Abstract-Strong Local-Field Enhancement of the Nonlinear Soft-Mode Response in a Molecular Crystal

Giulia Folpini, Klaus Reimann, Michael Woerner, Thomas Elsaesser, Johannes Hoja,  Alexandre Tkatchenko,

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.119.097404

The nonlinear response of soft-mode excitations in polycrystalline acetylsalicylic acid (aspirin) is studied with two-dimensional terahertz spectroscopy. We demonstrate that the correlation of ${\mathrm{CH}}_3$rotational modes with collective oscillations of $\pi$ electrons drives the system into the nonperturbative regime of light-matter interaction, even for a moderate strength of the THz driving field on the order of $50\mathrm{kV}/\mathrm{cm}$. Nonlinear absorption around 1.1 THz leads to a blueshifted coherent emission at 1.7 THz, revealing the dynamic breakup of the strong electron-phonon correlations. The observed behavior is reproduced by theoretical calculations including dynamic local-field correlations.

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