Far-IR quantum cascade lasers at 21.5 /spl mu/m wavelength

Summary form only given. We report the first QC laser operating above 20 /spl mu/m, in particular at a maximum wavelength of 21.5 /spl mu/m. No other III-V semiconductor laser has operated previously at such long wavelength. Laser action, obtained up to a maximum temperature of 130 K originates from inter-miniband transitions in appropriately "chirped" graded superlattices; we calculated a very large optical dipole matrix element of 6.1 nm and a lifetime of the upper laser state of only 0.7 ps. Such long wavelengths QC lasers require carefully designed injectors, whose accurate funnel-like shape avoids tunneling of carriers to higher energy states of the following periods of the heterostructure. This becomes more and more relevant at longer wavelengths, where the small transition energy becomes comparable to the energy separation of the states within the minibands. The waveguide design consists of a surface-plasmon guide on top and InGaAs on the bottom to achieve dielectric confinement; this results in a high confinement factor and a calculated waveguide attenuation. A double surface-plasmon structure was also grown, i.e. a system in which the bottom confinement is provided by a highly doped layer, acting essentially as a metal.