What Phenomena Fall in this Energy Range?

To a reasonable approximation the interactions of THz radiation with matter can be divided into three categories: interactions with low pressure gases, interactions with gases near atmospheric pressure, and interactions with liquids and solids. This separation is according to the $Q$ of the resonances, with phenomena in the first category having a $Q$ of ~10⁶, in the second a $Q$ of ~10², and in the last very low $Q$, or more often, continuum interactions. Most of the successful applications of this spectral region fall into the first category and are dependent on the high $Q$ for their success.

For an isolated system, its line-width $\Delta \nu$ is related to its relaxation time $\Delta \tau$ by $\Delta \nu \sim \frac{1}{\Delta \tau}$. For gasses this is a reasonable approximation and the concept of pressure broadening results from simple kinetic theory, with $\Delta \nu_{pb}$ typically ~10 MHz/Torr. At low pressure, Doppler broadening with $\frac{\Delta \nu}{\nu} \sim 10^{-6}$ (1 MHz at 1 THz) sets a lower bound. However, in solids and liquids the resonant systems are not isolated or are often collective and the line-widths are much broader.

Because of the six orders of magnitude difference in the line-widths of THz phenomena, appropriate technology for the respective scientific studies varies widely, as does the basic physics of the phenomena involved. Thus, line-width provides a useful and convenient classification for both the sciences associated with the THz as well as the appropriate technology for their study.