arXiv subjects

B. Gil

Publications and source records attributed to B. Gil.

27 records · Page 2Linked to original sources

arXiv2016

Efficient single photon emission from a high-purity hexagonal boron nitride crystal

Among a variety of layered materials used as building blocks in van der Waals heterostructures, hexagonal boron nitride (hBN) appears as an ideal platform for hosting optically-active defects owing to its large bandgap ($\sim 6$ eV). Here we study the optical response of a high-purity hBN crystal under green laser illumination. By means of photon correlation measurements, we identify individual defects emitting a highly photostable fluorescence under ambient conditions. A detailed analysis of the photophysical properties reveals a high quantum efficiency of the radiative transition, leading to a single photon source with very high brightness. These results illustrate how the wide range of applications offered by hBN could be further extended to photonic-based quantum information science and metrology.

cond-mat.mtrl-sci↗

arXiv2015

Hexagonal boron nitride is an indirect bandgap semiconductor

Hexagonal boron nitride is a wide bandgap semiconductor with a very high thermal and chemical stability often used in devices operating under extreme conditions. The growth of high-purity crystals has recently revealed the potential of this material for deep ultraviolet emission, with an intense emission around 215 nm. In the last few years, hexagonal boron nitride has been raising even more attention with the emergence of two-dimensional atomic crystals and Van der Waals heterostructures, initiated with the discovery of graphene. Despite this growing interest and a seemingly simple structure, the basic questions of the bandgap nature and value are still controversial. Here, we resolve this long-debated issue by bringing the evidence for an indirect bandgap at 5.955 eV by means of optical spectroscopy. We demonstrate the existence of phonon-assisted optical transitions, and we measure an exciton binding energy of about 130 meV by two-photon spectroscopy.

cond-mat.mtrl-sci↗

arXiv2012

Exciton radiative decay through plasmon modes in planar metal-semiconductor structures

We develop a non-local dielectric response theory to describe the temperature dependence of exciton lifetime in metal-semiconductor heterostructures. Coupling between excitons and surface plasmons results in a strongly nonmonotonous behaviour of exciton radiative decay rate versus temperature. Tuning the plasmon frequency one can control the efficiency of exciton emission of light.

cond-mat.mes-hall↗

arXiv2011

Excitonic parameters of GaN studied by time-of-flight spectroscopy

We refine excitonic parameters of bulk GaN by means of time-of-flight spectroscopy of light pulses propagating through crystals. The influence of elastic photon scattering is excluded by using the multiple reflections of the pulses from crystal boundaries. The shapes of these reflexes in the time-energy plane depict the variation of the group velocity induced by excitonic resonances. Modeling of the shapes, as well as other spectra, shows that a homogeneous width of the order of 10 μeV characterizes the exciton-polariton resonances within the crystal. The oscillator strength of A and B exciton-polaritons is determined as 0.0022 and 0.0016, respectively.

cond-mat.mtrl-sci↗

arXiv2011

Delay and distortion of slow light pulses by excitons in ZnO

Light pulses propagating through ZnO undergo distortions caused by both bound and free excitons. Numerous lines of bound excitons dissect the pulse and induce slowing of light around them, to the extend dependent on their nature. Exciton-polariton resonances determine the overall pulse delay and attenuation. The delay time of the higher-energy edge of a strongly curved light stripe approaches 1.6 ns at 3.374 eV with a 0.3 mm propagation length. Modelling the data of cw and time-of-flight spectroscopies has enabled us to determine the excitonic parameters, inherent for bulk ZnO. We reveal the restrictions on these parameters induced by the light attenuation, as well as a discrepancy between the parameters characterizing the surface and internal regions of the crystal.

cond-mat.mtrl-sci↗

arXiv2009

Relaxation and emission of Bragg-mode and cavity-mode polaritons in a ZnO microcavity at room temperature

The strong coupling regime in a ZnO microcavity is investigated through room temperature photoluminescence and reflectivity experiments. The simultaneous strong coupling of excitons to the cavity mode and the first Bragg mode is demonstrated at room temperature. The polariton relaxation is followed as a function of the excitation density. A relaxation bottleneck is evidenced in the Bragg-mode polariton branch. It is partly broken under strong excitation density, so that the emission from this branch dominates the one from cavity-mode polaritons.

cond-mat.other↗

arXiv2008

Comparison of strong coupling regimes in bulk GaAs, GaN and ZnO semiconductor microcavities

Wide bandgap semiconductors are attractive candidates for polariton-based devices operating at room temperature. We present numerical simulations of reflectivity, transmission and absorption spectra of bulk GaAs, GaN and ZnO microcavities, in order to compare the particularities of the strong coupling regime in each system. Indeed the intrinsic properties of the excitons in these materials result in a different hierarchy of energies between the valence-band splitting, the effective Rydberg and the Rabi energy, defining the characteristics of the exciton-polariton states independently of the quality factor of the cavity. The knowledge of the composition of the polariton eigenstates is central to optimize such systems. We demonstrate that, in ZnO bulk microcavities, only the lower polaritons are good eigenstates and all other resonances are damped, whereas upper polaritons can be properly defined in GaAs and GaN microcavities.

cond-mat.other↗

arXiv2008

Polarized emission of GaN/AlN quantum dots : single dot spectroscopy and symmetry-based theory

We report micro-photoluminescence studies of single GaN/AlN quantum dots grown along the (0001) crystal axis by molecular beam epitaxy on Si(111) substrates. The emission lines exhibit a linear polarization along the growth plane, but with varying magnitudes of the polarization degree and with principal polarization axes that do not necessarily correspond to crystallographic directions. Moreover, we could not observe any splitting of polarized emission lines, at least within the spectral resolution of our setup (1 meV). We propose a model based on the joint effects of electron-hole exchange interaction and in-plane anisotropy of strain and/or quantum dot shape, in order to explain the quantitative differences between our observations and those previously reported on, e.g. CdTe- or InAs-based quantum dots.

cond-mat.other↗

arXiv2006

Spin-Exchange Interaction in ZnO-based Quantum Wells

Wurtzitic ZnO/(Zn,Mg)O quantum wells grown along the (0001) direction permit unprecedented tunability of the short-range spin exchange interaction. In the context of large exciton binding energies and electron-hole exchange interaction in ZnO, this tunability results from the competition between quantum confinement and giant quantum confined Stark effect. By using time-resolved photoluminescence we identify, for well widths under 3 nm, the redistribution of oscillator strengths between the A and B excitonic transitions, due to the enhancement of the exchange interaction. Conversely, for wider wells, the redistribution is cancelled by the dominant effect of internal electric fields, which dramatically reduce the exchange energy.

cond-mat.other↗