Abstract-Ultrafast polarization control by terahertz fields via π-electron wavefunction changes in hydrogen-bonded molecular ferroelectrics

T. Miyamoto, D. Hata, T. Morimoto, H. Yamakawa, N. Kida, T. Terashige, K. Iwano, H. Kishida, S. Horiuchi,  H. Okamoto

https://www.nature.com/articles/s41598-018-33076-9

Rapid polarization control by an electric field in ferroelectrics is important to realize high-frequency modulation of light, which has potential applications in optical communications. To achieve this, a key strategy is to use an electronic part of ferroelectric polarization. A hydrogen-bonded molecular ferroelectric, croconic acid, is a good candidate, since π-electron polarization within each molecule is theoretically predicted to play a significant role in the ferroelectric-state formation, as well as the proton displacements. Here, we show that a sub-picosecond polarization modulation is possible in croconic acid using a terahertz pulse. The terahertz-pulse-pump second-harmonic-generation-probe and optical-reflectivity-probe spectroscopy reveal that the amplitude of polarization modulation reaches 10% via the electric-field-induced modifications of π-electron wavefunctions. Moreover, the measurement of electric-field-induced changes in the infrared molecular vibrational spectrum elucidates that the contribution of proton displacements to the polarization modulation is negligibly small. These results demonstrate the electronic nature of polarization in hydrogen-bonded molecular ferroelectrics. The ultrafast polarization control via π-electron systems observed in croconic acid is expected to be possible in many other hydrogen-bonded molecular ferroelectrics and utilized for future high-speed optical-modulation devices.

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

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

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

In noncentrosymmetric media, optical rectification is known to be a general mechanism of the generation of terahertz electromagnetic waves. Here, we show that effective terahertz radiation is possible via a different mechanism in a hydrogen-bonded organic molecular ferroelectric 5,6-dichloro-2-methylbenzimidazole (DCMBI). By the irradiation of a femtosecond laser pulse on a single crystal of DCMBI at room temperature, we observe a strong terahertz radiation. 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 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. {I.

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 ..

Abstract-Visualization of a nonlinear conducting path in an organic molecular ferroelectric by using emission of terahertz radiation

M. Sotome, N. Kida, Y. Kinoshita, H. Yamakawa, T. Miyamoto, H. Mori, and H. Okamoto

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.241102

Nonlinear electric transport and switching to a negative resistance state are typical electric-field-induced phenomena in correlated electron materials, while their mechanisms are generally difficult to solve. In the present study, we apply the terahertz-radiation imaging method to an organic molecular ferroelectric, α-type bis(ethylenedithio)tetrathiafulvalene iodide salt, and investigate the nature of its negative resistance phenomenon. When the negative resistance state is produced, the ferroelectric order is melted in an elongated region with the width of ∼100 μm and that region grows along the direction inclined by about 40° from the $b$ axis with the increase of nonlinear current. A comparison of the terahertz radiation intensity with the current magnitude revealed that the melted region forms a conducting path. We interpreted the diagonal growth of the conduction path by taking into account the anisotropy of the intermolecular transfer integrals.

• 
• 
• 
• 

Abstract-Visualization of a nonlinear conducting path in an organic molecular ferroelectric by using emission of terahertz radiation

M. Sotome, N. Kida, Y. Kinoshita, H. Yamakawa, T. Miyamoto, H. Mori, and H. Okamoto

https://journals.aps.org/prb/accepted/ba073K50P1a1e407c148870869f209b1cf6800e4b

A nonlinear electric transport and switching to a negative resistance state is one of typical electric-field-induced phenomena in correlated electron materials, while their mechanisms are generally difficult to solve. In the present study, we apply the terahertz-radiation imaging method to an organic ferroelectric, α-(BEDT-TTF)2I3 [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene] and investigate the nature of its negative resistance phenomenon. When the negative resistance state is produced, the ferroelectric order is melted in an elongated region with the width of ∼100 μm and that region grows along the direction inclined by about 40° from the b axis with the increase of nonlinear current. A comparison of the terahertz radiation intensity with the current magnitude revealed that the melted region forms a conducting path. We interpreted the diagonal growth of the conduction path by taking into account the anisotropy of the intermolecular transfer integrals.

Abstract-Novel electronic ferroelectricity in an organic charge-order insulator investigated with terahertz-pump optical-probe spectroscopy.

• http://www.nature.com/articles/srep20571
• H. Yamakawa
• , T. Miyamoto
• , T. Morimoto
• , H. Yada
• , Y. Kinoshita
• , M. Sotome
• , N. Kida
• , K. Yamamoto
• , K. Iwano
• , Y. Matsumoto
• , S. Watanabe
• , Y. Shimoi
• , M. Suda
• , H. M. Yamamoto
• , H. Mori
•  & H. Okamoto
• In electronic-type ferroelectrics, where dipole moments produced by the variations of electron configurations are aligned, the polarization is expected to be rapidly controlled by electric fields. Such a feature can be used for high-speed electric-switching and memory devices. Electronic-type ferroelectrics include charge degrees of freedom, so that they are sometimes conductive, complicating dielectric measurements. This makes difficult the exploration of electronic-type ferroelectrics and the understanding of their ferroelectric nature. Here, we show unambiguous evidence for electronic ferroelectricity in the charge-order (CO) phase of a prototypical ET-based molecular compound, α-(ET)2I3 (ET:bis(ethylenedithio)tetrathiafulvalene), using a terahertz pulse as an external electric field. Terahertz-pump second-harmonic-generation(SHG)-probe and optical-reflectivity-probe spectroscopy reveal that the ferroelectric polarization originates from intermolecular charge transfers and is inclined 27° from the horizontal CO stripe. These features are qualitatively reproduced by the density-functional-theory calculation. After sub-picosecond polarization modulation by terahertz fields, prominent oscillations appear in the reflectivity but not in the SHG-probe results, suggesting that the CO is coupled with molecular displacements, while the ferroelectricity is electronic in nature. The results presented here demonstrate that terahertz-pump optical-probe spectroscopy is a powerful tool not only for rapidly controlling polarizations, but also for clarifying the mechanisms of ferroelectricity.

Abstract-d carrier-density dependence of electron-hole scattering in silicon investigated by optical-pump terahertz-probe spectroscopy

T. Terashige, H. Yada, Y. Matsui, T. Miyamoto, N. Kida, and H. Okamoto

Phys. Rev. B 91, 241201(R) – Published 5 June 2015

http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.241201

We measured the optical conductivity σ̃(ω) spectra of photodoped silicon by optical-pump terahertz-probe spectroscopy and analyzed them with a two-carrier Drude model. Taking into account the values of electron (hole)-phonon scattering rates previously reported in chemically doped silicon, we evaluated the electron-hole scattering rates γe-h. From 293 to 90K, the magnitudes and temperature dependence of γe-hwere successfully reproduced by a theoretical model including the effects of Rutherford scattering, Coulomb screening, and Pauli exclusion. This suggests that these three factors dominate electron-hole scattering processes in silicon. Below 90K, γe-hbecomes larger than that of the theoretical curve, which is attributable to a prolongation of the relaxation time of hot carriers.

• 
• 
• 
• 

Abstract-Terahertz radiation induced by coherent phonon generation via impulsive stimulated Raman scattering in paratellurite

M. Sotome, N. Kida, R. Takeda, and H. Okamoto
https://journals.aps.org/pra/abstract/10.1103/PhysRevA.90.033842

We report on the observation of terahertz radiation in a noncentrosymmetric insulating oxide, paratellurite (α−TeO2), by irradiation of a femtosecond laser pulse at room temperature. In the power spectrum of the terahertz radiation, an intensity fringe pattern with a period of ∼0.25 THz shows up below 3 THz. It can be reproduced by taking into account the effective generation length for the terahertz radiation with a poor phase-matching condition. In addition, a temporal oscillation component appears in the radiated terahertz wave with a frequency of ∼3.71 THz, which is in good agreement with the center frequency of the Raman active longitudinal optical E mode. On the basis of comprehensive polarized optical and Raman spectroscopic studies, we explain the generation mechanism of the temporal oscillation component in terms of the coherent phonon generation via impulsive stimulated Raman scattering.

DOI: http://dx.doi.org/10.1103/PhysRevA.90.033842

• 
• 
• 
• 
• 
• 
• 

Abstract-Visualization of ferroelectric domains in boracite using emission of terahertz radiation

http://arxiv-web3.library.cornell.edu/abs/1409.2960

Y. Kinoshita, N. Kida, M. Sotome, R. Takeda, N. Abe, M. Saito, T. Arima, H. Okamoto

(Submitted on 10 Sep 2014)

We report on the emission of terahertz radiation by irradiation of femtosecond laser pulses in non-centrosymmetric paraelectric and ferroelectric phases of Co3B7O13I boracite. The Generation of the terahertz waves in both phases is caused by optical rectification via a second-order nonlinear optical effect. In the ferroelectric phase, we successfully visualized ferroelectric domains by analyzing the polarization state of the terahertz wave radiated from the crystal. In a large area of the crystal (∼ 500 × 500 μm2), the observed polarization vector of the radiated terahertz wave was tilted from directions of spontaneous polarization, i.e., [100]cub, [010]cub, and [001]cub in cubic setting, which can be explained by the presence of a ferroelectric 90∘ domain wall of the (101)cub plane.