Wideband THz Time Domain Spectroscopy based on Optical Rectification and Electro-Optic Sampling

Abstract

We present an analytical model describing the full electromagnetic propagation in a THz time-domain spectroscopy (THz-TDS) system, from the THz pulses via Optical Rectification to the detection via Electro Optic-Sampling. While several investigations deal singularly with the many elements that constitute a THz-TDS, in our work we pay particular attention to the modelling of the time-frequency behaviour of all the stages which compose the experimental set-up. Therefore, our model considers the following main aspects: (i) pump beam focusing into the generation crystal; (ii) phase-matching inside both the generation and detection crystals; (iii) chromatic dispersion and absorption inside the crystals; (iv) Fabry-Perot effect; (v) diffraction outside, i.e. along the propagation, (vi) focalization and overlapping between THz and probe beams, (vii) electro-optic sampling. In order to validate our model, we report on the comparison between the simulations and the experimental data obtained from the same set-up, showing their good agreement.G eneration and detection of THz electromagnetic waves (0.1-10 3 10 12 Hz) is not a novel topic 1-3 , but it has been arousing an ever-increasing interest only in the last decade. Recent studies demonstrated that many substances and particularly bio-polymers like proteins, amino and DNA possess specific global and sub-global modes in the THz band. Hence, particular consideration has been recently devoted to THz spectroscopy since it reveals information on the conformational stage of molecules and potentially enables their discrimination in various compounds. Indeed, THz pulses have been broadly used, ranging from spectroscopy and time-resolved pump-probe experiments to imaging applications 4,5 . Moreover, the negligible ionization power of the THz radiations compared to optical and X-rays technologies make them perfectly suitable for biological applications 6 . For the above-mentioned reasons, THz technology are penetrating in lots of areas, beyond fundamental Physics investigation, spanning from medical care and homeland security to cultural heritage conservation 7-13 . The most challenging technology development remains in the area of THz sources and detectors, since, in general, both wide band and high power are required at the same time. Thanks to the improvement of the pulsed laser technology, different types of stable THz sources have been developed so far, with peak power values ranging from kW to MW 14,15 . In particular, Optical Rectification (OR) 16 combined with Electro-Optic Sampling (EOS) detection is still the configuration of choice in the few standard high energy pulsed THz system. For this reason, in this paper we present an analytical model able to simulate the full electromagnetic propagation which takes place in the THz-TDS set-up 17 , sketched in Fig. 1 (refer to Methods), based on OR and EOS and employing ,110. ZnTe crystals. We will explain and justify the approach and the approximations which underlie the modelling of each stage and that allowed us to build a ...

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