Theoretical analysis of spectral gain in a terahertz quantum-cascade laser: Prospects for gain at 1 THz

In a recent letter [B. S. Williams Appl. Phys. Lett. 82, 1015 (2003)], Williams reported the development of a terahertz quantum-cascade laser operating at 3.4 THz or 14.2 meV. We have calculated and analyzed the gain spectra of the quantum-cascade structure described in their work, and in addition to gain at the reported lasing energy of similar or equal to14 meV, we have discovered substantial gain at a much lower energy of around 5 meV or just over 1 THz. This suggests an avenue for the development of a terahertz laser at this lower energy, or of a two-color terahertz laser. (C) 2003 American Institute of Physics.