ZnO-based Microcavity Research Articles

ZnO-Based Microcavities Sculpted by Focus Ion Beam Milling.

We reported an easy fabrication method to realize ZnO-based microcavities with various cavity shapes by focused ion beam (FIB) milling. The optical characteristics of different shaped microcavities have been systematically carried out and analyzed. Through comprehensive studies of cathodoluminescence and photoluminescence spectra, the whispering gallery mode (WGM) was observed in different shaped microcavities. Up further increasing excitation, the lasing action was dominated by these WGMs and matched very well to the simulated results. Our experiment shows that ZnO microcavities with different shapes can be made with high quality by FIB milling for specific applications of microlight sources and optical devices.

Strong light–matter interaction in ZnO microcavities

The strong light–matter interaction in ZnO-embedded microcavities has received great attention in recent years, due to its ability to generate the robust bosonic quasiparticles, exciton-polaritons, at or above room temperature. This review introduces the strong coupling effect in ZnO-based microcavities and describes the recent progress in this field. In addition, the report contains a systematic analysis of the room-temperature strong-coupling effects from relaxation to polariton lasing. The stable room temperature operation of polaritonic effects in a ZnO microcavity promises a wide range of practical applications in the future, such as ultra-low power consumption coherent light emitters in the ultraviolet region, polaritonic transport, and other fundamental of quantum optics in solid-state systems.

From Excitonic to Photonic Polariton Condensate in a ZnO-Based Microcavity

We report exciton-polariton condensation in a new family of fully hybrid ZnO-based microcavity demonstrating the best-quality ZnO material available (a bulk substrate), a large quality factor (~4000) and large Rabi splittings (~240 meV). Condensation is achieved between 4 and 300 K and for excitonic fractions ranging between 17% and 96%, which corresponds to a tuning of the exciton-polariton mass, lifetime, and interaction constant by 1 order of magnitude. We demonstrate mode switching between polariton branches allowing, just by controlling the pumping power, to tune the photonic fraction by a factor of 4.

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.

Room temperature polariton lasing vs photon lasing in a ZnO-based hybrid microcavity

We report on the room temperature polariton lasing and photon lasing in a ZnO-based hybrid microcavity under optical pumping. A series of experimental studies of the polariton lasing (exciton-photon detunings of δ = -119 meV) in the strong-coupling regime are discussed and compared to a photon lasing (δ = -45 meV) in the weak-coupling regime obtained in the same structure. The measured threshold power density (31.8 kW/cm2) of polariton lasing is one order of magnitude lower than that of the photon lasing (318.2 kW/cm2). In addition, the comparison between polariton lasing and photon lasing is done in terms of the linewidth broadening, blue-shift of the emission peak, and polarization.

Ballistic propagation of exciton–polariton condensates in a ZnO-based microcavity

We report on the observation of macroscopically coherent states of exciton–polaritons in a ZnO-based bulk planar microcavity up to 250 K. Excitation power-dependent photoluminescence investigations show a clear threshold behaviour and corresponding spectral narrowing of the emission for negative detunings, revealing clear signatures of a Bose–Einstein condensate. For positive detunings, no condensation occurred but the emission from an electron–hole plasma was detected. Above threshold interscattering phenomena of condensate polaritons between roughly equidistant energy levels have been observed. As a special feature we found ballistic propagation of the condensate in the pump-induced potential landscape, making these ZnO-based microcavities promising candidates for applications based on polariton transport. This effect is caused by strong repulsive interactions in our system, leading to an immense blueshift of the condensate emission and hence to pronounced dynamic effects.

Exciton–polaritons in a ZnO-based microcavity: polarization dependence and nonlinear occupation

We report on the occupation of the lower exciton–polariton branch in a ZnO-based microcavity as a function of the detuning between the exciton and the uncoupled cavity-photon mode and on the optical excitation density. We emphasize the difference in the dispersion and occupation of the lower polariton branch as a function of the linear polarization of the emitted light. For the negative detuning regime, we found an energy splitting between the transverse electric (TE)- and transverse magnetic (TM)-polarized states at in-plane wave vectors between 0.4×107 m−1 and 1.2×107 m−1, which is caused by the polarization dependence of the dispersion of the uncoupled cavity-photon mode. The maximum energy splitting of about 6 meV was observed for a detuning of about Δ=−70 meV. From the integrated photoluminescence peak, we deduce the occupation of the lower polariton branch as well as the scattering rates of exciton–polaritons into the lower polariton branch. We found that the energy splitting causes an enhanced scattering of exciton–polaritons into the lower polariton branch for the TM-polarized light compared with that of the TE-polarized light. By varying the excitation density, we observe a superlinear growth of the lower polariton branch occupation for negative and intermediate detuning regimes. For an accumulation of exciton–polaritons in the ground state at low temperatures (T=10 K), we found an intermediate detuning regime (−20 meV<Δ<+20 meV) as the optimum. With increasing temperature, this optimum detuning range shifts to larger negative values.

Characteristics of exciton-polaritons in ZnO- based hybrid microcavities

Wide bandgap semiconductors are promising materials for the development of polariton-based optoelectronic devices operating at room temperature (RT). We report the characteristics of ZnO-based microcavities (MCs) in the strong coupling regime at RT with a vacuum Rabi splitting of 72 meV. The impact of scattering states of excitons on polariton dispersion is investigated. Only the lower polariton branches (LPBs) can be clearly observed in ZnO MCs since the large vacuum Rabi splitting pushes the upper polariton branches (UPBs) into the scattering absorption states in the ZnO bulk active region. In addition, we systematically investigate the polariton relaxation bottleneck in bulk ZnO-based MCs. Angle-resolved photoluminescence measurements are performed from 100 to 300 K for different cavity-exciton detunings. A clear polariton relaxation bottleneck is observed at low temperature and large negative cavity detuning conditions. The bottleneck is suppressed with increasing temperature and decreasing detuning, due to more efficient phonon-assisted relaxation and a longer radiative lifetime of the polaritons.

Vertical cavity surface emitting laser action of an all monolithic ZnO-based microcavity

We report on room temperature laser action of an all monolithic ZnO-based vertical cavity surface emitting laser (VCSEL) under optical pumping. The VCSEL structure consists of a 2λ microcavity containing eight ZnO/Zn0.92Mg0.08O quantum wells embedded in epitaxially grown Zn0.92Mg0.08O/Zn0.65Mg0.35O distributed Bragg reflectors (DBRs). As a prerequisite, design and growth of high reflectivity DBRs based on ZnO and (Zn,Mg)O for optical devices operating in the ultraviolet and blue-green spectral ranges are discussed.

Large vacuum Rabi splitting in ZnO-based hybrid microcavities observed at room temperature

Wide-band gap ZnO semiconductors are attractive materials for the investigation of microcavity exciton polaritons due to the large exciton binding energy and oscillator strength. We report the growth and characterization of bulk ZnO-based hybrid microcavity. The phenomenon of strong exciton-photon coupling at room temperature has been observed in the ZnO-based hybrid microcavity structure, which consists of 30 pair epitaxially grown AlN/AlGaN distributed Bragg reflector (DBR) on the bottom side of the 3/2λ thick ZnO cavity and 9 pair SiO2/HfO2 DBR as the top mirror. The cavity quality factor is about 221. The experimental results show good agreement with theoretically calculated exciton-polariton dispersion curves based on transfer matrix method. From the theoretical and experimental exciton-polariton dispersion curves with two different cavity-exciton detuning values, the large vacuum Rabi splitting is estimated to be about 58 meV in the ZnO-based hybrid microcavity.

Optical study of bulk ZnO for strong coupling observation in ZnO-based microcavities

We report an experimental study of the excitonic properties of bulk ZnO and the strong coupling observation in hybrid ZnO-based microcavities. The strong coupling is highlighted with a Rabi splitting value of about 70 meV. The influence of the excitonic and band-to-band absorptions on the observation of this strong light-matter coupling regime is analysed through the evolution of the reflectivity spectra obtained on microcavities with different active layer thicknesses.

Cavity polaritons in ZnO-based hybrid microcavities

Among wide-bandgap semiconductors, ZnO is a very attractive candidate for blue-ultraviolet lasers operating at room temperature owing to its large exciton binding energy and oscillator strength. Especially, ZnO-based microcavity structures are most conducive for polariton lasing at room temperature. We report the observation of cavity polaritons in bulk ZnO-based hybrid microcavities at room temperature. The bulk ZnO-based hybrid microcavities are composed of 29 pairs of Al0.5Ga0.5N∕GaN distributed Bragg reflector (DBR) at the bottom of the λ-thick cavity layer and eight pairs of SiO2∕Si3N4 DBR as the top mirror, which provided cavity Q values of ∼100. Anticrossing behavior between the lower and upper polariton branches was observed at room temperature. From the polariton dispersion curve, the vacuum Rabi splitting was estimated to be ∼50meV. These results are promising toward the realization of ZnO-based microcavity polariton devices.

Polariton Bose condensation in microcavities

Critical conditions for the Bose condensation of exciton-polaritons in conventional microcavities containing quantum wells are derived and analyzed. Using the model of an interacting Bose-gas we have derived a simple formula for the critical temperature of condensation of the polaritons. In the alternative model of Bose condensation in a system having a discrete spectrum of eigen states resulting from the interaction of localized excitons with light, we have found the critical conditions as well. In the latter case the critical temperature of condensation is governed not only by the shape of the polariton in-plane dispersion curve but also by the size of the light-spot that excites the system. The regime of condensation is strongly dependent on the spatial scale of the in-plane potential disorder in the quantum well. We estimate the critical temperature of the polariton Bose condensation as up to 20 K in GaAs-based microcavities, 100 K in CdTe-based microcavities, and more than 300 K in hypothetic GaN- or ZnO-based microcavities. We conclude that the most serious obstacle for the experimental observation of polariton lasing is the kinetic blocking of the polariton relaxation down to its ground state in a microcavity. Solving numerically the classical Boltzmann equations for the exciton-polaritons in the cavity, we observe the strong kinetic blocking (“bottleneck”) effect. We show that the polariton relaxation becomes much more efficient in n-doped microcavities due to the electron–polariton scattering.

ZnO as a Material Mostly Adapted for Realisation of Room-Temperature Polariton Lasers

Wannier-Mott excitons in the wurzite-type semiconductor material ZnO are stable at room temperature, have an extremely large oscillator strength and emit purple light. This makes ZnO an excellent potential candidate for the fabrication of room-temperature lasers where the coherent light amplification is ruled by the fascinating mechanism of the Bose-condensation of the exciton-polaritons. We first report on the direct optical measurement of the exciton oscillator strength f in ZnO. The longitudinal transverse splitting of the exciton resonances Γ 5 (B) and Γ 1 (C) are found to achieve record values of 5 and 7 meV, respectively, that is two orders of magnitude larger than that in GaAs. Secondly, we propose a model ZnO-based microcavity structure that is found to be the most adapted structure for the observation of the polariton laser effect. We thus can compute the phase diagram of the lasing regimes. A record value of the threshold power of 2 mW per device (at power density of 3000 W/cm 2 ) at room temperature is found for the model laser structure.

ZnO as a material mostly adapted for the realization of room-temperature polariton lasers

Wannier-Mott excitons in the wurzite-type semiconductor material ZnO are stable at room temperature, have an extremely large oscillator strength, and emit blue light. This makes ZnO an excellent potential candidate for the fabrication of room-temperature lasers where the coherent light amplification is ruled by the fascinating mechanism of the Bose condensation of the exciton polaritons. We report the direct optical measurement of the exciton oscillator strength f in ZnO. The longitudinal transverse splitting of the exciton resonances Γ 5 (B) and Γ 1 (C) are found to achieve record values of 5 and 7 meV, respectively, that, is two orders of magnitude larger than in GaAs. Second, we propose a model ZnO-based microcavity structure that is found to be the most adapted structure for the observation of the polariton laser effect. We thus can compute the phase diagram of the lasing regimes. A record value of the threshold power of 2 mW per device (at power density of 3000 W/cm 2 ) at room temperature is found for the model laser structure.