Novel quantum cascade devices for long wavelength IR emission

There is presently a lack of efficient, compact, solid-state sources for th e spectral range 1-10 THz, also known as the "terahertz gap". In fact Gunn diodes fail at such high frequencies, while, from the other side, conventio nal semiconductor lasers are limited to the mid-infrared. Intersubband or i nterminiband transitions, which constitute the basis of the very successful quantum cascade (QC) lasers, possess the potential for the efficient gener ation of far-infrared light, although many important physical questions hav e to be addressed in the case of THz transition frequencies. Furthermore, t he problem of confining long wavelength radiation inside waveguides with th ickness compatible with existing growing techniques, minimizing at the same time the absorption losses, poses an interesting technological challenge. Recent significant progresses in this direction are presented here. Surface plasmon modes at the interface between a metal and the semiconductor are e xploited in the design of a high performance lambda similar to 17 mum (17.6 THz) superlattice QC laser. Thanks to adoption of this novel waveguide the total epitaxial thickness of the structure is reduced by a factor of 2 wit h respect to a conventional waveguide with semiconductor claddings. The emi ssion is made single mode with the adoption of a dual-metal Bragg grating w hich modulates the skin depth of the surface plasmon. The same approach is used in the realization of a lambda similar to 19 mum (15.8 THz) QC laser, which represents the longest wavelength III-V semiconductor laser to date. (C) 2001 Elsevier Science B.V. All rights reserved.