Breathing spectral holes in the vacuum of phonons in femtosecond pump-probe experiments

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Abstract
Temporal features of ultrafast pump-probe signals in a strongly coupled electron-phonon system are clarified theoretically by a simple two-level model. It is shown that a transient spectral hole is burnt in the ground state as a squeezed state by femtosecond optical pulses. The long-lived component in the temporal oscillation of transmission gains commonly observed for localized electron-phonon systems under degenerate pump-probe conditions is naturally explained as mainly due to the breathing and oscillating motion of this spectral hole.