Room-Temperature Polariton Lasing in Semiconductor Microcavities

We observe a room-temperature low-threshold transition to a coherent polariton state in bulk GaN microcavities in the strong-coupling regime. Nonresonant pulsed optical pumping produces rapid thermalization and yields a clear emission threshold of 1 mW, corresponding to an absorbed energy density of $29\mu \mathrm{J}{\mathrm{cm}}^{-2}$, 1 order of magnitude smaller than the best optically pumped (In,Ga)N quantum-well surface-emitting lasers (VCSELs). Angular and spectrally resolved luminescence show that the polariton emission is beamed in the normal direction with an angular width of $\pm 5°$ and spatial size around $5\mu \mathrm{m}$.

Figure 1

(a) Microcavity reflectivity at 300 K and $\theta \sim 5°$, with lower polariton mode marked (arrow). Dashed line shows nonresonant pump energy. Inset shows the layer structure. (b), (c) Theoretical angular dispersion both without (b) and with (c) the resonant exciton contribution to the GaN cavity (${\omega }_{\mathrm{LP}}$ dashed line, ${\omega }_{\mathrm{cav}}$ dash-dotted line) for a slightly negatively detuned cavity ($\Delta =-10\mathrm{meV}$). (d) Angle-resolved PL at low powers up to 60°, with lower (LP) and upper (UP) polaritons, exciton (X), and Bragg modes (B) marked.

Figure 2

(a), (b) Emission spectra at pump powers from $20\mu \mathrm{W}$ to 2 mW at 0°, shifted in (a) for clarity, (b) false color, log scale. (c) Integrated intensity vs pump power (solid points), with fit. (d) Spectral linewidth and (e) energy shift of peak emission with pump power (for different modes $L_{1-3}$).

Figure 3

Spatial distribution of polariton emission, just below (a) and just above threshold (b), and width (FWHM) of the distribution as a function of power (c). (d) $k$-space image of interferometrically recombined emission for time delay of 265 fs.

Figure 4

(a) Extracted PL integrated intensities vs detection angle on the lower polariton branch (LP, solid points) and subsidiary Bragg mode ($B_1$) at three input powers, 0.3, 0.9, and 1.4 mW. Lines guide the eye at each power. (b) Time-resolved emission at normal incidence vs pump power from 0.2 mW to 1.4 mW, with amplitudes of fast and slow components extracted (inset).