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-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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