New hope for terahertz: "Some researchers had even begun to suspect that a room-temperature, solid-state terahertz laser was physically impossible. The performance of experimental terahertz lasers built in the lab has suggested a linear correlation between operating temperature and frequency, in which halving the frequency requires roughly halving the temperature. This led some scientists to speculate that frequency and temperature are linked by some fundamental physical law, a strict proportionality that couldn’t be violated.
That hypothesis, however, turns out to be wrong. In the latest issue of the journal Nature Physics, a group of researchers at MIT and Sandia National Laboratories report a solid-state terahertz laser that operates at nearly twice the temperature that that putative proportionality would have predicted."