External cavity terahertz quantum cascade laser sources based on intra-cavity frequency mixing with 1.2–5.9 THz tuning range

We discuss the design and operation of widely-tunable terahertz sources based on Cherenkov intra-cavity difference-frequency generation in mid-infrared quantum cascade lasers. Laser chips are integrated into a Littrow-type external cavity system. Devices demonstrate continuous terahertz emission tuning at room temperature with a record tuning range from 1.2 THz to 5.9 THz and peak power output varying between 5 and 90 μW, depending on the operating frequency. Beam steering of terahertz Cherenkov emission with frequency is suppressed and mid-infrared-to-terahertz conversion efficiency is improved by bonding devices onto highresistivity silicon substrates that have virtually no refractive index dispersion and vanishinglysmall optical loss in 1–6 THz range.