Far-infrared surface-plasmon quantum-cascade lasers at 21.5 μm and 24 μm wavelengths

Quantum-cascade lasers operating above 20 μm (at λ∼21.5 μm and λ∼24 μm) wavelength are reported. Pulsed operation was obtained up to 140 K and with a peak power of a few milliwatts at cryogenic temperatures. Laser action originates from interminiband transitions in "chirped" superlattice active regions. The waveguides are based on surface-plasmon modes confined at a metal-semiconductor interface. The wavelengths were chosen in order to avoid major phonon absorption bands, which are particularly strong at energies just above the reststrahlen band. We also report on a 21.5-μm-wavelength laser based on a two-sided interface-plasmon waveguide.