Performance of combined h100i–h110i ZnTe crystals in an amplified THz time-domain spectrometer Dmitry Turchinovich * , Jaap I. Dijkhuis Atom Optics and Ultrafast Dynamics, Department of Physics and Astronomy, University of Utrecht, P.O. Box 80000, 3508TA Utrecht, The Netherlands Received 15 June 2006; received in revised form 22 August 2006; accepted 29 August 2006 Abstract In this work we report on the performance of combined index-matched ZnTe h110i–h100i THz emitters and detectors for reduction of etalon effects in a nonlinear crystal-based THz time-domain spectrometer, driven by an amplified femtosecond laser. We demonstrate that the application of a combined electrooptic detector crystal reduces the strength of the first etalon and the average spectral dynamic range oscillations by a factor of two. We also show that even in a simple emitter configuration the emitter-related etalon is significantly reduced due to the transient dielectric effects of carriers created by two-photon absorption in the emitter. Ó 2006 Elsevier B.V. All rights reserved. PACS: 07.57.Pt; 42.65.Re; 42.70.Nq; 72.30.+q Keywords: Terahertz spectroscopy; Optical rectification; Electrooptic sampling Generation of THz pulses via optical rectification (OR) of ultrashort laser pulses [1,2] and their detection via free- space electrooptic sampling (FEOS) [3,4] in nonlinear crys- tals of special orientation is a common approach in modern THz time-domain spectroscopy (THz-TDS). In OR the bandwidth of the incident powerful laser pulse is converted into the bandwidth of THz emission, while both the optical and THz signal co-propagate through the nonlinear crys- tal. In FEOS both THz and weak probe laser pulses co- propagate through the nonlinear crystal, leading to the THz field-induced phase retardation of the specially pre- polarized probe laser pulse. This phase retardation is pro- portional to the electric field strength of the detected THz signal. In case of cubic nonlinear crystals, such as ZnTe or GaP, the crystals with h110i crystal orientation are able to perform OR and FEOS at normal incidence. The crys- tals of h100i orientation do not possess the nonlinear prop- erties needed for OR and FEOS, although their linear THz and optical properties are identical to that of h110i-ori- ented crystals. The requirement for a successful generation or detection of a THz pulse in such a nonlinear crystal-based THz-TDS spectrometer is phase matching between the generating (detecting) optical pulse and generated (detected) THz sig- nal. Since the nonlinear crystals suitable for THz spectros- copy applications have strong optical phonon resonances in the THz range, the strong dispersion of THz refractive index limits the phase-matching frequency range (see e.g. [5]). Thick nonlinear crystals provide THz-optical phase matching around a narrow frequency band. They support only a fraction of the bandwidth of the generating (detect- ing) laser pulse, since optical and THz signals experience larger walk-off over long co-propagation distances. But the generated (detected) peak signal strength is generally high for long co-propagation distance. Thin nonlinear crystals provide good THz-optical phase matching within the full bandwidth of the generating 0030-4018/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.08.045 * Corresponding author. Tel.: +31 302532206; fax: +31 302537468. E-mail address: D.Turchinovich@phys.uu.nl (D. Turchinovich). www.elsevier.com/locate/optcom Optics Communications 270 (2007) 96–99