Spontaneous Polarization Buildup in a Room-Temperature Polariton Laser

We observe the buildup of strong ($\sim 50\%$) spontaneous vector polarization in emission from a GaN-based polariton laser excited by short optical pulses at room temperature. The Stokes vector of emitted light changes its orientation randomly from one excitation pulse to another, so that the time-integrated polarization remains zero. This behavior is completely different from any previous laser. We interpret this observation in terms of the spontaneous symmetry breaking in a Bose-Einstein condensate of exciton polaritons.

Figure 1

Formation of polariton lasing at $T=300\mathrm{K}$ in a GaN microcavity at in-plane wave vectors up to $k_{\max }=7\mu {\mathrm{m}}^{-1}$ for (a) just below (scaled up by $\times 1000$) and (b) just above threshold ($I_{\mathrm{th}}\sim 1\mathrm{mW}$). Inset shows dispersion. (c) Image of pumped sample above threshold. (d) Interference of far-field emission cone through a slightly misaligned Michelson interferometer above and below threshold. (e),(f) Polariton emission intensity just below and above threshold as a function of energy, together with Boltzmann fit (dashed line) giving an effective temperature of 360 K, and result from kinetic simulation (solid lines).

Figure 2

Above-threshold polarization-resolved emission when analyzing along (a) horizontal or vertical and (b) right or left circular bases. (c) Extracted linear polarization degree showing stochastic variation from pulse to pulse above and below threshold. (d) Histogram of the fraction of each polarization state $f$ along linear, diagonal, and circular bases of nearly 2000 polariton condensates. Open and closed circles show repeated measurement with reversed polarization split (e.g., $H/V$ and $V/H$), while crosses show below-threshold unpolarized emission statistics (curve divided by 2 to fit on scale, within detection sensitivity shaded gray and scaled dashed line).

Figure 3

(a) Diffusion of the pseudospin, $\mathbf{S}$ (Stokes polarization vector) of a polariton condensate. (b) Experimental (points) and modeled (lines) histogram of polarization vector lengths, showing shift from zero to $\sim 43\%$ above threshold. (c) Statistical distribution of the linear polarization degree of the condensate showing experiment versus theory for different dephasing times. (d) Root mean square linear polarization degree as a function of pump power (thick black line). Result of theory described in the text (crosses, solid red line is guide to eye) uses linewidth (blue circles) as input parameter.