Index Tuning for Precise Frequency Selection of Terahertz Quantum Cascade Lasers

We demonstrate precise shifts of terahertz quantum cascade laser frequencies by micron-scale changes to the duty cycle of a distributed-feedback grating, which alters the mode index, as an alternative to nanometer scale changes to the grating pitch. This method allows fabrication of lasers within a couple of gigahertz of a specific target frequency, enabling the temperature/bias to be independently optimized. Waveguides incorporating a central stripe were found to promote lasing on the fundamental lateral (TM00) mode, while an added oxide refill and metal cap made the lasers insensitive to wire bond locations for improved longitudinal mode control.