Strong coupling of polaritons at room temperature in a GaAs/AlGaAs structure

Abstract

We report direct measurement of the dispersion relation of polaritons in GaAs/AlGaAs microcavity structures at room temperature, which clearly shows that the polaritons are in the strong coupling limit. The Rabi splitting of the polariton states decreases as the polariton gas increases in density, but even when the polariton gas becomes a coherent, Bose-condensate-like state, the polaritons retain a strong exciton component, as seen in the nonlinear energy shift of the light emission. This opens up the possibility of polaritonic devices at room temperature in a material system which can be grown with very high quality and uniformity.

Figures (16)

• Figure 1: Strong coupling at room temperature in Samples P1 and W1. (a) Left plot: angle-resolved PL measurements of the polariton at very low pumping power for Sample P1 at a location with LP exciton fraction of approximately 0.12 at $k=0$. Right plot: the intensity as a function of energy along the slice at constant angle corresponding to the vertical dashed line in (a). To reduce the noise in the experimental data, we averaged over 20 pixels around the slice. Dashed lines: fit to two Lorentzian peaks. (b) The same measurements at very low pumping power for Sample W1, for an LP exciton fraction of approximately 0.24 at $k=0$. The extra curved lines seen at low energy and high angle in (b) are ripples in the reflectivity spectrum at the edge of the stopband of the DBR cavity of this sample.
• Figure 2: Angle-resolved PL for different pump powers, at a location on Sample P1 with 0.54 exciton fraction of the LP at $k=0$. (a) Pump power $P=0.01P{_{\mathrm{th}}}$, (b)$P = 0.83P{_{\mathrm{th}}}$, (c)$P=1.00P{_{\mathrm{th}}}$ and (d)$P= 1.13P{_{\mathrm{th}}}$. The threshold power $P{_{\mathrm{th}}}$ is defined in Section III of the Supplementary Material.
• Figure 3: Blue shift and linewidth of the PL lines for Sample P1 at a location with 0.54 LP exciton fraction at $k=0$. (a) The intensity at $k=0$ of the polaritons as a function of optical pump intensity, corrected for the reflection from the top surface, (b) The energies of the polariton lines at $k=0$ as a function of the pump power. The red dashed line represents the cavity zero energy extracted from the three-level model fits. The line marked "Cavity $k=0$ energy" is the value from the fit of Figure \ref{['fig:different_detunings']}(b); the vertical pink range gives the uncertainty of this value. The bare heavy-hole exciton energy is very near to this value. (c) Circles: Full width at half maximum at $k=0$. A single Lorentzian fit was used for the data corresponding to solid circles since we see only main peaks at high power due to condensation. The vertical black dashed lines (with uncertainty given by the gray regions) indicate the power beyond which the system can no longer be identified as being in the strong-coupling regime.
• Figure 4: Angle-resolved PL for different locations on Sample P1 compared to simulation. Left side of each image: Angle-resolved PL measurements of the polariton at very low pumping power, corresponding to the estimated exciton fractions of the lower polariton at $k=0$ given by the upper labels. Right sides: simulated data using the model discussed in the text. The parameters for the fits are given in the Supplementary Material. The solid black, red, and yellow lines represent the LP, MP, and UP, respectively. The green dashed line represents the heavy-hole exciton energy, the blue dashed line represents the light-hole exciton energy, and the purple dashed line represents the cavity energy. The extra curved lines seen at low energy and high angle in (c) are ripples in the reflectivity spectrum at the edge of the stopband of the DBR cavity of this sample.
• Figure S1: Sample Structures. (a) The design of Samples P1, P2, and P3, which only differ in the number of top DBR periods $N$, with $N= 23$, $N=29$ and $N=32$, respectively. (b) The sample design of Sample W1. The QWs are arranged in groups of four, with Samples P1, P2, and P3 each containing three groups of QWs, while Sample W2 contains five groups of QWs.