In conventional lasers the lasing threshold is achieved when the population difference between the initial and final states of the lasing transition reaches a critical value determined by the equality between gain and optical losses. In such lasers, lasing requires population inversion. This generally is achieved by optical or electrical pumping.
However, lasers without population inversion have been proposed. For instance, S. E. Harris, Physical Review Letters, Vol. 62(9), p. 1033 (1989), shows on theoretical grounds that under certain conditions that include interference between lifetime-broadened discrete levels which decay to the same continuum, lasing without population inversion may be possible in extreme UV and X-ray laser systems, or in systems that comprise artificially layered materials. A. Imamoglu et al., Optics Letters, Vol. 19(21), p. 1744 (1994) propose another scheme for lasing without population inversion. The scheme utilizes interferences in double-quantum well intersubband transitions. Neither publication offers experimental verification. See also R. F. Kazarinov et al., Soviet Physics Semiconductors, Vol. 5, p. 707 (1971), and R. F. Kazarinov et al., Soviet Physics Semiconductors, Vol. 6, p. 120 (1972), which disclose the possibility of the amplification of electromagnetic waves in a semiconductor structure with a superlattice. The structure proposed by Kazarinov et al. is subject to field break-up into field domains, and electromagnetic wave amplification has not been observed in the proposed structure.
It would be highly desirable to have available a tunable laser in the mid-IR wavelength range (e.g. about 3-13 .mu.m), due to the existence of atmospheric transmission windows at 3-5 .mu.m and 8-13 .mu.m. Many gases and vapors have pronounced absorption features in these wavelength regions, and thus could readily be detected. Such a tunable laser could be advantageously used for trace-gas sensing for, e.g., environmental, industrial or medical applications. See, for instance, U. Martinelli, Laser Focus World, March 1996, p. 77.
Recently, a new class of lasers, designated "quantum cascade" or "QC" lasers, was disclosed. See U.S. Pat. Nos. 5,457,709 and 5,509,025. See, for instance, also J. Faist et al., Science, Vol. 264, p. 553 (1994); J. Faist et al., Applied Physics Letters, Vol. 66, p. 538 (1995); J. Faist et al., Applied Physics Letters, Vol. 67, p. 3057 (1995); C. Sirtori et al., Applied Physics Letters, Vol. 68, p. 1745 (1996); C. Sirtori et al., Applied Physics Letters, Vol. 69, p. 2810 (1996); and J. Faist et al., Applied Physics Letters, Vol. 68, p. 3680 (1996). All of the above are incorporated herein by reference. The operating wavelength of a QC laser can be tailored over a wide range of wavelengths (including wavelengths in the mid-IR range) by controlling layer thickness in the multilayer semiconductor structure. QC lasers utilize intersubband population inversion to achieve lasing, with the lasing wavelength substantially not being electric field tunable.
In view of the potential advantages of an electric field-tunable semiconductor laser, especially one that is capable of operation in the mid-IR wavelength regime, it would be highly desirable to have available such a laser. This application discloses such a laser, the laser utilizing a novel mechanism to establish population inversion.