Continuous Frequency Tuning with near Constant Output Power in Coupled Y-Branched Terahertz Quantum Cascade Lasers with Photonic Lattice

We demonstrate continuous frequency tuning in terahertz quantum cascade lasers with double metal waveguides using a Y-branched coupler. Two THz QCLs placed side-by-side couple by evanescent fields across the air gap between them. Each QCL waveguide comprises a 48-mu m-wide coupler and S-bend section, which are connected to an 88-mu m-wide Y-branch through an impedance matching tapered section. Photonic lattices are patterned on top of the coupler section in each QCL using focused ion-beam milling to control the spectral characteristics. The waveguide design used for individual QCL sections is optimized using finite element modeling and the spectral characteristics are modeled using a transfer matrix model. Continuous frequency tuning of similar to 19 GHz is demonstrated while maintaining an output power of similar to 4.2-4.8 mW and a heat sink temperature of 50 K. The tuning is controlled electrically through Stark shift and cavity pulling effects by driving both QCLs simultaneously and represents the widest electrically controlled continuous tuning performance from a THz QCL without significant change in output power.