Tunable terahertz quantum cascade lasers with an external cavity

Tunable quantum cascade lasers operating in the terahertz frequency range are demonstrated. By using an external cavity based on the reflection from a movable mirror, both broad and fine tuning of the frequency are achieved by varying the cavity length. Coarse tuning up to 3cm−1 is obtained near the center frequency of 4.8THz (∼160cm−1), and continuous mode-hop-free tuning is observed over ∼ 0.4cm−1, nearly corresponding to the cavity free spectral range.