Characterization of two-threshold behavior of the emission from a GaAs microcavity

We compare two regimes indicative of polariton lasing and photon lasing of a planar GaAs/GaAlAs microcavity with zero detuning between the bare cavity mode and the quantum-well exciton. In particular, we investigate the cavity emission subsequent to nonresonant pulsed excitation. For the ground state emission from the lower energy-momentum dispersion branch we find a two-threshold behavior in the input-output curve where each transition is accompanied by characteristic changes of the in-plane mode dispersion. We demonstrate that the thresholds are unambiguously evidenced in the photon statistics of the emission based on the second-order correlation function. Moreover, the distinct two-threshold behavior is confirmed in the evolution of the emission pulse duration. Our findings show that a comprehensive study of spectral and temporal characteristics of the emission from a semiconductor microcavity can be used to characterize the different emission regimes.

Figure 1

Dispersion curves measured at different excitation powers (linear color scale). In each panel, the PL emission is normalized to 1 and plotted in a linear color scale; the color code is indicated. The lines give calculated lower polariton LP (black), cavity photon mode C (white solid), and exciton X (white dotted) dispersion, respectively. It should be noted that the observation of both the LP mode and emission from condensed polaritons in panel (b) is due to the particle density changing throughout an excitation pulse.

Figure 2

Magnified dispersion curves over a wider momentum range, measured with a microscope objective with NA $=0.42$. (a) The dispersion relation at the first threshold ($P_{\text{exc}}=16\text{mW}$) is essentially flat over the mapped $k_{\left|\right|}$ space. (b) At negative cavity-exciton detuning, the condensate dispersion follows the LP dispersion at large momenta. (c) Above the second threshold ($P_{\text{exc}}=190\text{mW}$) the emission comes predominantly from the (slightly blueshifted) cavity photon mode, as underlined by the parabolic shape of the dispersion. In each panel, the PL emission is normalized to 1 and plotted in a linear color scale, the color code is indicated at the top; the lines give calculated LP (black) and cavity photon C (white) mode dispersion, respectively. Black circles (red triangles) follow the energy at the intensity maxima corresponding to the lower polariton (condensed polariton) mode. The signal at large $k_{\left|\right|}$ values has been magnified as indicated, for better visibility.

Figure 3

Excitation power dependence of (a) the integrated intensity of the cavity emission with in-plane momenta $|k_{\left|\right|}|\le 0.13\mu$m${}^{-1}$, (b) the emission energy at $k_{\left|\right|}=0$, and (c) the spectral linewidth (FWHM); the vertical dashed lines indicate the two thresholds. The solid (open) symbols in panel (b) represent the energy of the mode with stronger (weaker) intensity; the pronounced jumps observed in the emission energy are attributed to nonresonant pulsed excitation.

Figure 4

(a) Power-dependent equal-time correlation function $g^{\left(2\right)}(\tau =0)$. The vertical dashed lines represent the thresholds, the blue (red) line indicates the thermal (coherent) state. (b) Emission pulse width (Gaussian standard deviation) vs excitation power. (c)$-$(e) Temporal pulse profile measured just below the first (c), at the second (d), and above the second threshold (e), respectively, as indicated by circles in (b). Note that for all time-resolved measurements, a 1-nm-wide bandpass filter has been used to single out the main emission mode.