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

The terahertz spectral range (lambda = 30-300 mu m) has long been devoid of compact, electrically pumped, room-temperature semiconductor sources(1-4). Despite recent progress with terahertz quantum cascade lasers(2-4), 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(5-7). 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 lambda(1) = 7.6 mu m and lambda(2) = 8.7 mu m, and produces terahertz output at lambda = 60 mu m through intracavity difference-frequency generation.