  We present terahertz quantum cascade lasers operating at a defect mode of a
photonic crystal bandgap. This class of devices exhibits single mode emission
and low threshold current compared to standard metal-metal lasers. The mode
selectivity is an intrinsic property of the chosen fabrication design. The
lower lasing threshold effect, already reported in photonic crystal quantum
cascade lasers, is further enhanced in the ultra-flat-dispersion defect line.
The presented results pave the way for integrated circuitry operating in the
terahertz regime and have important applications in the field of quantum
cascade lasers, spectroscopy and microcavity lasers.
