1. 1. R. F. Kazarinov and R. A. Suris, Possibility of amplification of electromagnetic waves in a semiconductor with superlattice, Sov. Phys. Semicond. 5, 707 (1971). 2. J. Faist, F. Capasso, D. L. Sivco, C. Sirtori, A. L. Hutchinson, and A. Y. Cho, Quantum cascade lasers, Science 264, 553 (1994).