Radiation and Matter

At a fundamental level what distinguishes one spectral region from another is how radiation interacts with matter. While the underlying electromagnetic wave theory simply scales with frequency and wavelength, we associate dramatically dissimilar phenomena with different regions of the spectrum, ranging all the way from audio signals that drive classical vibrations we can hear and feel to cosmic rays.

So what is it that distinguishes the THz spectral region; what kinds of science, technology, and applications have arisen, and what kinds of scientific and technological applications can we foresee? One THz corresponds to an energy of 0.004 eV, a temperature of 50 K, a wavelength of 0.3 mm, and, in common spectroscopic terms, 33.3 cm^-1. If the commonly used bounds extending from perhaps 0.1 THz (near where a large proportion of current THz work is done) to 10 THz are adopted, the corresponding temperature scale ranges from 5 K to 500 K.

We will argue below that the relative size of $hν$ and $kT$ is important. Thus, these wide definitions of the THz regime include both the $hν/kT>>1$ and $hν/kT<<1$ limits, with a correspondingly broad range of phenomena. Likewise, this definition of the THz includes wavelengths from 3 mm to 30 µm. Thus, size considerations (whose scales are ordinarily set by the wavelength) lead to low order mode (i.e. microwave) devices in the longer wavelength portions of this region and to high order mode (i.e. laser) devices at the shorter wavelengths. For now, we will simply note here that jumping what is sometimes referred to as the “gap in the electromagnetic spectrum” is not equivalent to filling it.