Coherent elastic waves in a one-dimensional polymer hypersonic crystal
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(a) The temporal evolution of the measured reflectivity signal of the probe pulse, with wavelength and incident angle , when the acoustic wavepacket is propagating through the PVP/PS superlattice. The values and marked by dashed vertical lines correspond, respectively, to the time between probe and pump pulses when the picosecond strain pulse is injected into a superlattice from the Si substrate and when the strain pulse reaches the superlattice/air interface. The inset shows the experimental scheme. (b) The fast Fourier transforms of the signals shown in (a) for corresponding and . (c) The calculated spectra of the displacement in the superlattice caused by the injected acoustic wavepacket for wave vectors which are active in the optically probed signals for corresponding pairs of and .
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The solid lines are the calculated dispersion relations in the direction perpendicular to the plane of the polymer superlattice (the period of the SL ). The crossings of the solid and dotted vertical lines correspond to the momentum conservation for phonon–photon interaction giving the frequency components which are detected optically in the performed experiments for corresponding pairs and : , , and correspond to the pairs (, ), (, ), and (, ), respectively. The dashed diagonal lines indicate the main “0” and sideband “” and “−1” spectral components which arise from the folding of the acoustic dispersion due to the superlattice. The grayscale (color scheme) shows the intensity of the acoustic modes for the displacement calculated in the system. The circular points indicate the frequencies of the modes detected in the experiment [taken from Fig. 1(b)].
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