[14p-B2-1~19] 3.9 テラヘルツ全般

▲ [14p-B2-10] Absorption saturation of inter-molecular vibrational bands of molecular crystal observed by intense THz pulse irradiation

〇(P)Khumaeni Ali^1,2、Kitahara Hideaki¹、Gotoh Daiki¹、Furuya Takashi¹、Yamamoto Kohji¹、Tani Masahiko¹ (1.Research Center for Development of Far-Infrared Region, University of Fukui、2.Department of Physics, Diponegoro University, Semarang, Indonesia)

キーワード：bio-molecular crystal, absorption saturation, terahertz generation based nonlinear optics

Terahertz induced nonlinear effects have been observed in variety of semiconductors, such as Si [1]. Jewariya et al. reported the nonlinear response of amino acid microcrystalline l-arginine using an intense monocycle THz pulse [2]. The nonlinear response of the absorption bands provides us information of the molecular interactions, such as hydrogen-bondings and the unharmonicity of the interaction potential, etc. Here we report another example of saturable absorption of bio-molecular crystal in THz frequency region. The sample used in this study was l-cystine crystal, the oxidized dimer form of the amino acid cysteine. The sample was prepared in the form of a pellet with a thickness of 1.2 mm and a diameter of 13 mm. The THz pulse was generated by using tilted pulsed front scheme with a Mg-doped stoichiometric LiNbO₃ crystal. The maximum THz electric field E_max at the focal point of THz beam was estimated to be 450 kV/cm. The terahertz electric field was varied by using a pair of wire-grid polarizers. A femtosecond laser system (800 nm, 100 fs, 1 kHz, maximum pulse energy ~ 3 mJ) was used as the optical source for the generation and detection of THz emission. THz radiation is guided with four off-axis parabolic mirrors of different focal lengths to the EO detection optics consisting of ZnTe crystal (t=1 mm) and a balanced photodetector. The time-domain THz waveform was obtained in a standard THz-TDS scheme. The sample was placed at the focal point of THz beam in the four parabolic mirror system. All the measurements were conducted at room temperature at around 20^OC. Figure 1(b) shows the transmittance spectra of l-cystine at various THz electric fields. The inter-molecular absorption bands observed at 0.24 THz and 0.71 THz [3] show saturation of absorption with increasing the THz electric field. However, their frequency shifts and broadening of the absorption bands are small, indicating the unharmonicity of these vibration modes are not strong. This result shows THz field in the order of ~100 kV/cm is already strong enough to investigate the unharmonicity of inter-molecular potential of small bio-molecules.
References:
S. Li, G. Kumar, T.E. Murphy, Optica 2, 553 (2015).
M. Jewariya, M. Nagai, and K. Tanaka, Phys. Rev. Lett. 105, 203003-1 (2010).
N. N. Barndt, et al, Vibrational Spectroscopy 47, 53-58 (2008).