Terahertz Quantum Cascade Laser Source Based on Intra-Cavity Difference-Frequency Generation

The terahertz spectral range (=30-300µm) has long been devoid of compact electrically pumped room temperature semiconductor sources1-4. Despite recent progress with terahertz quantum cascade lasers2-4, 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 lasers5-7. This approach offers broad spectral tunability and does work at room temperature; however it requires powerful laser pumps and a more complicated optical setup, 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 1=7.6 µm and 2=8.7 µm, and produces terahertz output at =60 µm through intra-cavity difference-frequency generation. Since mid-infrared quantum cascade lasers (QCLs) have been shown to operate continuous-wave (CW) above 400K9, we believe that this approach can eventually lead to a room temperature electrically pumped CW semiconductor terahertz (THz) source.