Compact, Thermoelectrically Cooled Surface Emitting THz QCL Sources

Terahertz quantum cascade lasers (THz QCLs) have emerged as the primary high brilliance source in the THz regime. Their main drawback has been in their limited operating temperature requiring the use of cryogenic liquids or expensive mechanical coolers. THz QCLs have recently reached maximum operating temperatures reachable by single-stage thermoelectric coolers, but multi-stage Peltier elements are still required for increased average power outputs [1], [2]. High temperature THz QCLs are usually large area, edge emitting ridge devices that promise the highest operating temperature. This places a limit for practical applications through their high thermal dissipation, poor outcoupling and divergent and patterned far field. Here we present THz QCL devices based on a 2-well active region reaching 210K [2]. The devices were scaled in size to operate at a reduced driving current of 2.5A to enable higher duty cycles without thermally overwhelming the four-stage Peltier coolers. To improve power output and far field we employ a fully dry-etched patch array antenna optimized for 3.9THz enabling surface emission in a 35°x20° beam and increased output coupling. To reduce mirror losses and to avoid cleaving in the post-processing we introduce a loop at the back facet to fold the cavity back onto itself (Fig. 1d). These devices are subsequently mounted on special mounts and soldered onto 4-stage Peltier elements inside compact high heat load (HHL) housings measuring only 31.75mmx31.75mmx18.5mm (Fig. 1a). Spectra were taken with a commercial Bruker Vertex 80v and a room temperature DTGS detector and show single frequency emission at 3.9THz with good signal-to-noise ratio (Fig. 1c), corresponding to an emitted average power of ~ 10uW at a duty cycle of 0.25%. The signal from the laser inside the HHL could be readily imaged with an uncooled micro-bolometer THz camera and two plano-convex TPX lenses (Fig. 1b).