Experimental realization of a polariton beam amplifier

In this paper we demonstrate a versatile concept for a planar cavity polariton beam amplifier using nonresonant excitation. In contrast to resonant excitation schemes, background carriers are injected which form excitons, providing both gain and a repulsive potential for a polariton condensate. Using an attractive potential environment induced by a locally elongated cavity layer, the repulsive potential of the injected background carriers is compensated, and a significant amplification of polariton beams is achieved without beam distortion.

Figure 1

Excitation pattern applied for the experiments. A laser spot consisting of two concentric semicircles is imprinted onto the sample for the generation of a directed polariton beam. A repulsive potential mediated by background carriers created by a Gaussian laser spot allows for the realization of beam amplification experiments.

Figure 2

Polariton condensate distribution for different excitation power levels of the Gaussian laser spot. (a) Without scattering potential the incoming polariton beam leaves the trap without further deflection in the $L_{\mathrm{y}}$ direction. The upper part of the panel shows the polariton distribution on a logarithmic scale. (b)–(d) With applied repulsive potential pronounced scattering of the incoming polariton beam (red arrow) is observed, as indicated by the white arrows.

Figure 3

Sketch of the microcavity polariton trap sample. Schematic drawing of the mesa trap structure for providing lateral confinement for polaritons. The laser-written semicircle potential and the Gaussian pump spot inside the trap are indicated in red. The polariton flow is depicted in blue. The height of the blue line represents the polariton density and illustrates the deflectionless amplification of the polariton flow at the Gaussian pump spot.

Figure 4

Generation of a directed polariton beam in the vicinity of a polariton trap. The diameter of the trap amounts to $d=30\mu \text{m}$ and is indicated by the white dashed line. The location of the Gaussian laser spot is denoted by the white circle in (b). (a) Polariton condensate distribution with Gaussian laser spot turned off. (b)–(d) Polariton condensate distribution with Gaussian laser spot at different excitation power levels. Spectral resolution is provided by an interference filter centered at $1570.2$ meV with a FWHM of 2 meV.

Figure 5

Spectrally resolved cross section of a sample region 1 $\mu \text{m}$ in diameter centered around the polariton beam propagating in the $L_x$ direction as indicated in Fig. 4 by the white box. The white dashed line indicates the boundary of the polariton trap. Excitation power levels of the Gaussian laser spot are the same as in the corresponding panels of Fig. 4. The red box indicates the integration area on the CCD chip used for the analysis of the gain presented in Fig. 6. The emission at 1.5667 eV in (d) occurs only for excitation power levels significantly above the condensation threshold and corresponds to trapped polariton states.

Figure 6

(a) Number of counts within the area indicated by the red box in Fig. 5 normalized by the number of counts without excitation of the Gaussian laser spot. The black dashed line is a guide to the eye. (b) Signal intensity arising from all trapped polariton states. Excitation power is normalized to the polariton condensation threshold of the trapped polariton states under excitation with only the Gaussian laser spot.

Figure 7

Calculated spectrally and spatially resolved polariton distribution for four different target densities. Target densities are normalized to the condensation threshold of the trapped states without the impinging polariton beam. The potential geometry of the trap is indicated by the white line.

Figure 8

The time integrated density of (a) the propagating and (b) trapped polariton condensates dependent on the target power. The target power is normalized to the condensation threshold without any competitive polariton propagation $P_{\mathrm{thr}}$. The increased condensation threshold in the presence of a propagating condensate is indicated by dashed lines.