Half Cycle Terahertz Pulse Generation by Prism-Coupled Cherenkov Phase-Matching Method
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Half Cycle Terahertz Pulse Generation by Prism-Coupled Cherenkov Phase-Matching Method Kodo Kawase & Shingo Ichino & Koji Suizu & Takayuki Shibuya
Received: 2 January 2011 / Accepted: 5 May 2011 / Published online: 17 May 2011 # Springer Science+Business Media, LLC 2011
Abstract Nonlinear optical terahertz wave generation is a promising method for realizing a practical source with wide frequency range and high peak power. Unfortunately, many nonlinear crystals have a strong absorption in the terahertz frequency region. This limits efficient and widely tunable terahertz wave generation. The Cherenkov phase-matching method is one of the most promising techniques for overcoming these problems. We propose a prism-coupled Cherenkov phase-matching method, in which a prism with a suitable refractive index at terahertz frequencies is coupled to a nonlinear crystal. We demonstrate prism-coupled Cherenkov phase-matching terahertz generation using the DAST and LiNbO3 crystals. With a DAST crystal, we obtain a spectral flat tunability up to 10 THz by difference frequency generation. With a LiNbO3 crystal, we observe a spectral flat broadband terahertz pulse generation up to 5 THz pumped by a femto second fiber laser. The obtained temporal waveform is an ideal half cycle pulse suitable for reflection terahertz tomography. Keywords Terahertz wave . Nonlinear optics . Cherenkov phase-matching . Tomography
1 Introduction A parametric process based on second-order nonlinearities has been used to generate tunable, monochromatic, coherent terahertz (THz) sources using nonlinear optical (NLO) crystals [1–4]. In general, however, NLO materials have high absorption coefficients in the THz region, which prevents the efficient generation of THz radiation. NLO crystals also have a difference in the refractive index between the optical and THz frequencies, and this inhibits effective collinear frequency conversion. Quasi-phase-matching methods can K. Kawase : S. Ichino : K. Suizu : T. Shibuya Nagoya University, Ecotopia, Furo-cho, Nagoya 464–8603, Japan K. Kawase (*) : T. Shibuya RIKEN, Advanced Science Institute, 519-1399 Aramaki-Aoba, Sendai 980–0845, Japan e-mail: [email protected]
J Infrared Milli Terahz Waves (2011) 32:1168–1177
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address this problem, but the tunable range is limited due to the periodicity of the device. Surface-emitting THz generation can overcome these problems [5]; absorption is minimized because the THz-frequency wave is generated at the crystal surface: the Cherenkov-type radiation is generated as surface emission [6–11]. We have previously demonstrated a Cherenkov phase-matching method for monochromatic THz generation via difference frequency generation (DFG) using lithium niobate crystals [12–15]. We were able to generate monochromatic THz radiation with wide tunability in the range of 0.1–7.2 THz. The lithium niobate crystal has a strong absorption line at 7.5 THz, and the refractive index changes considerably around the line. The Cherenkov condition would broke down slightly above 7.5 THz bec
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