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Quantum Cascade Lasers

Acronym: QCL

Definition: semiconductor lasers relying on intersubband transitions, normally emitting in the mid-infrared spectral region

More general terms: semiconductor lasers, mid-infrared laser sources, terahertz sources

German: Quantenkaskadenlaser

Categories: optoelectronics, lasers

How to cite the article; suggest additional literature


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.

schematic of quantum cascade laser
Figure 1: Strongly simplified schematic of the gain region of a quantum cascade laser. The diagram shows the electron energy versus position in the structure, which contains three quantum wells. The overall downward trend of energy towards the right-hand side is caused by an applied electric field. In reality, each gain region must be divided into an active region and an injector.

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

Output Wavelengths

Most quantum cascade lasers emit mid-infrared light – 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 [12].

Output Power and Efficiency

Whereas continuously operating room-temperature devices [4] 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.

Dynamic Properties

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.


The emission linewidth is usually rather small – which is often very useful for applications in spectroscopy. A contributing factor for that is the small linewidth enhancement factor.

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.

Terahertz quantum cascade lasers are also interesting for various imaging applications. See the article on terahertz radiation.

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.


The RP Photonics Buyer's Guide contains 28 suppliers for quantum cascade lasers. Among them:

Questions and Comments from Users


Why is the injection region needed in addition to the active region in a quantum cascade laser?

Answer from the author:

The function of the quantum wells is vital for the operation of such a laser. That means they definitely need to be embedded into regions with higher electron energy; without those regions, you would not have any quantum wells and there's a completely different electronic structure. Therefore, the only way is to use tunneling for the injection of carriers from one quantum well into the other.

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[1]R. F. Kazarinov et al., “Possibility of amplification of electromagnetic waves in a semiconductor with a superlattice”, Fiz. Tekh. Poluprovod. 5 (4), 797 (1971)
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[9]R. P. Green et al., “Linewidth enhancement factor of terahertz quantum cascade lasers”, Appl. Phys. Lett. 92 (7), 071106 (2008), doi:10.1063/1.2883950
[10]P. Q. Liu et al., “Highly power-efficient quantum cascade lasers”, Nature Photon. 4, 95 (2010), doi:10.1038/nphoton.2009.262
[11]Y. Bai et al., “Quantum cascade lasers that emit more light than heat”, Nature Photon. 4, 99 (2010), doi:10.1038/nphoton.2009.263
[12]M. A. Belkin et al., “Room temperature terahertz quantum cascade laser source based on intracavity difference-frequency generation”, Appl. Phys. Lett. 92 (20), 201101 (2008), doi:10.1063/1.2919051
[13]G. Scalari et al., “THz and sub-THz quantum cascade lasers”, Laser & Photon. Rev. 3 (1-2), 45 (2009), doi:10.1002/lpor.200810030
[14]F. Capasso, “High-performance midinfrared quantum cascade lasers” (review article, open access), Opt. Eng. 49 (11), 111102 (2010), doi:10.1117/1.3505844
[15]S. Kumar, “Recent progress in terahertz quantum cascade lasers”, J. Sel. Top. Quantum Electron. 17 (1), 38 (2011), doi:10.1109/JSTQE.2010.2049735
[16]C. Sirtori, “Wave engineering with THz quantum cascade lasers”, Nature Photon. 7, 691 (2013), doi:10.1038/nphoton.2013.208
[17]M. S. Vitiello et al., “Quantum cascade lasers: 20 years of challenges”, Opt. Express 23 (4), 5167 (2015), doi:10.1364/OE.23.005167

(Suggest additional literature!)

See also: semiconductor lasers, infrared light, mid-infrared laser sources, terahertz sources, spectroscopy
and other articles in the categories optoelectronics, lasers


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