Low-threshold quantum-cascade lasers without injector regions, emitting at /spl lambda/ /spl sim/ 6.7 /spl mu/m

We report on an unusual quantum-cascade laser design without injector miniband, designed as a five-level staircase. The aim of this design scheme is to prevent a thermal backfilling of the lower laser state, by using a double or triple LO-phonon resonance, respectively. These lasers show exceedingly small threshold current densities at low temperatures (0.2 kA/cm² at 77 K) and a threshold voltage of about 10 V. At 300 K, threshold current densities of ~2 kA/cm² are achieved and the maximum operating temperature in pulsed mode is 380 K