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 relaxation1,2, 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 lasers3,4, interband cascade lasers5 and quantum cascade lasers6,7. 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 × 108 m−1.
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The authors gratefully acknowledge helpful comments, advice and support from J. Chen, F.-S. Choa, S. R. Forrest, S. Gooding, K.-T. Shiu, I. Waldmueller and Y. Yao. This work was supported in part by the Mid-Infrared Technologies for Health and the Environment (MIRTHE) centre (NSF-ERC no. EEC-0540832) and the European Union Marie Curie Research Training Network Physics of Intersubband Semiconductor Emitters (POISE) programme. K.J.F. gratefully acknowledges the support of the National Science Foundation Graduate Research Fellowship Program.
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Franz, K., Menzel, S., Hoffman, A. et al. High k-space lasing in a dual-wavelength quantum cascade laser. Nature Photon 3, 50–54 (2009) doi:10.1038/nphoton.2008.250
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