Pulse generation and spectral optimization of broadband terahertz quantum cascade lasers

By introducing side-absorbers to metal-metal waveguides of broadband terahertz quantum cascade lasers, we are able to control and completely suppress higher-order lateral waveguide modes. This change leads to an octave spanning laser spectrum with a 600 GHz wide flat top. In addition, the absence of higher order lateral modes has a strong effect on the pulse generation of injection seeded QCLs. We achieve a seeded bandwidth of 1 THz and observe a stable THz pulse train with pulse durations of ~2 ps. Furthermore, time-domain spectroscopy based dispersion measurements reveal the strong impact of optical gain on the total group velocity dispersion in such a QC laser. These measurements enable the design of dispersion compensation structures with the goal of developing octave spanning QCL based THz frequency combs.