Low voltage-defect quantum cascade lasers based on excited-states injection at λ ∼ 8.5  μm.

A quantum cascade laser emitting at λ∼8.5  μm based on the excited-state injection is presented. The operating voltage is reduced for a low-voltage defect in the excited-state design, compared with the conventional ground-state injection design. The threshold voltage and voltage defect are as low as 6.3 V and 54 mV for a 30-stage active region, respectively. Devices were fabricated through standard buried-heterostructure processing to decrease the heat accumulation. A continuous-wave optical power of 340 mW is obtained at 283 K with a threshold current density of 2.7  kA/cm ² . Such a design has the potential to further improve the wall plug efficiency for increased voltage efficiency.