the NWs with a thinner shell caused by a stronger carrier scattering at the core/shell interface. If the AlGaAs shell thickness is larger than 50 nm, the luminescence efficiency of the GaAs NW cores starts to be deteriorated, ascribed to the defect formation inside the AlGaAs shell evidenced by transmission electron microscopy.

cond-mat.mtrl-sci↗

arXiv2014

Alloy inhomogeneity and carrier localization in AlGaN sections and AlGaN/AlN nanodisks in nanowires with 240-350 nm emission

The Al-Ga intermixing at Al(Ga)N/GaN interfaces in nanowires and the chemical inhomogeneity in AlxGa1-xN/AlN nanodisks (NDs) are attributed to the strain relaxation process. This interpretation is supported by the three-dimensional strain distribution calculated by minimizing the elastic energy in the structure. The alloy inhomogeneity increases with Al content, leading to enhanced carrier localization signatures in their optical characteristics i.e. red shift of the emission, s-shaped temperature dependence and linewidth broadening. Despite these alloy fluctuations, the emission energy of AlGaN/AlN NDs can be tuned in the 240-350 nm range with internal quantum efficiencies around 30%.

cond-mat.mtrl-sci↗

arXiv2014

The dynamics of amplified spontaneous emission in CdSe/ZnSe quantum dots

We have used the variable stripe technique and pump-probe spectroscopy to investigate both gain and the dynamics of amplified spontaneous emission from CdSe quantum dot structures. We have found modal gain coefficients of 75 and 32 1/cm for asymmetric and symmetric waveguide structures, respectively. Amplified spontaneous emission decay times of 150 and 300 ps and carrier capture times of 15 and 40 ps were measured for the structures with high and low material gains respectively. The difference in the capture times are related to the fact that for the symmetric waveguide, carriers diffuse into the active region from the uppermost ZnMgSSe cladding layer, yielding a longer rise time for the pump-probe signals for this sample.

cond-mat.mtrl-sci↗

arXiv2010

One-dimensional ZnO exciton polaritons with negligible thermal broadening at room temperature

Phonon damping is the main source of pure dephasing in the solid state, limiting many fundamental quantum effects to low temperature observations. Here we show how excitons in semiconductors can be totally decoupled from the phonon bath, even at room temperature, thanks to their strong interaction with photons. To do so, we investigated ZnO microwires, a new semiconductor nanostructure made of large band-gap material where the light can be trapped and guided into whispering gallery modes. In this system, the very large coupling regime between exciton and photon results in unusual exciton-polariton of one-dimensional character and Rabi splitting as large as 300meV. We find that polariton modes of excitonic fraction up to 75% exhibit no thermal broadening up to room temperature. We show that this remarkable behavior is due to the very large Rabi splitting as compared to the LO phonon energy.

cond-mat.mes-hall↗

arXiv2009

Dynamics of long-range order 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 slower, staying up to a few hundreds of picoseconds. This behavior can be qualitatively reproduced, using a stochastic classical field model describing interaction between the polariton condensate and the exciton reservoir within a disordered potential.

cond-mat.quant-gas↗

arXiv2009

Coexisting Non-Equilibrium Condensates with Long-Range Spatial Coherence in Semiconductor Microcavities

Real and momentum space spectrally resolved images of microcavity polariton emission in the regime of condensation are investigated under non resonant excitation using a laser source with reduced intensity fluctuations on the timescale of the exciton lifetime. We observe that the polariton emission consists of many macroscopically occupied modes. Lower energy modes are strongly localized by the photonic potential disorder on a scale of few microns. Higher energy modes have finite k-vectors and are delocalized over 10-15 microns. All the modes exhibit long range spatial coherence comparable to their size. We provide a theoretical model describing the behavior of the system with the results of the simulations in good agreement with the experimental observations. We show that the multimode emission of the polariton condensate is a result of its nonequilibrium character, the interaction with the local photonic potential and the reduced intensity fluctuations of the excitation laser.

cond-mat.other↗

arXiv2008

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 potential seen by the polaritons strongly limits the spatial extension of the ground state. Above the critical density, in presence of weak disorder, this limitation is spontaneously overcome by the non linear interaction, resulting in an extended synchronized phase. This mechanism is clearly evidenced by spatial and spectral studies, coupled to interferometric measurements. In case of strong disorder, several non phase-locked (independent) condensates can be evidenced. The transition from synchronized phase to desynchronized phase is addressed considering multiple realizations of the disorder.

cond-mat.other↗

arXiv2008

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

The first realization of a polariton condensate was recently achieved in a CdTe microcavity [Kasprzak et al., Nature 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.

cond-mat.mes-hall↗

arXiv2008

Quantised Vortices in an Exciton-Polariton Fluid

One of the most striking quantum effects in a low temperature interacting Bose gas is superfluidity. First observed in liquid 4He, this phenomenon has been intensively studied in a variety of systems for its amazing features such as the persistence of superflows and the quantization of the angular momentum of vortices. The achievement of Bose-Einstein condensation (BEC) in dilute atomic gases provided an exceptional opportunity to observe and study superfluidity in an extremely clean and controlled environment. In the solid state, Bose-Einstein condensation of exciton polaritons has now been reported several times. Polaritons are strongly interacting light-matter quasi-particles, naturally occurring in semiconductor microcavities in the strong coupling regime and constitute a very interesting example of composite bosons. Even though pioneering experiments have recently addressed the propagation of a fluid of coherent polaritons, still no conclusive evidence is yet available of its superfluid nature. In the present Letter, we report the observation of spontaneous formation of pinned quantised vortices in the Bose-condensed phase of a polariton fluid by means of phase and amplitude imaging. Theoretical insight into the possible origin of such vortices is presented in terms of a generalised Gross-Pitaevskii equation. The implications of our observations concerning the superfluid nature of the non-equilibrium polariton fluid are finally discussed.

cond-mat.other↗

arXiv2006

Probing exciton localization in non-polar GaN/AlN Quantum Dots by single dot optical spectroscopy

We present an optical spectroscopy study of non-polar GaN/AlN quantum dots by time-resolved photoluminescence and by microphotoluminescence. Isolated quantum dots exhibit sharp emission lines, with linewidths in the 0.5-2 meV range due to spectral diffusion. Such linewidths are narrow enough to probe the inelastic coupling of acoustic phonons to confined carriers as a function of temperature. This study indicates that the carriers are laterally localized on a scale that is much smaller than the quantum dot size. This conclusion is further confirmed by the analysis of the decay time of the luminescence.

cond-mat.other↗

arXiv2002

Effect of growth conditions on optical properties of CdSe/ZnSe single quantum dots

In this work, we have investigated the optical properties of two samples of CdSe quantum dots by using submicro-photoluminescence spectroscopy. The effect of vicinal-surface GaAs substrates on their properties has been also assessed. The thinner sample, grown on a substrate with vicinal surface, includes only dots with a diameter of less than 10 nm (type A islands). Islands of an average diameter of about 16 nm (type B islands) that are related to a phase transition via a Stranski-Krastanow growth process are also distributed in the thicker sample grown on an oriented substrate. We have studied the evolution of lineshapes of PL spectra for these two samples by improving spatial resolution that was achieved using nanoapertures or mesa structures. It was found that the use of a substrate with the vicinal surface leads to the suppression of excitonic PL emitted from a wetting layer.

cond-mat.mtrl-sci↗