1. C. Gmachl, F. Capasso, J. Faist, D. L. Sivco, and A. Y. Cho are at Bell Laboratories, Lucent Technologies, 600 Mountain Avenue, Murray Hill, NJ 07974, USA. E. E. Narimanov and A. D. Stone are in the Department of Applied Physics, Yale University, Post Office Box 208284, New Haven, CT 06520, USA. J. U. Nöckel is at the Max-Planck-Institut fur Physik komplexer Systeme, Nöthnitzer Str. 38, D-01187 Dresden, Germany.