Terahertz Radiation with Multi-Narrowband Components via Photoinduced Melting of Charge Order in an Electronic-Type Ferroelectric Organic alpha-(BEDT-TTF)(2)I-3

In noncentrosymmetric materials, a difference frequency generation from a femtosecond laser pulse or equivalently an optical rectification via a second-order nonlinear optical effect is known to be a dominant process for the emission of broadband terahertz radiations. Here, we report that a terahertz radiation with long-lived terahertz multi-narrowband-components appears in a photoinduced phase transition in an organic molecular compound, alpha-(BEDT-TTF)(2)I-3 [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene]. This compound is a charge-order insulator below 135 K accompanied by the electronic-type ferroelectricity. By an irradiation of a near-infrared femtosecond laser pulse on the ferroelectric chargeorder phase in alpha-(BEDT-TTF)(2)I-3, the terahertz radiation consisting of a single-cycle component and several kinds of long-lived narrowband oscillatory components are detected. The frequencies of the oscillatory components accord with those of the infrared-active phonon modes coupled with charge degrees of freedom located in the terahertz region. By comparing the excitation density dependence of the electric-field waveforms of the terahertz radiation with that of the time characteristics of the reflectivity changes reflecting the photoinduced charge-order melting, we revealed the mechanisms of the observed terahertz radiation. The single-cycle component is attributed to two mechanisms, the optical rectification and the reduction of the ferroelectric polarization by the photoinduced melting of the charge order. With the increase of the excitation density, the former mechanism tends to be suppressed and the latter mechanism dominates the terahertz radiation in the strong excitation. The narrowband oscillatory components of the terahertz radiation are assigned to the coherent molecular oscillations with charge modulations, which are driven via the disappearance of the molecular displacements stabilizing the charge order.