Quantum Cascade Lasers
The quantum cascade laser is a special kind of semiconductor laser, usually emitting mid-infrared light. Such a laser operates on laser transitions not between different electronic bands but on intersubband transitions of a semiconductor structure. Figure 1 shows what happens to an electron injected into the gain region: in each period of the structure, it undergoes a first transition (blue arrow) between two sublevels (minibands) of a quantum well (which is the laser transition on which stimulated emission occurs), then a non-radiative transition (red arrow) to the lowest sublevel, before tunneling (gray arrow) into the upper level of the next quantum well. By using several tens or even 100 quantum wells in a series (a cascade), a higher optical gain and multiple photons per electron are obtained at the expense of a higher required electrical voltage. The operation voltage can easily be of the order of 10 V, whereas few volts are sufficient for ordinary laser diodes.
As the transition energies are defined not by fixed material properties but rather by design parameters (particularly by layer thickness values of quantum wells), quantum cascade lasers can be designed for operating wavelengths ranging from a few microns to well above 10 μm, or even in the terahertz region. Note that a single injected carrier can generate (in the ideal case of perfect quantum efficiency) one photon for each quantum well region.
In a quantum cascade laser, the mentioned quantum well structure is embedded in a waveguide, and the laser resonator is mostly of DBR or DFB type. There are also external-cavity lasers, where a wavelength tuning element such as a diffraction grating is part of the resonator.
Typical Properties of Quantum Cascade Lasers
Most quantum cascade lasers emit mid-infrared light (which means wavelengths between 3 μm and 50 μm according to ISO 20473:2007) – they are thus a kind of mid-infrared laser sources. However, quantum cascade lasers can also be made for generating terahertz waves (→ terahertz sources). Such devices constitute particularly compact and simple (electrically pumped) sources of terahertz radiation. Even room temperature terahertz generation can been achieved via internal difference frequency generation .
Output Power and Efficiency
Whereas continuously operating room-temperature devices  are normally limited to moderate output power levels in the milliwatt region (although more than a watt is possible), multiple watts are easily possible with liquid-nitrogen cooling. Even at room temperature, watt-level peak powers are possible when using short pump pulses.
The power conversion efficiency of quantum cascade lasers is typically of the order of a few tens of percent. Recently, however, devices with efficiencies around 50% have been demonstrated [10, 11], although only for cryogenic operation conditions.
The carrier lifetime in quantum cascade lasers is much shorter than in ordinary laser diodes, for example; it is limited by phonon scattering phenomena. That also has consequences for the dynamic properties: there is very strong damping of relaxation oscillations (overdamped transient dynamics). For that reason, quantum cascade lasers can be modulated with very high intrinsic bandwidth limited of several tens of gigahertz.
Applications of Quantum Cascade Lasers
Perhaps the most important applications for quantum cascade lasers will be in the area of laser absorption spectroscopy of trace gases, e.g. for detecting very small concentrations of pollutants in air. In addition to the suitable wavelength range, QCLs usually feature a relatively narrow linewidth and good wavelength tunability, making them very suitable for such applications.
Another application field of THz QCLs is free-space communications. Although terahertz beams exhibit substantially stronger beam divergence than optical beams, one can still use directed beams for short-distance transmission through air.
An example for a military application is the use for infrared countermeasures, i.e., irritating heat-seeking missiles attacking airplanes by sending mid-infrared light to their sensors.
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