Control of electronic states by a nearly monocyclic terahertz electric-field pulse in organic correlated electron materials

Recent developments in femtosecond laser technology have enabled the generation of a nearly monocyclic strong terahertz (THz) pulse with an amplitude greater than 100 kV cm(-1). Such a THz pulse can be used to control not only the elementary excitations in solids such as phonons, magnons, and excitons but also the electronic phases. To achieve ultrafast phase control in the sub-ps time domain with a THz pulse, correlated electronic materials that demonstrate electronic phase transitions without large structural changes induced by external stimuli, such as by temperature, pressure, and light can be used. In this paper, we review recent studies on electronic phase controls using a nearly monocyclic THz pulse in organic molecular compounds with correlated electron systems, TTF-CA (TTF: tetrathiafulvalene and CA: p-chloranil), alpha-(ET)(2)I-3 (ET: bis(ethylenedithio)tetrathiafulvalene), and kappa-(ET)(2)Cu[N(CN)(2)]Br. TTF-CA undergoes a neutral-to-ionic phase transition as the temperature decreases. It demonstrates an electronic-type ferroelectricity in the ionic phase, in which ferroelectric polarization is generated from intermolecular charge transfers across the neutral-to-ionic phase transition. THz-pulse pump second-harmonic-generation probe and optical-reflectivity probe measurements show that ferroelectric polarization in the ionic phase can be rapidly modulated by a THz pulse via charge transfers induced by an electric field. In alpha-(ET)(2)I-3, rapid polarization modulation by a THz pulse was also achieved in the ferroelectric charge-order phase. Detailed analyses of reflectivity changes induced with THz electric fields revealed that the ferroelectric polarization originated from intermolecular charge transfers and was oriented diagonally to the crystal axes. These results demonstrate that the ferroelectricity of this compound was electronic, similar to that of the ionic phase of TTF-CA. In the para-electric neutral phase of TTF-CA, a macroscopic polarization was generated by a THz pulse via the dynamics induced by an electric field on microscopic ionic domains. In kappa-(ET)(2)Cu[N(CN)(2)] Br, a transition from a Mott insulator to a metal by a THz pulse was demonstrated by observing Drude-like low-energy spectral weights induced by the electric field. A THz pulse induced doublon-holon pair production by quantum tunnelling processes, which collapsed the original Mott gap in under a picosecond. These results suggest that strong THz-pulse irradiation is an effective approach for the ultrafast control of electronic phases in correlated electron materials.