Intense Terahertz Pulse-Induced Nonlinear Responses in Carbon Nanotubes
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Intense Terahertz Pulse-Induced Nonlinear Responses in Carbon Nanotubes Ryo Shimano & Shinichi Watanabe & Ryusuke Matsunaga
Received: 30 January 2012 / Accepted: 24 May 2012 / Published online: 5 June 2012 # Springer Science+Business Media, LLC 2012
Abstract By using intense terahertz(THz) monocycle pulses, nonlinear light-matter interaction in aligned semiconducting single-walled carbon nanotubes(SWNTs) embedded in a polymer film was investigated. THz electric-field-induced ultrafast Stark effect of onedimensional excitons in SWNTs was observed at room temperature, suggesting the potential functionality of SWNTs for high speed electro-optic devices operating at telecom wavelength with a THz bandwidth. When the peak electric field amplitude exceeds 200 kV/cm, the generation of excitons by the THz pump becomes prominent. The mechanism is described by the above-gap excitation of electrons and holes in SWNTs due to the impact excitation process induced by the intense THz electric field. Keywords Terahertz time-domain spectroscopy . Exciton . Single-walled carbon nanotubes . Ultrafast . Stark effect . Impact excitation
1 Introduction With the development of terahertz time-domain spectroscopy (THz-TDS), the study of low energy electromagnetic properties in the range of several millielectronvolts in a variety of materials has been accelerated. Recently, much progress has been made in the technique of intense THz pulses generation, opening new avenues of research in the nonlinear responses of materials in the THz range [1, 2]. In the frequency range of a few THz, the optical rectification of a tilted-pulse-front femtosecond laser pulse with a stoichiometric LiNbO3 (sLN) crystal has made it possible to generate the intense THz pulses with 10 μJ energy [3, 4]. In the range of several tens THz, THz pulses with amplitudes exceeding 100 MV/cm R. Shimano (*) : S. Watanabe : R. Matsunaga Department of Physics, The University of Tokyo, 7-3-1 Hongo Bunkyo-ku, Tokyo 113-0033, Japan e-mail: [email protected] Present Address: S. Watanabe Department of Physics, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan
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J Infrared Milli Terahz Waves (2012) 33:861–869
were generated via difference-frequency mixing technique [5]. By tightly focusing the collimated THz beam emitted from a sLN into a diffraction-limited spot size [6], the peak electric field (E-field) amplitude of THz pulse as large as 0.7MV/cm in the frequency range below 2 THz has been achieved with the low pump laser pulse energy of 0.75 mJ, as we describe below in detail. By minimizing the distortion of the THz beam after the sLN crystal, the THz pulses with amplitudes as much as 1.2 MV/cm have been generated with use of a 4 mJ laser system [7]. In such a high field regime, the nature of light-matter interaction can change from perturbation regime to the field-induced tunnel-ionization regime. In atomic systems, these two regimes can be discriminated by the so-called Keldysh parameter γ 0 (Ip/2U
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