I General Introduction

In this book, we define TeraHertz (THz) waves as the part of the electromagnetic spectrum with wavelengths ranging from 3mm down to 30 μm, that is, from 100GHz to 10 THz covering the upper part of millimeter waves (30–300GHz), the whole range of submillimeter waves (300GHz to 3 THz), and the lower end of infrared waves (from 3THz to visible light). THz science and technology is a relatively young area both in research and applications. THz applications started from radio astronomy in the 1970s. This was based on the property that molecules and atoms can be identified by their radiation spectrum caused by their rotational and vibrational resonances. Since then the THz band has found many other potential applications because it provides unprecedented bandwidth and opportunities for completely new sensor applications. It is feasible and potential for many ground-based commercial applications as well as for Earth science applications: remote sensing of the Earth’s surface and atmosphere, broadband high data-rate indoor (e.g., smart home) and short-range outdoor wireless communications, short-range, long-range, and multi-function automotive radars and ultra wide band (UWB) high-resolution radars, telematics for road traffic and transport, both between vehicles and between vehicles and infrastructures, imaging for security, medical, and other purposes. In free space, transmission of THz waves typically requires line-of-sight between the transmitter and the receiver. The length of a terrestrial communication hop cannot be very long since the water vapor of the atmosphere is highly absorbent, ranging from not more than a few meters to hundreds of meters. On the other hand, this high attenuation enables one to limit THz-communication distances to secure distances. Depending on the relative humidity (between 25 and 100%), the atmospheric attenuation at sea level and at a temperature of 25 ∘C ranges from 0.3 to 1 dB km–1 at 100GHz and from 50 to 250 dB km–1 at 700GHz. At water vapor absorption peaks, for example, at 557GHz, the attenuation may reach values over 104 dB km–1. So far the employment of THz systems for applications has been slow because of the immature technology. The frequency range from 0.1 to 10 THz is often called the terahertz gap, because technologies for generating and detecting this radiation are much less mature than those at microwave or infrared