Quantum cascade (QC) lasers operating at terahertz frequencies were demonstrated two years ago, and, since then, their development has proceeded at a very rapid pace. The gain medium of the first devices was based on chirped superlattices, and a resonator relying on the surface plasmon concept was employed to achieve a large optical confinement with concomitant low propagation losses. Laser action was obtained at 4.4 THz, in pulsed mode and at temperatures up to 50 K. Improved fabrication allowed continuous-wave (cw) operation and increased the operating temperature to 75 K. Using a similar active region, lasing at 3.5 THz was achieved. More recently, various groups have introduced several new design concepts such as bound-to-continuum transitions and extraction of carriers via resonant phonon scattering, leading to pulsed operation up to 140 K, output powers of up to 50 mW, and cw operation up to 93 K. The lowest emission frequency is now 2.1 THz, tackling the technologically important region of 1.5-2.5 THz. Stable single-mode emission under all operating conditions has also recently become a reality thanks to the adoption of distributed feedback resonators. This rapid and substantial progress underlines the growing potential of QC lasers in THz photonics.
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