Sensing at Terahertz Frequencies
In this chapter a review of sensing applications working at terahertz (THz) frequencies is performed. Firstly, an introductory section putting in context the THz regime and highlighting particularities and potential applications at this frequency band is
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Abstract In this chapter a review of sensing applications working at terahertz (THz) frequencies is performed. Firstly, an introductory section putting in context the THz regime and highlighting particularities and potential applications at this frequency band is outlined. Then, a comprehensive examination of sensing solutions following different technologies investigated during the last two decades is presented. Special attention is given to the fibre optics solutions and free-space approaches based on metasurfaces. Finally, plasmonic sensing platforms and waveguide solutions are discussed as well.
1 Introduction The terahertz (THz) band is usually defined as the spectral region that lies between microwaves and far-infrared. It is generally accepted that it extends from 0.1 to 10 THz [1], but there is not a total consensus about its exact limits yet. For example, some scientists extend the upper limit up to 30 THz, in contradiction with the classical infrared band limits used in optics that puts the far-infrared boundary at 1 THz. Likewise, the lower limit of 0.1 THz does not agree with the classical band classification of microwave engineering, wherein millimetre-waves cover the frequency span from 30 to 300 GHz. This disagreement comes from the intrinsic multidisciplinary character of THz, with scientists and technologists of multiple and varied disciplines sharing knowledge and working together to devise new devices and concepts in this relatively new band. That being said, and being aware that it is still under debate, in this chapter we will consider that the THz band goes from 0.1 to 10 THz, which is the most accepted definition so far. Figure 1 shows the electromagnetic spectrum wherein the different frequency ranges are delimited. Since depending on the background of the reader, he or she might be used to work in terms of wavelength, wavenumber, or frequency, the correspondence units are also outlined. P. Rodríguez-Ulibarri ⋅ M. Beruete (✉) Public University of Navarra, Pamplona, Spain e-mail: [email protected] © Springer International Publishing Switzerland 2017 I.R. Matias et al. (eds.), Fiber Optic Sensors, Smart Sensors, Measurement and Instrumentation 21, DOI 10.1007/978-3-319-42625-9_14
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Fig. 1 Electromagnetic spectrum and its associated applications
Until recently, the THz range was known as the “THz gap” due to the lack of efficient and cost-effective generators and detectors. In fact, for a long time there were only two main niches for THz applications, astronomy and spectroscopy. The classical THz sources were based on tubes (carcinotrons, klynstrons and backward wave oscillators) and free electron lasers (FEL). Those solutions were bulky, expensive and had low efficiency. Similarly, classical THz detectors usually relied on bolometers which need to be cooled at cryogenic temperatures. The situation changed completely with the emergence of new THz generation techniques based on optical rectification and photomixing plus new genuine THz sources l
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