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 sources 1 , 2 , 3 , 4 . Despite recent progress with terahertz quantum cascade lasers 2 , 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 lasers 5 , 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 laser 8 with the active region engineered to possess giant second-order nonlinear susceptibility associated with intersubband transitions in coupled quantum wells. The laser operates at λ 1 = 7.6 µm and λ 2 = 8.7 µm, and produces terahertz output at λ = 60 µm through intracavity difference-frequency generation.