Quantum Cascade lasers and metal waveguides at λ > 20 μm

The realization of Quantum Cascade lasers at very long wavelengths is of particular interest due to the lack of narrowband, powerful sources in the far-infrared range of the electromagnetic spectrum. We report Quantum Cascade lasers operating above 20 μm (at λ= 21.5 μm and λ= 24 μm) wavelengths, with pulsed operation up to 140 K and with a peak power of few milliwatts at cryogenic temperatures. Increased accuracy in the band-structure design becomes one of the key factors to assure high electron injection efficiency and to prevent hot-carrier effects. For this reason we developed a technique which allows the observation of intersubband spontaneous emission in unipolar Quantum Cascade lasers above threshold, a helpful instrument for device optimization. Finally, at these very long wavelengths various types of waveguide concepts have to be adopted in order to reduce the otherwise prohibitive layer thickness, enhance the optical confinement and control the waveguide loss. We report Quantum Cascade lasers with double metal-semiconductor waveguide resonators for operating wavelengths of 19, 21 and 24 μm. The waveguides are based on surface-plasmon modes confined at the metal-semiconductor interfaces on both sides of the active region/injector stack and are not restricted by a cut-off wavelength for the TM polarized intersubband radiation.