High k -space lasing in a dual-wavelength quantum cascade laser

The understanding of charge carrier distributions is fundamental to our knowledge of laser systems. In semiconductor lasers, because of the propensity of charge carriers to undergo extremely fast momentum relaxation 1 , 2 , they accumulate at band extrema—that is, they have a small wavevector close to k ≈ 0 in direct-gap semiconductors. Conventional understanding suggests that the device-level physics occurs at these band extrema, including population inversion for lasing. This behaviour is universal in diode lasers 3 , 4 , interband cascade lasers 5 and quantum cascade lasers 6 , 7 . Here, we report on a quantum cascade laser with an energy configuration able to establish local population inversion high in k -space. We observe dual-wavelength emission from two discrete optical transitions. Temperature-dependent performance attributes show that the two transitions are highly coupled; competition for charge carriers is apparent from the anti correlated behaviour. The two optical transitions represent a conventional quantum cascade laser transition at k ≈ 0 and another laser transition from non-thermal electrons near k ≈ 3.6 × 10 8 m −1 . Scientists report a dual-wavelength quantum cascade laser that lases at wave factors k ≈ 0 and k ≈ 3.6 × 10 8 m −1 . The finding may change the conventional idea that population inversion of lasing occurs only at k ≈ 0 and give ways on designing intersub-band devices with high k -space.