ZnTe-20-20 110-cut crystal for large-area electro-optic ZnTe terahertz emitters

Описание: ZnTe-20-20 110-cut crystal for large-area electro-optic ZnTe terahertz emitters - [email protected]

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Large-area electro-optic ZnTe terahertz emitters
T. Löffler, T. Hahn, M. Thomson, F. Jacob, and H. G. Roskos
Physikalisches Institut, Johann Wolfgang Goethe-Universität,
Max-von-Laue-Strasse 1, D-60438 Frankfurt am Main, Germany
Abstract: We present a detailed experimental and theoretical study of terahertz (THz) generation and beam propagation in an optoelectronic THz system consisting of a large-area (ZnTe) electro-optic emitter and a standard electro-optic detector, and provide a comparison to typical biased GaAs emitters. As predicted by theory, in the absence of saturation the generated THz pulse energy is inversely proportional to the area of the optical pump beam incident on the emitter, although the detected on-axis
electric field amplitude of the subsequently focused THz beam is
practically independent of this area. This latter result promotes the use of larger emitter crystals in amplifier-laser-based THz systems in order to minimize saturation effects. Moreover, the generation of an initially larger THz beam also provides improved spatial resolution at intermediate foci between emitter and detector.

Broadband optoelectronic terahertz (THz) generation and detection with femtosecond (fs) optical pulses is a powerful and well-studied method for performing far-infrared spectroscopy, imaging and sensing [1]. Among the various approaches developed to date, those employing low-repetition-rate high-pulse-energy amplifier lasers as the optical pulse source offer distinct advantages due to (i) the comparatively high THz pulse energies
achieved, and (ii) the ability to efficiently generate synchronous pulses in other regions of the electromagnetic spectrum. This opens the way for important THz applications such as nonlinear spectroscopy [2,3] and single-shot imaging [4]. An overview of amplifier-laser-based THz systems is provided in Ref. [5,6], including a discussion of large-area GaAs antennae emitters [7-8], THz beam propagation [6,9] and applications of such systems [10]. For the case of THz generation in electro-optic crystals, detailed analyses exist on the influence of the crystal orientation and thickness [11,12].
However, despite the fact that large-area electro-optic crystals (i.e. with illuminated dimensions much greater than the THz-wavelength) have already been used in conjunction with amplifier lasers in applications [10,13] and for the testing of organic electro-optic materials [14], there is still a lack of analysis in the literature in terms of the optimum optical pump beam geometry. In this paper we examine the performance of amplifier-laser-based
THz systems with respect to the optical pump beam size, both theoretically and experimentally, and provide a comparison with the performance of standard large-area biased GaAs emitters.
For high-repetition-rate (and therefore, low pulse-energy) THz systems, it is common experimental practice to tightly focus the optical pump beam into the emitter crystal in order to maximize conversion efficiency. This is a rational strategy because the energy conversion efficiency of the second-order non-linear generation process is proportional to the opticalpump fluence (in the absence of saturation effects) [15]. In addition, the effective size of the emitter (i.e. that of the pump beam inside the crystal) is significantly smaller than the THzwavelength, hence the emitter is effectively a point source and no significant influence on the subsequent THz beam propagation is expected by further focusing of the pump beam.
The situation changes drastically when an amplifier laser system with high pump pulse energies in the µJ-mJ range is employed. In this case, the spot size of the pump beam must be significantly increased to avoid damaging the emitter crystal due to dielectric breakdown. In addition, saturation effects in the generation process become important for such high pulse energies. In order to overcome such problems, electro-optic emitter crystals are usually employed with an unfocused pump beam with a diameter in the mm-cm range. As demonstrated both theoretically and experimentally in this paper, increasing the size of the optical pump beam does not actually reduce the detected (on-axis) THz electric field, due to the improved focusing properties of larger beams, with the resulting optimal strategy being to defocus the optical pump beam on the emitter insofar as practicable whilst minimizing saturation effects.
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