Second-Order Time Correlations within a Polariton Bose-Einstein Condensate in a CdTe Microcavity

Second-order time correlations of polaritons have been measured across the condensation threshold in a CdTe microcavity. The onset of Bose-Einstein condensation is marked by the disappearance of photon bunching, demonstrating the transition from a thermal-like state to a coherent state. Coherence is, however, degraded with increasing polariton density, most probably as a result of self-interaction within the condensate and scatterings with noncondensed excitons and polaritons. Such behavior clearly differentiates polariton Bose condensation from photon lasing.

Figure 1

Second-order time correlations measured for polariton photoluminescence at $k_{\parallel }=0$, for four different pump intensities. Suppression of the photon bunching is seen as the condensation threshold is passed, demonstrating transition from thermal towards coherent state. Far above threshold (d) the coherence is degraded (photon bunching reappears) due to interactions within the polariton condensate and scatterings with noncondensed excitons and polaritons.

Figure 2

(a) $g_m^{(2)}(\tau )$ measured for the excitation laser. The photon bunching at zero time delay is not observed showing second-order coherence of the laser field. (b)–(d) $g_m^{(2)}(\tau )$ measured for the polariton photoluminescence at $k_{\parallel }=0$. The photon bunching of the order 0.5% is measured below the condensation threshold. Its value gradually increases with the excitation density (c),(d), and see Fig. 3. The error bars correspond to $N^{-1/2}$.

Figure 3

$g_m^{(2)}(\tau =0)$, for the polariton photoluminescence at $k_{\parallel }=0$ as a function of the normalized excitation intensity (blue triangles). The $g_m^{(2)}(0)$ gradually increases with the pump intensity. In order to interpret this behavior the interplay between the coherence time and the relaxation time should be taken into consideration (see main text). Far above threshold the decoherence is manifested by increase of the linewidth (open brown circles) and by increase of the $g_m^{(2)}(0)$. The orange circle is the $g_m^{(2)}(0)$ of the excitation laser. Green squares are average values of $g_m^{(2)}(\tau \ne 0)$ correlation peaks. Inset: The number of coincidences as a function of $\tau$ measured for the polariton photoluminescence at $k_{\parallel }=0$ in the pulsed excitation regime; far above threshold (up) a significant photon bunching at $\tau =0$ is measured.

Figure 4

Acceleration of the polariton relaxation towards the ground state with increasing the pump intensity measured in the intensity correlation experiment. The narrowing of the correlation peaks due to the reduction of the decay time is shown (Inset: below threshold: dashed-dot line and at threshold: solid line). Above condensation threshold the width of the correlation peaks is limited by the $t_{\mathrm{AC}}$ and not by the $t_{\mathrm{rel}}$.