Terahertz quantum-cascade-laser source based on intracavity difference-frequency generation

The terahertz spectral range (λ = 30–300 µm) has long been devoid of compact, electrically pumped, room-temperature semiconductor sources1, 2, 3, 4. Despite recent progress with terahertz quantum cascade lasers2, 3, 4, existing devices still require cryogenic cooling. An alternative way to produce terahertz radiation is frequency down-conversion in a nonlinear optical crystal using infrared or visible pump lasers5, 6, 7. This approach offers broad spectral tunability and does work at room temperature; however, it requires powerful laser pumps and a more complicated optical set-up, resulting in bulky and unwieldy sources. Here we demonstrate a monolithically integrated device designed to combine the advantages of electrically pumped semiconductor lasers and nonlinear optical sources. Our device is a dual-wavelength quantum cascade laser8 with the active region engineered to possess giant second-order nonlinear susceptibility associated with intersubband transitions in coupled quantum wells. The laser operates at λ1 = 7.6 µm and λ2 = 8.7 µm, and produces terahertz output at λ = 60 µm through intracavity difference-frequency generation.