In the experiments the spectrum of the coherent

In the experiments, the spectrum of the coherent polariton emission was monitored as function of the time delay t0 between the white light excitation pulse and the strain pulse arrival time to the QW. The latter is defined as shown in Fig. 1(e). The results are shown in Fig. 2 for the UP [panel (a)] and LP [panels (b) and (c)]. Clearly, the polariton spectrum, which under stationary conditions consists of the well-defined UP and LP resonances (Fig. 1(b)), undergoes enormous changes when the strain pulse is hitting the QW. THz modulation of the UP alzheimer\’s disease (Fig. 2(a)) by several meV occurs. Regardless of the smaller LP modulation amplitude (Fig. 2, panels (b) and (c)), for certain values of delay t0 the LP spectra show a remarkable, well defined structure with spectral fringes at the flanks of the main resonance. The most pronounced sidebands with up to 3 fringes are observed for high amplitude strain pulses (Fig. 2(b)) in the delay intervals t0=10–25ps and t0=90–105ps, which correspond to the linear parts of the strain evolution. Fig. 2(d) shows the reflectivity spectrum R(t0, E) for a delay t0=17ps, at which several fringes are clearly detected.
We associate the experimentally observed spectral sidebands with the specifics of the nonadiabatic THz optical frequency modulation. To illustrate that this observation is a general phenomenon, we consider a scalar harmonic oscillator decaying with time-dependent frequency ω(t) and decay time . We demonstrate that the oscillator spectrum shows a series of sidebands in the case when ω(t) is changing linearly with time and the frequency shift within exceeds the stationary spectral width. A case-specific numerical analysis, where the temporal evolution ω(t) is taken to be identical to the experiments, shows good agreement with the experimental results [16].

Destruction and recurrence of excitons by acoustic shock waves on picosecond time scales
The same sample and experimental technique as described in Section 2 and shown in Fig. 1 was also used in the next experiment. The main difference concerns the detuning value ΔE0=− in the absence of the strain pulses. In the present experiment the part of the wafer was chosen where ΔE0=0, which corresponds to the strongest mixing of the pure exciton and photon resonances resulting in the vacuum Rabi splitting 4meV as can be seen from Fig. 1(f).
Fig. 3 shows contour plots of the reflectivity as function of time delay and photon energy for four different pump fluences on the metal film. Time zero corresponds to the moment when at low pump fluence (2.5mJ/cm2) the maximum compressive strain hits the quantum well, and the dotted lines mark the times at which changes by the strain can be resolved. Beforehand, in all cases the two polariton modes with 4meV Rabi splitting are seen.
At the lowest pump fluence (Fig. 3(a)) the induced modulations occur rather slowly, so that the evolution of the two polariton resonances with time can be followed well in the spectra. The polariton mode energies undergo a smooth oscillation with increasing delay, following the compressive and tensile parts of the acoustic wave packet. This smooth behavior reflects a continuous change of the exciton–photon mixing in the polariton states by the strain pulse modulating mostly the exciton energy, while the impact on the resonator is negligible due to the smallness of changes in cavity length and elasto-optical constant by the strain [24]. Note that in the delay range up to a few 10ps sidebands (see Section 2) appear on the low energy side of both polaritons, relative to which they shift in parallel with time. These sidebands become more pronounced with the increase of the excitation density to 6.25 and further to 10mJ/cm2 (Fig. 3(b and c)). Therefore, the resonances lose their spectral sharpness because of the modulation induced chirping of the optical transitions, as described before and in Ref. [15]. Still the resonance centers can be followed well in time, from which the polariton energies are seen to change smoothly. In particular, the observation of two polaritons for all delays underlines that the exciton state is preserved and strong coupling is maintained.