New level for continuous-wave terahertz lasers
Since the first quantum cascade (QC) laser was demonstrated in 1994 and implemented in THz regime in 2002, they have become one of the most important solid state light sources in this frequency range. The metal-metal (MM) waveguide was a key improvement in applying the quantum cascade concept from mid-infrared to THz range, allowing for a sub-wavelength field confinement. Nevertheless, this confinement leads to a highly divergent beam from the facet. Different strategies like photonic crystal (PhC) or metallic grating distributed feedback (DFB) patterning for in plane or vertical emission have been developed. However, most of these progresses were demonstrated in pulsed operation mode. Continuous-wave (CW) operation performance is still limited and far from being optimized, despite the fact that it is of a crucial demand in astrophysics, biology, sensing, environmental and pollutant monitoring, or security screening.