Negative free carrier absorption in terahertz quantum cascade lasers.

We analyze the peculiar case where the free carrier absorption arising from LO phonon absorptionassisted transitions becomes negative and therefore turns into a gain source for quantum cascade lasers. Such an additional source of gain exists when the ratio between the electronic and the lattice temperatures is larger than one, a condition that is usually fulfilled in quantum cascade lasers. We find a gain of few cm^-1's at 200 K. We report the development of a terahertz quantum cascade laser operating in the negative free carrier absorption regime. [ABSTRACT FROM AUTHOR]