Recent progress in quantum cascade lasers and applications

Quantum cascade ('QC') lasers are reviewed. These are semiconductor injecti on lasers based on intersubband transitions in a multiple-quantum-well (QW) heterostructure, designed by means of band-structure engineering and grown by molecular beam epitaxy. The intersubband nature of the optical transiti on has several key advantages. First, the emission wavelength is primarily a function of the QW thickness. This characteristic allows choosing well-un derstood and reliable semiconductors for the generation of light in a wavel ength range unrelated to the material's energy bandgap. Second, a cascade p rocess in which multiple-often several tens of-photons are generated per el ectron becomes feasible, as the electron remains inside the conduction band throughout its traversal of the active region. This cascading process is b ehind the intrinsic high-power capabilities of the lasers. Finally, intersu bband transitions are characterized through an ultrafast carrier dynamics a nd the absence of the linewidth enhancement factor, with both features bein g expected to have significant impact on laser performance. The first experimental demonstration by Faist et al in 1994 described a QC- laser emitting at 4.3 mum wavelength at cryogenic temperatures only. Since then, the lasers' performance has greatly improved, including operation spa nning the mid- to far-infrared wavelength range from 3.5 to 24 mum, peak po wer levels in the Watt range and above-room-temperature (RT) pulsed operati on for wavelengths from 4.5 to 16 mum. Three distinct designs of the active region, the so-called 'vertical' and 'diagonal' transition as well as the 'superlattice' active regions, respectively, have emerged, and are used eit her with conventional dielectric or surface-plasmon waveguides. Fabricated as distributed feedback lasers they provide continuously tunable single-mod e emission in the mid-infrared wavelength range. This feature together with the high optical peak power and RT operation makes QC-lasers a prime choic e for narrow-band light sources in mid-infrared trace gas sensing applicati ons. Finally, a manifestation of the high-speed capabilities can be seen in actively and passively mode-locked QC-lasers, where pulses as short as a f ew picoseconds with a repetition rate around 10 GHz have been measured.