The experimental setup is illustrated in Fig. 5A. During the experiment, the sample was mounted inside a cold finger cryostat and kept at a temperature of 5 K. The optical imaging setup supported two modes of operation, which were toggled by the presence or absence of a Fourier lens: With the Fourier lens present, the Fourier plane of the collection objective was imaged on the entrance slit of the spectrometer, resulting in angular-resolved emission spectra. Furthermore, in the Fourier imaging mode, a small aperture was positioned at the first real-space image plane of the Fourier lens. The dimensions of the aperture were chosen such that only light emitted through the grating outcoupler was allowed to pass. This spatial filter allowed us to completely separate the polariton signal from emission originating from the vicinity of the remote excitation spot. In both modes of operation, the signal was filtered by a polarizer to collect only light from the transverse electric–polarized WG-polaritons [see discussion in the study of Rosenberg et al. (15)] and by an 800-nm long-pass filter, which block scattering of the 774.5-nm excitation source. To ensure no buildup of long-lived exciton or charge reservoirs between excitation pulses, in all experiments, the applied voltage was modulated and synchronized with the excitation pulse and the exposure window, and was turned off after every exposure until the next one. The voltage on time was 1 μs long, starting 700 ns before the moment of optical excitation. It had an off time of 4 μs between optical excitations, which is an ample time for the subnanosecond recombination of all excitons. Between optical pulses, during the voltage off-time, there was no field-induced charge separation and the excitons were essentially unpolarized. Figure 5B shows the timing scheme of our experiments. The procedure described above ensures both spatial and temporal separation of polaritons and excitons/charges. The separation occurs because, after the excitation pulse, the polaritons propagate very fast toward the spatially remote output grating, where their emission is being captured within the temporal window of the gated camera. The temporal width of the polariton cloud is given by the temporal width of the laser pulse, which was <300 ps. The excitons/charges move much slower from the excitation point (with typical velocities of 1 to 10 μm/ns) (31, 32) and could not make it to the remote output grating within 300 ps. If carriers/excitons ever reach the grating within one excitation cycle, they do that much after the polariton signal has gone, so there were no local mutual interactions between excitons/carriers and the measured polaritons originating from the same pulse.

(A) Experimental setup. The sample is positioned inside a cold finger cryostat and kept at T = 5 K. The sample is nonresonantly excited from the side. The emitted PL can be analyzed angularly (a) and spatially (b) by flipping the Fourier lens in and out, respectively. A spatial filter positioned at the real-space image of the Fourier lens filters out signal that originates outside of the grating area. (B) Timing diagram of the acquisition method. A 200-MHz laser excites the sample nonresonantly. The maximum trigger of the ICCD camera is 50 kHZ; therefore, only one of four pulses is measured with an acquisition window of 10 ns. The voltage is applied for 1 μs, starting 700 ns before each camera gate.

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