Bulk GaN Microcavity Research Articles

Exciton-polariton laser

We present a review of the investigations realized in the last decades of the phenomenon of the Bose-Einstein condensation (BEC) in the system of two-dimensional cavity polaritons in semiconductor nanostructures. The conditions at which the excitons interacting with cavity photons form new type of quasiparticles named as polaritons are described. Since polaritons can form in a microcavity a weakly interacting Bose gas, similarly to the exciton gas in semiconductors, the microcavity exciton-polariton BEC emerged in the last decades as a new direction of the exciton BEC in solids, promising for practical applications. The high interest in BEC of exciton-polaritons in semiconductor microcavities is related to the ultra-low threshold lasing which has been demonstrated, in particular, for an electrically injected polariton laser based on bulk GaN microcavity diode working at room temperature.

All-dielectric GaN microcavity: Strong coupling and lasing at room temperature

The strong light-matter coupling regime and lasing in a GaN microcavity fabricated by incorporating a high optical quality GaN membrane inside an all-dielectric mirror cavity is demonstrated at room temperature. A nonlinear increase of the emission and line narrowing marks the onset of polariton lasing regime with significantly reduced threshold compared with previous reports for bulk GaN microcavity. This combination of low lasing thresholds and ease of fabrication allows incorporation of quantum wells and electrical contacts into the active region, paving the way for electrically driven room temperature (RT) polariton laser devices.

Influence of optical confinement and excitonic absorption on strong coupling in a bulk GaN microcavity grown on silicon

We report the optical study of a lambda-thick GaN microcavity grown by molecular beam epitaxy on a silicon substrate. Angle-resolved reflectivity measurements evidence the strong coupling regime at room temperature on the half cavity (without the top mirror), but at low temperature, the high excitonic absorption quenches the optical cavity mode at the excitonic energies. On the whole microcavity, the improved quality factor leads to the observation of the polariton emission whatever the temperature. No bottleneck is observed at 70K even at low pumping power and large negative detuning. The impact of the optical confinement and the excitonic absorption, studied through reflectivity measurements are accurately reproduced by the transfer-matrix formalism. The optimization of the design in this structure leads to a large Rabi splitting (52meV) at room temperature.

Strong light-matter coupling in bulk GaN-microcavities with double dielectric mirrors fabricated by two different methods

Two routes for the fabrication of bulk GaN microcavities embedded between two dielectric mirrors are described, and the optical properties of the microcavities thus obtained are compared. In both cases, the GaN active layer is grown by molecular beam epitaxy on (111) Si, allowing use of selective etching to remove the substrate. In the first case, a three period Al0.2Ga0.8N/AlN Bragg mirror followed by a λ/2 GaN cavity are grown directly on the Si. In the second case, a crack-free 2 μm thick GaN layer is grown, and progressively thinned to a final thickness of λ. Both devices work in the strong coupling regime at low temperature, as evidenced by angle-dependent reflectivity or transmission experiments. However, strong light-matter coupling in emission at room temperature is observed only for the second one. This is related to the poor optoelectronic quality of the active layer of the first device, due to its growth only 250 nm above the Si substrate and its related high defect density. The reflectivity spectra of the microcavities are well accounted for by using transfer matrix calculations.

Modelling of strong coupling regime in bulk GaN microcavities using transfer matrix and quasiparticle formalisms

The evolution of the polaritonic Rabi splitting versus the excitonic broadenings is analyzed within two different formalisms: the transfer-matrix and the quasiparticle models. The splittings are deduced from angle-resolved reflectivity measurements performed at 5 K and 300 K on two GaN microcavities grown on silicon. Although the quasiparticle model allows a qualitative analysis when the quality factor of the cavity is high, the transfer-matrix formalism is more appropriate for the interpretation of the reflectivity spectra, whatever the configuration of the structure.

Theory of an electrically injected bulk polariton laser

We present the first simulation of an electrically injected polariton laser device based on Bose–Einstein condensation of exciton polaritons in a bulk GaN microcavity. Our simulator self-consistently describes carrier flow, exciton formation and dissociation, and relaxation of exciton polaritons toward the ground state. We find a density-current threshold of about 50 A/cm2 at room temperature, which could be further improved by using polariton lasers based on quantum wells.

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.

Polariton relaxation bottleneck and its thermal suppression in bulk GaN microcavities

We discuss the presence of a polariton bottleneck in bulk GaN microcavities. Angle resolved photoluminescence measurements were performed using low excitation power densities for several negative detunings between the exciton and photon modes. At low temperatures, we observe an enhancement in the emission intensity at angles corresponding to the anticrossing of the lower and upper polariton modes, a clear demonstration of the polariton relaxation bottleneck. This feature becomes less prominent with increasing temperature, eventually disappearing at room temperature. We conclude that polariton-acoustic phonon scattering is the dominant polariton relaxation mechanism in bulk GaN microcavities, as supported by theoretical simulations.

Polariton emission in GaN microcavities

Abstract We present a detailed experimental study aimed at demonstrating the polariton nature of the photoluminescence emission in a bulk GaN microcavity grown on (111) silicon. The comparison of the photoluminescence with coincident reflectivity measurements at different temperatures shows an anticrossing behaviour with a Rabi splitting of the order of 35 meV up to room temperature. Relevant confirmations of the mixing between excitons and photons are found in the analysis of the spectral linewidth and of the time resolved kinetics.

A polariton laser based on a bulk GaN microcavity

Abstract Simulations of polariton relaxation in a bulk GaN microcavity are performed by solving semi-classical Boltzmann equations. The structure which is considered is that of a recently grown microcavity showing strong coupling at 300 K. A kinetic phase diagram is calculated. We show that such a cavity can operate as a low-threshold polariton laser at room temperature.

Room-Temperature Polariton Lasing in Semiconductor Microcavities

We observe a room-temperature low-threshold transition to a coherent polariton state in bulk GaN microcavities in the strong-coupling regime. Nonresonant pulsed optical pumping produces rapid thermalization and yields a clear emission threshold of 1 mW, corresponding to an absorbed energy density of 29 microJ cm-2, 1 order of magnitude smaller than the best optically pumped (In,Ga)N quantum-well surface-emitting lasers (VCSELs). Angular and spectrally resolved luminescence show that the polariton emission is beamed in the normal direction with an angular width of +/-5 degrees and spatial size around 5 microm.

Strong coupling in bulk GaN microcavities grown on silicon

AbstractWe have studied GaN microcavities grown on silicon by angle resolved reflectivity at 5 and 300 K. A Bragg mirror made up of λ/4 AlGaN and AlN layers has been grown on a silicon substrate. The GaN active layer whose thickness is equal to λ/2 is covered by a 100 Å thick aluminium layer which acts as the top mirror. Transfer matrix simulations are carried out to interpret the reflectivity spectra from which crucial parameters of the active layer such as the oscillator strengths, the excitonic energies and the associated broadenings of A and B excitons are deduced. At low temperature the strong coupling regime between with A and B excitons and the cavity photonic mode is clearly observed. At room temperature the splitting is evaluated to 45 meV. In order to get further insight in the physics of nitride‐based microcavities and especially to improve the exciton‐photon coupling, the influence of the Bragg or metallic mirrors on the cavity is analysed. The inhomogeneous broadening of excitons together with the strain gradient experienced by the active layer along the growth axis are also investigated. All of these effects on the Rabi splitting are analysed and discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Strong light–matter coupling in GaN microcavities grown on silicon(111) at room temperature

We present experimental results demonstrating strong light–matter coupling at room temperature in bulk GaN microcavities grown on silicon(111). Simple low finesse planar microcavities show a Rabi energy as high as 60 meV at room temperature. We also demonstrate room temperature strong-coupling from a bulk GaN microcavity with epitaxial distributive Bragg reflectors (DBRs). Furthermore, for this structure at low temperature, the strong coupling of both the A and B excitonic features of GaN with the cavity mode are clearly resolved for the first time in such a microcavity. At room temperature a Rabi energy of 50 meV is observed, and well reproduced using numerical analysis describing the interaction of both the A and B excitonic states with the photonic mode. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Strong coupling of light withAandBexcitons in GaN microcavities grown on silicon

We present experimental results demonstrating strong-light matter coupling at low and room temperature in bulk GaN microcavities with epitaxial (Al,Ga)N Bragg mirrors grown on silicon (111). At low temperature, the strong coupling of both the $A$ and $B$ excitonic features of GaN with the cavity mode is clearly resolved in the microcavity. At room temperature a Rabi energy of $50\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ is observed and well reproduced using transfer-matrix reflectivity calculations describing the interaction of both the $A$ and $B$ excitonic states with the photonic mode.

Room temperature Strong coupling in low finesse GaN microcavities

AbstractWe present experimental results demonstrating strong-light matter coupling at low and room temperature in bulk GaN microcavities. Angle dependent reflectivity measurements demonstrate strong-coupling with a Rabi-energy of 50meV at room temperature which is well reproduced with transfer matrix simulations. The absence of strong coupling in the photoluminescence is attributed to the low finesse of the microcavity (Q=60) and is confirmed by simulations which indicate a quality factor of 90 is required to observe strong-coupling in the emission.

Observation of Rabi splitting in a bulk GaN microcavity grown on silicon

We report the experimental observation of the strong-coupling regime in a nitride-based microcavity. The active layer in the optical cavity consists of a \ensuremath{\lambda}/2 GaN layer sandwiched between a dielectric mirror and the silicon substrate, which acts as the bottom mirror. Reflectivity measurements have been performed under various angles of incidence at $T=5\mathrm{K},$ producing evidence of strong-coupling behavior between the exciton and the cavity mode. A Rabi splitting of 31 meV is obtained experimentally. Transfer-matrix simulations have allowed us to account for the exciton-photon interaction. From these calculations, the oscillator strengths of the $A$ and $B$ excitons are evaluated and these values are in good agreement with those previously determined in bulk GaN.