THz transients and the corresponding pulse envelopes for different propagation scenarios (IMAGE)
Caption
Fig. 2. THz transients (left) and the corresponding pulse envelopes (right) for different propagation scenarios. The blue lines give the THz propagation in vacuum, the black lines through neat isopropyl alcohol (IPA), and the red lines through IPA including a concentration of electrons of 100 µM. (a) THz transients as measured in the experiments. (b, c) THz transients after Fourier filtering for frequencies above (high-pass filter) and below (low-pass filter) the polaron resonance frequency at 1.5 THz. (d-f) Envelopes corresponding to the transients in panels (a-c). The pulse components below the polaron resonance (panel (c)) travel at an average phase velocity of the vacuum speed of light, as is seen from the red and blue curves being in phase [Fig. 2(c)]. This behavior points to a diverging phase velocity. Correspondingly, the pulse envelope significantly shifts towards earlier times, with an onset on the left-hand-side even faster than in the case of the vacuum envelope [Fig. 3(f)]. The temporal broadening of the envelope is due to a self-induced current inside the sample, which emits a THz electric field. At frequencies above the polaron resonance, the envelope of the THz pulse transmitted through the excited sample arrives at later times than through the unexcited sample [Fig. 3(e)].
Credit
MBI/Dr. M. Runge
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