Spontaneous Coherent Phase Transition of Polaritons in CdTe Microcavities

We report on evidence for polariton condensation out of a reservoir of incoherent polaritons. Polariton population and first-order coherence are investigated by spectroscopic imaging of the far-field emission of a CdTe-based microcavity under nonresonant pumping. With increasing pumping power, stimulated emission with thresholdlike behavior and spectral narrowing is observed in the strong exciton-photon coupling regime. We show that it comes from a narrow ring in $k$ space, exhibiting enhanced spatial and angular coherence at the stimulation onset.

Figure 1

Real-space and $k$-space imaging setup: the ray path for real-space imaging is represented by the dashed line, while the ray path for $k$-space imaging is represented as a gray surface for the emission cone of angle $\theta$.

Figure 2

Far-field emission for two excitation powers: first row and second row of images are obtained below and above the stimulation threshold, respectively. (a) and (d) display the far-field emission in the $(\theta ,\varphi )$ Fourier plane with a linear scale of gray. The dotted circle plots the cone of angle $\theta =17°$ where the stimulated emission occurs. (b) and (e) display the emission measured directly in the $(\theta ,E)$ plane on a logarithmic scale of gray. (c) and (f) display the intensity profile along the azimuth angle $\varphi$ for $\theta =17°$. (g) displays the evolution of the spatial degree of coherence $c_s$ versus the excitation power. The threshold for increased coherence is the same as that for the stimulated emission.

Figure 3

(a),(b) Images obtained with the Billet interferometer for $\Delta k_{\parallel }=0.9\mu {\mathrm{m}}^{-1}$ below and above stimulation threshold, respectively. The gray scale is linear. (c),(d) Profiles of the interference pattern area of the above images. (e) Contrast of interference versus displacement $\Delta k_{\parallel }$.