Wei Xie

Whispering gallery modes in indium oxide hexagonal microcavities

The work is funded by the NSFC 973 projects and nSTCSM of China Grant Nos. 2004CB619004 and n2006CB921506. The authors thank the Australian Government nDepartment of Innovation, Industry, Science and Research nfor funding this collaborative research under the International nScience Linkages China Program.

Quasi-whispering gallery modes of exciton-polaritons in a ZnO microrod

We report the photoluminescence (PL) investigations of quasi-whispering gallery mode (quasi-WGM) polaritons in a ZnO microrod at room temperature. By using the confocal micro-PL spectroscopic technique, we observe the clear optical quasi-WGMs. These quasi-WGMs appeared in the ultraviolet (UV) emission region where the cavity modes strongly couple with excitons and form polaritons. The quasi-WGMs polaritons can be well described by the plane wave interference and the coupling oscillator model.

Spin-Resolved Purcell Effect in a Quantum Dot Microcavity System

We demonstrate the spin selective coupling of the exciton state with cavity mode in a single quantum dot (QD)-micropillar cavity system. By tuning an external magnetic field, each spin polarized exciton state can be selectively coupled with the cavity mode due to the Zeeman effect. A significant enhancement of spontaneous emission rate of each spin state is achieved, giving rise to a tunable circular polarization degree from -90% to 93%. A four-level rate equation model is developed, and it agrees well with our experimental data. In addition, the coupling between photon mode and each exciton spin state is also achieved by varying temperature, demonstrating the full manipulation over the spin states in the QD-cavity system. Our results pave the way for the realization of future quantum light sources and the quantum information processing applications.

Weak lasing in one-dimensional polariton superlattices

Significance Bose–Einstein condensation of polaritons in periodically modulated cavities is a very interesting fundamental effect of the physics of many-body systems. It is also promising for application in solid-state lighting and information communication technologies. By a simple microassembling method, we created periodically modulated polariton condensates at room temperature, and observed the stabilization of the coherent condensate due to the spontaneous symmetry-breaking transition. This manifests a previously unidentified type of phase transition, leading to a novel state of matter: the weak lasing state. The optical imaging in both direct and reciprocal space provides clear evidence for the weak lasing in the specific range of the pumping intensities. Bosons with finite lifetime exhibit condensation and lasing when their influx exceeds the lasing threshold determined by the dissipative losses. In general, different one-particle states decay differently, and the bosons are usually assumed to condense in the state with the longest lifetime. Interaction between the bosons partially neglected by such an assumption can smear the lasing threshold into a threshold domain—a stable lasing many-body state exists within certain intervals of the bosonic influxes. This recently described weak lasing regime is formed by the spontaneously symmetry breaking and phase-locking self-organization of bosonic modes, which results in an essentially many-body state with a stable balance between gains and losses. Here we report, to our knowledge, the first observation of the weak lasing phase in a one-dimensional condensate of exciton–polaritons subject to a periodic potential. Real and reciprocal space photoluminescence images demonstrate that the spatial period of the condensate is twice as large as the period of the underlying periodic potential. These experiments are realized at room temperature in a ZnO microwire deposited on a silicon grating. The period doubling takes place at a critical pumping power, whereas at a lower power polariton emission images have the same periodicity as the grating.

Polariton lasing of quasi-whispering gallery modes in a ZnO microwire

We report the experimental observation of the quasi-whispering-gallery mode (quasi-WGM) polariton lasing in a one dimensional ZnO microrod cavity at room temperature. Under nonresonant optical pulsed excitation, we measure the pumping power dependence of the emission intensity and the blueshift of the polariton modes, and a fairly low lasing threshold for the quasi-WG mode is determined. We also measure the lasing threshold versus the photon-exciton detuning by dispersion simulations. These experiment results show that both of the WGMs with higher Q factors and the quasi-WGMs with lower Q factors can lase when the detuning between the optical modes and the exciton energies is optimum.

Single-crystalline polyhedral In2O3 vertical Fabry–Pérot resonators

High-quality In2O3 polyhedrons with different morphologies were fabricated and studied as vertical Fabry–Perot (FP) microcavities. By using the microphotoluminescence technique, we identified a series of FP resonant modes in such systems, whose energies lie in the visible spectral range and are scalable with the cavity size. Experimental results are in good agreement with full-wave numerical simulations and can be well fitted with a plane wave interference model. Compared with the conventional one dimensional nanowire/nanobelt FP cavities, such vertical FP microcavities have much less optical loss and provide efficient optical modulations, which may find many applications in developing optical devices.

Polariton lasing in a ZnO microwire above 450 K

Exciton-polariton lasing in a one-dimensional ZnO microcavity is demonstrated at high temperature of 455u2009K. The massive occupation of the polariton ground state above a distinct pump power threshold is clearly demonstrated by using the angular resolved spectroscopy under non-resonant excitation. The temperature dependence of the polariton lasing threshold is well interpreted by two competing mechanisms, i.e., the thermodynamic and kinetic mechanisms. Michelson interference measurements are performed to investigate the temporal and spatial coherence of polariton laser, with the coherence time and coherence length being τc∼0.97u2009ps and rc∼0.72μm at 440u2009K and 400u2009K, respectively.

Robust exciton-polariton effect in a ZnO whispering gallery microcavity at high temperature

A robust exciton-polariton effect in a ZnO whispering gallery microcavity well above room temperature is presented. The lower polariton branches are tuned by current induced thermal effect. The red shift can be as large as ∼40u2009meV. It is found that the strong coupling can be preserved and the polariton-phonon interaction quenching effect remains up to ∼550u2009K, while the Rabi splitting is about 330u2009meV. The tuning speed is in the order of millisecond, showing its potential in polariton-based optoelectronic device application.

Optical and spin polarization dynamics in GaSe nanoslabs

We report nearly complete preservation of “spin memory” between optical absorption and photoluminescence (PL) in nanometer slabs of GaSe pumped with 0.2 eV excess energy. At cryogenic temperatures, the initial degree of circular polarization (ρ0) of PL approaches unity, with the major fraction of the spin polarization decaying with a time constant >500 ps in sub-100-nm GaSe nanoslabs. Even at room temperature, ρ0 as large as 0.7 is observed, while pumping 1 eV above the band edge yields ρ0 = 0.15. Angular momentum preservation for both electrons and holes is due to the separation of the non-degenerate conduction and valence bands from other bands. In contrast to valley polarization in atomically thin transition metal dichalcogenides, here optical spin polarization is preserved in nanoslabs of 100 layers or more of GaSe.We report nearly complete preservation of spin memory between optical absorption and photoluminescence (PL) in nanometer slabs of GaSe pumped with up to 0.2 eV excess energy. At cryogenic temperatures, the initial degree of circular polarization (

Indium oxide octahedra optical microcavities

rho_0