Waveguide design for bi-modal operation of THz quantum cascade lasers

The design and fabrication of a bi-modal semi-insulating surface-plasmon waveguide for a quantum cascade laser emitting at 3.75 THz is presented. Different transverse modes are excited under different electric current injection due to their different overlapping with the laterally nonuniform gain profile in the active region. HFSS and COMSOL program are used to simulate the near/far field and waveguide loss for various waveguide structures, respectively. It is found that a 150 μm wide surface-plasmon waveguide allows either co-excitation or selective excitation of the first two transverse modes. The total optical loss (i.e., the combination of waveguide and mirror loss) is found below 20 cm^-1. An electrically controllable dynamic beam pattern steering is predicted. The near field and far field measurements of a fabricated quantum cascade laser device confirm the theoretical results. The dynamic switching of far-field beam pattern by an angle of 25° is observed when the injected current density changes from 1.9 kA/cm² to 2.3 kA/cm².