L. E. Vorobjev, S. N. Danilov, and D. A. Firsov, "Continuously tuned far-infrared semiconductor laser and its use for investigations,"
in Conference on Lasers and Electro-Optics, J. Bowers, D. Miller, D. Scifres, and A. Weiner, eds.,
Vol. 9 of OSA Technical Digest
(Optical Society of America, 1996), paper CTuO5.