Vertical sub-wavelength mode confinement in THz quantum cascade lasers

We exploit the modal confinement properties of metal-metal ridge waveguides to reduce the thickness of the active laser cores in both terahertz and mid-infrared quantum cascade lasers. Devices with active regions over 60 times thinner than the free-space emission wavelength are demonstrated. The devices surprisingly show only a modest increase in threshold current density compared with conventional-thickness devices.