Mode calculations for a terahertz quantum cascade laser

We calculate the loss and confinement factors of modes in terahertz quantum cascade laser structures at frequencies of 1-4 THz. The determination of the total loss splits naturally into the calculation of free carrier losses in the active region and the waveguide losses. For both, we employ the Drude model. In the case of waveguide losses, we incorporate it into the formalism of the optical scattering matrix and trace the net threshold gain for laser operation in the waveguide for various frequencies as a function of thickness and doping of the buried contact layer. The results indicate that at lower frequencies and high doping, the preferred mode switches character from extended to tightly confined. This may have consequences for the creation of simplified longer-wavelength devices.