Quantum Cascade Lasers Enhance Mid-IR Spectroscopy

Most vapor molecules have a unique absorption spectrum in the mid-IR region, so this range is of particular interest for trace gas analysis and chemical sensing. And quantum cascade (QC) lasers are ideal candidate light sources for mid-IR spectroscopy: Their emission wavelength reaches from the midto the far-IR with good power efficiency. QC lasers are based on the intersubband transitions between quantized subbands in multiplequantum-well heterostructures, a design that is fundamentally different from that of conventional semiconductor lasers using interband transitions across a bandgap. Because QC lasers use intersubband optical transitions, they were once thought to be inefficient, and their emission spectra were thought to be inherently limited in tunability and breadth. However, research in recent years has shown that different quantum designs can be implemented to significantly improve the performance of these lasers. Figure 1a demonstrates how, in the conventional semiconductor laser, electrons transition between conduction band and valence band. Photon emission results from the recombination of carriers across the bandgap. In QC lasers, transitions take place between subbands in one band, usually the conduction band, in multiple-quantumwell heterostructures (Figure 1b). Applying different voltages can achieve voltagetunable emission wavelength via a Stark effect. The detailed design that achieves this includes a coupling state, which is inserted between the injector ground state and the upper laser state. Alternatively, Figure 1c shows the transitions from the multiple subbands to yield a broadband spectrum. In recent years, with better performance, QC lasers have been successfully applied to trace gas analysis and spectroscopic sensors.