A Quantum Cascade Laser (QCL) is a type of semiconductor laser that emits light in the mid- to far-infrared portion of the electromagnetic spectrum. Quantum cascade lasers offer many benefits: They are tunable across the mid-infrared spectrum from 5.5 to 11.0 µm (900 cm-1 to 1800 cm-1); provide a rapid response time; and provide spectral brightness that is significantly brighter than even a synchrotron source.
Quantum cascade lasers comprise alternating layers of semiconductor material, forming quantum energy wells that restrict the electrons to specific energy states. As an electron moves across the lasing medium, it moves from one quantum well to the next, driven by the voltage applied across the device. At precise locations, called the "active region," the electron transitions from one energy state to a lower one and, in the process, emits a photon. The electron continues through the structure and when it encounters the next active region it transitions again and emits another photon. The QCL may have as many as 75 active regions, and each electron generates that many photons as it traverses the structure.
The output wavelength is determined by the structure of the layers rather than the lasing material, making it possible for device fabricators to tailor the wavelength in a way that diode lasers cannot be tailored. Diode laser output wavelength is limited to ~2.5 µm, but QCLs operate at much longer wavelengths: Mid-wave infrared production devices up to 11 µm are available, and some 25-µm emitters have been made on an experimental basis.
Because they require a relatively low amount of power and are small in size, QCL-based microscopy systems are said to be able to replace larger and slower FTIR (Fourier Transform Infrared) and mass spectroscopy systems for both lab and field work.