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-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.

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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.

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