The terahertz electromagnetic spectrum comprehends the electromagnetic radiations whose wavelength ranges from 0.1 mm to 1 mm, therefore also known as submillimiter band. These radiations fall into the so-called terahertz gap, owing to the fact that there is a lack of generation and detection technologies for these frequencies that are between the radio-waves and the mid-infrared, i.e. between the spectral regions optimized by electronics and optics respectively. Historically, terahertz spectroscopy technologies have been exploited mainly in the field of astronomy, imaging and security, but recent technological improvements are gradually closing the gap and making this band available for more and more applications for sensing and communication.