High Resolution Waveguide Terahertz Time-Domain Spectroscopy
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High Resolution Waveguide Terahertz Time-Domain Spectroscopy Michael Theuer & Joseph S. Melinger
Received: 20 June 2011 / Accepted: 18 July 2011 / Published online: 21 August 2011 # Springer Science+Business Media, LLC 2011
Abstract Terahertz time-domain spectroscopy accesses the frequency range between 100 GHz and 5 THz by using the coherent generation and detection based on femtosecond laser sources. On the way to obtain fingerprint absorption spectra of molecular solids, terahertz waveguides have proven to be a valuable tool to extend the results to narrow and high resolution linewidths of crystalline solids. We will discuss the development, properties and applications of terahertz waveguide geometries for spectroscopic applications, in particular high-resolution measurements using parallel-plate waveguides. Keywords Terahertz . Far infrared . Waveguide . Spectroscopy
1 Introduction Many interesting molecular spectroscopic transitions can be observed in the terahertz (THz) frequency range covering frequencies between 100 GHz and 10 THz. These frequencies in the far-infrared correspond to vacuum wavelengths between 3 mm and 30 μm or wavenumbers reaching from 3.3 cm−1 to 333 cm−1. The rich gas phase and condensed phase spectroscopy in this region was first explored using incoherent sources and narrowband coherent sources. The later development of sub-picosecond THz sources led to the creation of THz time-domain spectroscopy (THz-TDS), which provides coherent
M. Theuer (*) Department of Terahertz Measurement and Systems, Fraunhofer-Institute for Physical Measurement Techniques IPM, 67663 Kaiserslautern, Rhineland-Palatinate, Germany e-mail: [email protected] URL: http://www.ipm.fraunhofer.de J. S. Melinger Electronics Science and Technology Division, Naval Research Laboratory, Code 6812, Washington, DC 20375, USA e-mail: [email protected]
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J Infrared Milli Terahz Waves (2011) 32:1267–1284
detection over a broad bandwidth of 0.1 THz to 5 THz [1]. Early THz-TDS experiments demonstrated the broadband coherent detection of pressure broadened rotational transitions of water vapor [2]. This initial work was followed by the demonstration of coherent echo transients resulting from broadband coherent excitation of rotational bands in small polar molecules [3–5]. Since these early studies a variety of small molecule vapors have been studied, in some cases with frequency resolution as high as 500 MHz [6]. The so-called low frequency vibrational modes of molecular solids also occur in the THz region. These vibrations tend to be global in the sense that they involve the motion of nearly all the atoms in the molecule. As such they are strongly influenced by intermolecular interactions including hydrogen bonding [7] and van der Waals forces [8]. These interactions determine the character of the vibrational motion as intramolecular modes, intermolecular modes, or mixed modes containing both intra- and intermolecular character. Indeed, a large number of THz-TDS studies have been performed on rela
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