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

Abstract

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 CH 3 rotational modes with collective oscillations of π electrons drives the system into the nonperturbative regime of lightmatter interaction, even for a moderate strength of the THz driving field on the order of 50 kV=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. DOI: 10.1103/PhysRevLett.119.097404 Soft modes are a particular type of polar low-frequency lattice vibrations in crystals. They display pronounced frequency downshifts when the material approaches a structural phase transition such as in displacive ferroelectricity. The strong coupling between electronic interband and vibrational transition dipoles can strongly enhance the vibrational oscillator strength, leading to the so-called polarization catastrophe in a phase transition from the para-to the ferroelectric state of a crystal [1]. A basic theoretical approach to describe such phenomena is the concept of local-field corrections (LFC) due to the Lorentz field determined by the macroscopic polarization of all contributing (transition) dipoles [2][3][4][5].To illustrate this concept, we consider a cubic molecular crystal having an electronic and a vibrational oscillator on the two simple cubic sublattices [ Fig. 1(a)]. The two oscillators experience the same Lorentz field LP [ Fig. 1(b)], leading to an increasing dipole-dipole interaction between the two sets of oscillators upon decreasing the crystal's lattice constant a. In the linear optical response, this results in both a Lorentz-field induced redshift of the two oscillators and a significant transfer of electronic oscillator strength to the vibrations. The resulting hybrid mode, i.e., the soft-mode, is connected with electronic polarizations and currents during its oscillation period, a fact that has been shown most directly in recent femtosecond x-ray diffraction experiments [6]. Parallel to the increase of vibrational oscillator strength, one expects an enhancement of its optical nonlinearity compared to uncoupled oscillators [7].The local field E loc in the presence of the Lorentz field is given by E loc ¼ E þ LP, where E represents the external field, L the Lorentz factor, and P the polarization of the material [ Fig. 1(b)]. In a molecular crystal, both the electronic and vibrational dipoles contribute to P. For strong dipole-dipole coupling, the LP term can be even larger than the externally applied field E, having two major consequences: (i) The time structure of E loc ðtÞ is affected by the polarization kinetics PðtÞ, following, e.g., a freeinduction decay (FID) with a finite decoherence time; (ii) a nonlinear saturation of PðtÞ for strong external fields EðtÞ results in ...

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Phonon driven charge dynamics in polycrystalline acetylsalicylic acid mapped by ultrafast x-ray diffraction

“…Upon THz excitation of a methyl (CH 3 ) rotational mode which couples strongly to the π -electron system of the aspirin molecules, one observes a strong blue-shift of the rotational frequency from its equilibrium value of 1.1 THz to 1.7 THz. This behavior represents a manifestation of the dynamic breakup of the strong electron-phonon correlations and has been reproduced by theoretical calculations including dynamic local-field correlations 10,11 . While nonlinear THz spectroscopy maps the nonlinear response of the coupled system, it provides only indirect information on the electronic charge relocations connected with the vibrational excitation.…”

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