Terahertz photonic integrated circuit for frequency tuning and power modulation.

The quantum cascade laser is a powerful solid-state source of terahertz-frequency radiation. However, integrating multiple photonic functions into a monolithic platform in this frequency range is non-trivial due to the scaling of photonic structures for the long terahertz wavelengths and the low frequency tuning coefficients of the quantum cascade lasers. Here, we have designed a simple terahertz-frequency photonic integrated circuit by coupling a racetrack resonator with a ridge laser in the longitudinal direction to design a notch filter. The transmission properties of this filter structure are dependent on the phase matching and losses in the coupled racetrack and results in a comb of stopband frequencies. We have optimized the comb separation by carefully selecting the cavity dimensions of the racetrack resonator to suppress longitudinal modes in the ridge laser enabling single-mode emission. The emission frequencies and output power from laser are controlled through appropriate control of drive currents to the ridge and the racetrack resonator. The emission frequency is electrically tuned over ∼81 GHz exploiting Stark shift of the gain as a function of drive current at the ridge laser, coinciding with an output power variation of ∼27% of the peak power (at a heat sink temperature of 50 K). The output power from the ridge also varied by ∼30% and the frequency was tuned by a further 10 GHz when the driving conditions at the ridge laser are invariant and the current at the racetrack resonator was varied. To our best knowledge, this is the first report of a frequency engineering, tuning and power modulation of terahertz-frequency quantum cascade lasers using a photonic integrated circuit.