Exciton-polariton condensate induced by evaporative cooling in a three-dimensionally confined microcavity

We realize the evaporative cooling of a polariton gas and observe the formation of polariton condensate in a potential trap based on a ZnO microwire at room temperature. Due to the three-dimensional confinement, the system is characterized by a well-defined discrete spectrum of polariton states which allows accurate measurements of the polariton population distribution. With increasing pumping power, the polariton gas system is cooled down very efficiently and relaxes closely to the thermal distribution via the polariton evaporative cooling. At an optimized excitation condition, the effective temperature of the polariton gas falls down to 22 K with the condensate fraction reaching nearly 50% in room temperature experiments.

Figure 1

(a) A scanning electron microscope image of a single ZnO microwire. The two black circled areas are the guides to the defective areas of the microwire. (b) Schematic diagram of the evaporative cooling process for the polaritons in a potential trap real space, where $U_0$ is the potential barrier, and its value ($\sim 16$ meV) can be obtained from the experimental data in Fig. 3. The black solid line depicts the potential profile caused by the two adjacent defective areas.

Figure 2

(a) Four typical angle-resolved PL mappings under different excitation powers. (b) The ground state PL intensity and the ratio of the ground state intensity to the sum intensity as a function of pumping power. (c) The emission frequency blueshift (shown as rectangles) and linewidth (shown as circles) narrowing as a function of pumping power. (d) The system effective temperature as a function of the pumping power. Inset: Experimental normalized emission intensity distribution among the discrete energy levels of polaritons at different pumping powers.

Figure 3

(a),(b) Interference pattern of the polariton emission with excitation power $P=1.5$ nW and $P=4.5$ nW. The black dashed lines are the boundary of the potential well. (c) The visibility of the interference fringes measured as a function of the polariton energy. (d),(e) The visibility versus delay between the two arms of the Michelson interferometer under (d) low and (e) high excitation power, respectively. (f) The angle-resolved PL mapping of the evaporated polaritons detected at the barrier. (g) The corresponding PL mapping of trapped polaritons detected at the trap area. The dashed lines in (f) and (g) are the guide lines for the polariton dispersion curves. The experimental configuration is shown schematically in the middle part of the figure.

Figure 4

(a) Four typical PL intensity mappings in momentum space under different excitation powers. (b) The ground state population as a function of the pumping power. (c) The effective temperature of the polariton gas in the quasi-0D system.

Figure 5

The calculated typical angle-resolved PL mappings without the evaporative cooling mechanism.