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

The terahertz spectral range (λ = 30-300 νm) has long been devoid of compact, electrically pumped, room-temperature semiconductor sources. Despite recent progress with terahertz quantum cascade lasers, 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 lasers. 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 laser 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 νm and λ₂ = 8.7 νm, and produces terahertz output at λ = 60 νm through intracavity difference-frequency generation.