Novel quantum cascade devices for long wavelength IR emission

There is presently a lack of efficient, compact, solid-state sources for the 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, conventional semiconductor lasers are limited to the mid-infrared. Intersubband or interminiband transitions, which constitute the basis of the very successful quantum cascade (QC) lasers, possess the potential for the efficient generation of far-infrared light, although many important physical questions have to be addressed in the case of THz transition frequencies. Furthermore, the problem of confining long wavelength radiation inside waveguides with thickness 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 exploited in the design of a high performance λ ∼ 17 μm (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 with respect to a conventional waveguide with semiconductor claddings. The emission is made single mode with the adoption of a dual-metal Bragg grating which modulates the skin depth of the surface plasmon. The same approach is used in the realization of a λ ∼ 19 μm (15.8 THz) QC laser, which represents the longest wavelength III-V semiconductor laser to date.