Ultra-broadband THz semiconductor laser based on heterogeneous quantum cascade gain medium

The broad gain medium is obtained by placing different active-region (AR) designs in a same waveguide, creating a heterogeneous QCL. The AR design is based on a four quantum well design which is then rescaled in order to cover the wanted bandwidth. A crucial role is played by the double-metal (DM) optical waveguide providing no cutoff and excellent figure of merit over a broad wavelength range. Three different ARs centred at 2.1 THz, 2.4 THz and 2.9 THz, respectively, are stacked as displayed in the paper.