Fig. 1. The microcavity structure and optical property of ZnO QWs. (a) Schematic diagram for vertical cavity structure of ZnO QWs. The four-period ZnO QWs are grown on the sapphire substrate and then top Zr₂O₃/SiO₂ DBR was deposited on it. (b) PL spectrum of bare ZnO films and ZnO-based QWs at RT. Due to the spatial potential confinement, the radiation energy of exciton in QWs is larger than that of FX in the bare film. (c) Low-temperature (77 K) PL spectrum of QWs, which shows three phonon replica and free exciton.

Fig. 2. Angle-resolved PL spectra of ZnO/MgZnO QWs (left) and theoretical fitting curve (right). The dispersion exhibits a large inhomogeneous broadening with the Rabi splitting of about 60 meV that deduced from fitting. The solid lines represent the coupled UP (blue line) and LP branch (red line), respectively. Meanwhile, the black solid and the horizontal dashed lines point the exciton energies and the bare uncoupled cavity mode dispersion for reference.

Fig. 3. Angle-resolved dispersion of exciton-polariton and the simulation results. Color maps of the measured PL spectra are normalized for the purpose of highlighting the variation of polariton dispersion curves (left panel). The measurement is in agreement with the numerically calculated results of rigorous coupled-wave analysis (right panel).

Fig. 4. Analysis of the LP characteristics of polaritons as a function of pumping intensity. (a) As the pump intensity is very lower, the polaritons distributed in k-space are very few. (b), (c) With increasing excitation power, the emission pattern of LP is enhanced dramatically for the case of 96 kW⋅cm⁻² and 106 kW⋅cm⁻². (d) At high pumping power (111 kW⋅cm⁻²), the LP branch exhibits a magnitude population occupancy that accounts for the relaxation between LP and UP.

Fig. 5. Color maps of angle-resolved PL spectra of ZnO QWs at different positions with different detunings. (a) For the large negative detune of −22 meV, the polariton emission is very weak, which can be attributed to the low coupling efficiency between cavity-photon and exciton. The solid (dashed) line shows the calculated dispersion of polariton (bare cavity modes and exciton state). (b) and (c) Images of polariton taken in the cases of δ = −15 meV and −8.8 meV, respectively. (d) The emission pattern of polariton just shows a UP branch solely, as the detuning is about 4.5 meV.