M. Mihailovic

Determination of the refractive indices of AlN, GaN, and AlxGa1−xN grown on (111)Si substrates

The refractive indices of several AlxGa1−xN alloys deposited on silicon are determined by ellipsometry and reflectivity experiments at room temperature. The AlGaN layers are grown on (111)Si substrate by molecular-beam epitaxy on top of an AlN/GaN/AlN buffer in order to reduce the strain of the alloy. The Al composition is deduced from energy dispersive x-ray spectroscopy and photoluminescence experiments. The refractive index n and the extinction coefficient k are determined in the 300–600 nm range. For the transparent region of AlxGa1−xN, the refractive index is given in form of a Sellmeier law.

Temperature dependence of optical properties of h-GaN films studied by reflectivity and ellipsometry

Spectroscopic ellipsometry (SE) carried out at 300 K together with reflectivity measurements performed from 5 to 300 K are used to determine the temperature dependence of the refractive index of hexagonal GaN films between 360 and 600 nm. The refractive index is well described with a Sellmeier dispersion law and its variation with temperature is given. Below the band gap, the three excitonic features (labelled A, B and C) appearing in the reflectivity spectra are analysed within a multi-polariton model which includes the spatial dispersion. The transition energy, broadening parameter and oscillator strength are derived. The temperature dependence of A and B broadening parameters is analysed.

Experimental observation of strong light-matter coupling in ZnO microcavities: Influence of large excitonic absorption

We present experimental observation of the strong light-matter coupling regime in ZnO bulk microcavities grown on silicon. Angle resolved reflectivity measurements, corroborated by transfer-matrix simulations, show that Rabi splittings in the order of 70 meV are achieved even for low finesse cavities. The impact of the large excitonic absorption, which enables a ZnO bulk-like behavior to be observed even in the strong coupling regime, is illustrated both experimentally and theoretically by considering cavities with increasing thickness.

Fabrication and characterization of a room-temperature ZnO polariton laser

A ZnO planar optical microcavity displaying room-temperature polariton lasing over a wide range of cavity-exciton detunings has been fabricated. The cavity combines optimum crystalline quality, given by a ZnO single-crystal substrate, and optimum photonic quality, obtained by the use of two dielectric SiO2/HfO2 Bragg mirrors. A maximum cavity quality factor of about 4000 has been measured. Typically, the polariton lasing transition is accompanied by an increase of the output intensity by more than two orders of magnitude, a reduction of the emission linewidth and a relatively small blueshift of the lower polariton branch (less than 5% of the Rabi splitting).

Laser emission with excitonic gain in a ZnO planar microcavity

The lasing operation of a ZnO planar microcavity under optical pumping is demonstrated from T=80 to 300 K. At the laser threshold, the cavity switches from the strong coupling to the weak coupling regime. A gain-related transition, which appears while still observing polariton branches and, thus, with stable excitons, is observed below 240 K. This shows that exciton scattering processes, typical of II-VI semiconductors, are involved in the gain process.

LO-phonon-assisted polariton lasing in a ZnO-based microcavity

Polariton relaxation mechanisms are analyzed experimentally and theoretically in a ZnO-based polariton laser. A minimum lasing threshold is obtained when the energy difference between the exciton reservoir and the bottom of the lower polariton branch is resonant with the LO phonon energy. Tuning off this resonance increases the threshold, and exciton-exciton scattering processes become involved in the polariton relaxation. These observations are qualitatively reproduced by simulations based on the numerical solution of the semiclassical Boltzmann equations.

Modelling of thermally detected optical absorption and luminescence of (In,Ga)N/GaN heterostructures

Thermally detected optical absorption (TDOA) and photoluminescence (PL) experiments are performed at 0.35 and 4 K, respectively, on InxGa1-xN(x less than or equal to 0.12) layers grown on GaN by molecular beam epitaxy. The modelling of absorption allows us to extract the absorption coefficients and bandgap energies of (In,Ga)N alloy. A bowing parameter equal to 2.4 eV is deduced. The knowledge of the GaN complex refractive index, previously measured, enables us to account for the Fabry-Perot interferences which structure the TDOA and PL spectra. A procedure is proposed to remove the latter in the PL spectrum of nitride based heterostructures, The model is based on the description of the light propagation in an active layer sandwiched between two heterostructures. The parameters deduced from the absorption line shape adjustment are used to take the absorption and optical path into account in the different layers of the samples

Polariton condensation phase diagram in wide-band-gap planar microcavities: GaN versus ZnO

The polariton condensation phase diagram is compared in GaN and ZnO microcavities grown on mesa-patterned silicon substrate. Owing to a common platform, these microcavities share similar photonic properties with large quality factors and low photonic disorder, which makes it possible to determine the optimal spot diameter and to realize a thorough phase diagram study. Both systems have been investigated under the same experimental conditions. The experimental results and the subsequent analysis reveal clearly that longitudinal optical phonons have no influence in the thermodynamic region of the condensation phase diagram, while they allow a strong (slight) decrease of the polariton lasing threshold in the trade-off zone (kinetic region). Phase diagrams are compared with numerical simulations using Boltzmann equations, and are in satisfactory agreement. A lower polariton lasing threshold has been measured at low temperature in the ZnO microcavity, as is expected due to a larger Rabi splitting. This study highlights polariton relaxation mechanisms and their importance in polariton lasing.

Fabrication and Optical Properties of a Fully-Hybrid Epitaxial ZnO-Based Microcavity in the Strong-Coupling Regime

In order to achieve polariton lasing at room temperature, a new fabrication methodology for planar microcavities is proposed: a ZnO-based microcavity in which the active region is epitaxially grown on an AlGaN/AlN/Si substrate and in which two dielectric mirrors are used. This approach allows us to simultaneously obtain a high-quality active layer together with a high photonic confinement as demonstrated through macro-, micro-photoluminescence (µ-PL) and reflectivity experiments. A quality factor of 675 and a maximum PL emission at k=0 are evidenced thanks to µ-PL, revealing an efficient polaritonic relaxation even at low excitation power.

Spectroscopy of a Bulk GaN Microcavity Grown on Si(111)

We report the experimental observation of the exciton–photon strong coupling regime in a GaN microcavity. The structure has been grown by molecular beam epitaxy on a Si(111) substrate. The upper mirror is a SiO2/Si3N4 dielectric mirror and the silicon substrate acts as the bottom one. Angle resolved reflectivity and photoluminescence experiments have allowed to demonstrate the exciton–photon strong coupling regime, characterized by a Rabi splitting of 31 meV at 5 K. From the modeling of experiments, the oscillator strengths of excitons A and B are evaluated and compared to the values previously published. Then, the design of the bulk microcavity is optimized in order to maintain the strong coupling regime at room temperature; our calculations predict a Rabi splitting of 33 meV at 300 K in this case. A second kind of structure based on GaN/AlGaN quantum wells is also proposed, leading to an expected splitting of 19 meV at 300 K.