Ultrafast Control of Ferroelectricity with Dynamical Repositioning of Protons in a Supramolecular Cocrystal Studied by Femtosecond Nonlinear Spectroscopy.

Photoexcitation effects on ferroelectricity were investigated in a hydrogen-bonded supramolecular cocrystal by using nonlinear femtosecond spectroscopy. Irradiation of a terahertz pulse that caused proton vibrations modulated the second harmonic (SH) signal in proportion to the applied electric field of the pulse and the magnitude of the change in the SH signal reached about 10%. By contrast, a mid-infrared pulse exciting the molecular vibration nonlinearly enhanced the SH intensity by about 15%. The proton dynamics were analyzed on the basis of the nonlinear equation of motion of the molecules. The calculated results indicate that the resultant dynamical repositioning of the protons caused the averaged increase in SH intensity. Thus, photoexcitations can be viewed as a novel technique for controlling the ferroelectric cocrystal at room temperature. [ABSTRACT FROM AUTHOR]