Abstract-Narrow-band terahertz radiation by impulsive stimulated Raman scattering in an above-room-temperature organic ferroelectric benzimidazole

M. Sotome, N. Kida, S. Horiuchi, and H. Okamototo,

https://journals.aps.org/pra/accepted/5307fYe8D141d06616b992a17369297dd547b7888

We observe a terahertz radiation from a hydrogen-bonded organic molecular ferroelectric 5,6-dichloro-2-methylbenzimidazole excited by a femtosecond laser pulse at room temperature. The emitted terahertz wave consists of three oscillatory components, the frequencies of which agree with those of Raman- and infrared-active phonon modes. This suggests that the terahertz radiation is attributed to polarization modulations by infrared-active phonons excited via the impulsive stimulated Raman scattering processes. By taking into account the Raman polarizability tensor and dipole-moment for each phonon, we succeeded in reproducing not only the spectrum of the terahertz radiation, but also its time characteristic. The analysis method is discussed in detail. Our result provides a new way for the light-induced terahertz radiation in organic ferroelectrics. The second-order optical nonlinearity in noncentrosymmetric media is useful for the frequency conversion of lights, which include not only the sum and difference frequency generation but also the optical parametric effect [1,2]. In this context, ferroelectric materials are being intensively studied [1]. In oxide ferroelectrics such as LiNbO${}_3$ and KH${}_2$PO${}_4$, highly efficient second-order optical nonlinearity has been reported, which is enough for practical use as nonlinear optical materials. Explorations of organic ferroelectrics are also important, since organic materials have advantages due to their low cost and environmentally benign characteristics. However, it had been reported that their Curie temperatures are much lower than room temperature. Recently, above-room-temperature ferroelectricity has been found in hydrogen-bonded molecular crystals having $\pi$-electron systems [3] such as 4,5-dihydroxy-4-cyclopentence-1,2,3-trione (croconic acid) [4] and 2-phenylmalondialdehyde [5]. In these materials, cooperative proton displacements and asymmetric $\pi$-electron configurations along the hydrogen-bonded direction cause the ferroelectric polarization. In croconic acid and 2-phenylmalondialdehyde, we have recently demonstrated that terahertz electromagnetic wave can be radiated by an irradiation of a femtosecond laser pulse [6,7] and that the terahertz-radiation can be ascribed to a difference frequency generation (DFG) within an incident laser pulse, which is described by the second-order nonlinear optical susceptibility${\chi }^{\left(2\right)}$ This process is sometimes called optical rectification (OR) [8] and is recognized as a typical terahertz-radiation mechanism in various noncentrosymmetric media such as ZnTe [9] and 4-N,N-dimethylamino-4'-N'-methyl stilbazolium tosylate (DAST) [10]. In the present study, we report that effective terahertz radiation is possible via a different mechanism in a hydrogen-bonded organic molecular ferroelectric, 5,6-dichloro-2-methylbenzimidazole (DCMBI) [11]. By the irradiation of a femtosecond laser pulse on a single crystal of DCMBI, we observe an emission of a terahertz wave consisting of several oscillatory components, which cannot be attributed to the OR mechanism. The central frequencies of those oscillations correspond to three phonon modes, which are both Raman- and infrared-active, indicating that the terahertz radiation ..