FREQUENCY-TUNABLE TERAHERTZ-WAVE GENERATION FROM GaP USING Cr:FORSTERITE LASERS
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FREQUENCY-TUNABLE TERAHERTZ-WAVE GENERATION FROM GaP USING Cr:FORSTERITE LASERS J. Nishizawa1, T. Sasaki1, K. Suto1, T. Tanabe2, T. Yoshida1, T. Kimura1 and K. Saito2 1
Semiconductor Research Institute, Aoba 519-1176, Aramaki, Sendai 980-0845, Japan 2 Department of Material Science, Tohoku University, Aoba-yama 6-6-11-1021, Sendai 980-8579, Japan e-mail: [email protected]
Abstract We have constructed THz spectrometers using the widely frequency-tunable THz-wave generated from GaP crystal pumped at 1.2 μm region using two Cr:forsterite lasers and compared with that pumped at 1 μm region using a YAG laser and an optical parametric oscillator (OPO). The systems have sufficient resolution for observation of solids and liquids at room temperature. We have measured terahertz absorption spectra of all twenty kinds of amino acids which form proteins.
Key words: terahertz wave, GaP, Raman, Cr:forsterite, phase matching, spectroscopy 779
Nishizawa et al.
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1. Introduction Following the proposal of THz-wave generation via the resonance of lattice or molecular vibrations by Nishizawa [1,2], Nishizawa and Suto realized a semiconductor GaP Raman laser [3] and generated a 12-THz wave with a peak power as high as 3 W using a GaP Raman oscillator containing a GaAs mixing crystal [4]. Following the same principle, Kawase and Ito realized frequency-tunable high-power THz-wave generator using LiNbO3 crystals [5]. For more practical generation of THz-waves and for their application in the field of chemical, biological, medical, etc., for example detection and treatment of cancers [6], we developed widely frequency-tunable high-power THz-wave generator (GaP-Raman THz generator) based on difference- frequency generation (DFG) in GaP crystals using a YAG laser and an optical parametric oscillator (OPO-source system) [7-9] and constructed an automatic THz spectral measurement system [10]. We have measured the THz spectra of bio-molecules, such as saccharides, DNA/RNA-bases, etc., in the frequency region from 0.4- to 5.8-THz (13 - 193
cm-1) with a spectral
resolution of 3.2 GHz (0.1 cm-1) using this spectrometer[10-14]. We have also demonstrated THz spectral imagings of biomolecules and human tissue [12]. Recently, we have constructed the GaP-Raman THz spectrometer using Cr:forsterite lasers (Cr:F-source system) instead of the YAG and the OPO lasers (OPO-source system). In this paper, we describe details of the Cr:F- source system and compare it with the OPO-source system. Absorption coefficient spectra of amino acids measured at room temperature by using the Cr:F-source GaP-Raman THz spectrometer are shown.
Frequency-Tunable Terahertz-Wave Generation from GaP Using Cr:Forsterite Lasers
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2. Terahertz Spectrometer Fig. 1 shows an experimental setup of Cr:F-source GaP-Raman THz generator. Two Cr:Forsterite lasers pumped by a Q-switched Nd:YAG laser with a 10 Hz repetition were used as pump and signal beam for the DFG. While the wavelength of the pump beam was fixed to 1210 nm, that of the signal beam was tuned between 1210 an
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