Quantum Cascade Lasers
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Acronym: QCL
Definition: semiconductor lasers relying on intersubband transitions, normally emitting in the mid-infrared spectral region
Categories: 
- mid-infrared laser sources
- quantum cascade lasers
- mid-infrared fiber lasers and bulk lasers
- terahertz sources
- quantum cascade lasers
- free-electron lasers
- gas lasers
- photoconductive antennas
Related: semiconductor lasersinfrared lightmid-infrared laser sourcesterahertz sourcesspectroscopy
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What is a Quantum Cascade Laser?
A quantum cascade laser is a special kind of semiconductor laser, usually emitting mid-infrared or terahertz radiation. In contrast to ordinary laser diodes, which rely on electron–hole recombination between different electronic bands (interband transitions), a quantum cascade laser is a unipolar device: it uses only electrons and operates on intersubband transitions within the conduction band of a tailored semiconductor heterostructure.
The gain medium consists of a large number (typically several tens) of repeated periods of quantum wells and barriers, grown with precisely controlled layer thicknesses. Each period can be divided conceptually into two parts:
- Active region: one or more quantum wells where the upper and lower laser levels are formed.
- Injector region: a sequence of wells and barriers engineered so that electrons are efficiently transferred from the lower level of one period into the upper level of the next period.
Figure 1 illustrates what happens to an electron injected into the gain region. In each period of the structure, the following happens:
- The electron is injected into the upper sublevel (miniband) in the active region and undergoes a radiative transition (blue arrow) to a lower sublevel — this is the laser transition on which stimulated emission occurs.
- It then relaxes by a fast non-radiative transition (red arrow), typically via phonon emission, to the lowest sublevel of that period.
- It then tunnels (gray arrow) through a barrier into the upper laser level of the next period, in the neighboring quantum well.
Because this sequence is repeated in every period, a single electron can generate (in the ideal case of perfect quantum efficiency) one photon in each active region it traverses. Using several tens or even about 100 such periods in series (a “cascade”) therefore yields high optical gain and multiple photons per injected electron, at the expense of a comparatively high required electrical voltage. Operation voltages of the order of 10 V are common, whereas only a few volts are sufficient for ordinary laser diodes.
Since the transition energies are determined not by fixed bulk material properties but by the designed layer thicknesses and compositions of the quantum wells and barriers, quantum cascade lasers can be engineered for operating wavelengths ranging from a few micrometers to well above 10 μm, and even into the terahertz region.
The quantum well structure is embedded in a waveguide that confines the optical mode and forms part of the laser resonator. The resonator is often of the DFB or DBR type to obtain single-mode operation. There are also external-cavity implementations, where a wavelength tuning element such as a diffraction grating is placed outside the chip as part of the resonator.
Device Architectures
Quantum cascade lasers are available in several resonator configurations, determining their spectral purity and tunability:
- Fabry–Pérot (FP) QCLs: These devices use the cleaved facets of the semiconductor chip as mirrors. They typically emit multiple longitudinal modes simultaneously (multimode operation) with high output power. They are suitable for applications where spectral purity is not critical, such as infrared illumination or excitation of broad absorption features.
- Distributed feedback (DFB) QCLs: A diffraction grating is integrated into the waveguide structure to enforce single-mode operation at a specific wavelength. DFB QCLs are the standard for high-precision gas sensing, as their emission frequency can be fine-tuned (typically over a few wavenumbers) by adjusting the chip temperature and injection current to scan across a specific molecular absorption line.
- External-cavity (EC) QCLs: The laser chip is anti-reflection coated on one facet, and an external optical grating provides feedback. This configuration allows for single-mode operation with a very broad wavelength tuning range (often exceeding 10% of the center wavelength), making them ideal for spectroscopy of complex mixtures or research applications requiring flexibility.
Typical Properties of Quantum Cascade Lasers
Output Wavelengths
Most quantum cascade lasers emit mid-infrared light (i.e. wavelengths between 3 µm and 50 µm according to ISO 20473:2007), and are therefore a type of mid-infrared laser sources. However, QCLs can also be engineered to generate terahertz waves (→ terahertz sources). These devices provide particularly compact, electrically pumped sources of terahertz radiation. Even room-temperature terahertz emission is possible via internal difference-frequency generation [12].
Output Power and Efficiency
Continuously operating room-temperature devices [4] typically reach milliwatt-level output powers (though watt-level continuous power has been achieved in specific designs). With liquid-nitrogen cooling, however, multiple watts of continuous output are readily attainable. At room temperature, watt-level peak powers are also possible when short electrical pulses are used.
The power-conversion efficiency of QCLs is usually on the order of a few tens of percent. Recently, devices with efficiencies around 50% have been demonstrated [10, 11], although these values have so far been achieved only under cryogenic operating conditions.
Dynamic Properties
Carrier lifetimes in quantum cascade lasers are much shorter than in conventional interband laser diodes. These lifetimes are mainly limited by rapid phonon-assisted scattering processes. As a consequence, QCLs exhibit heavily damped relaxation oscillations — their transient dynamics are typically overdamped. This in turn enables very high intrinsic modulation bandwidths, often in the range of several tens of gigahertz.
Linewidth
The emission linewidth of a QCL is usually quite small, which is highly advantageous for many kinds of spectroscopy. One reason for this is the very small linewidth enhancement factor characteristic of intersubband gain media.
Mode-locked Operation
Quantum cascade lasers can also operate in a mode-locked regime, although achieving stable pulse formation is considerably more challenging than in conventional interband semiconductor lasers. The main difficulty (primarily for passive mode locking) arises from the ultrafast carrier dynamics in QCLs with extremely short upper-state lifetimes.
Despite these challenges, several approaches to mode-locked operation have been demonstrated:
- Active mode locking: By applying an external radio-frequency modulation — usually through an integrated electro-optic or acousto-optic modulator — QCLs can produce trains of ultrashort pulses [18, ???, 26]. This method does not rely on slow saturable absorption and is therefore compatible with the fast gain dynamics of QCLs.
- Hybrid (active–passive) schemes: Devices incorporating engineered saturable absorbers or sections with nonlinear losses have shown signatures of passive pulse shaping when combined active modulation [21].
- Frequency-comb operation: Even when not producing isolated short pulses in the time domain, QCLs can operate as optical frequency combs [19, ???]. In such cases, the phases of many longitudinal modes become locked through nonlinear processes such as four-wave mixing within the active region. QCL frequency combs are especially promising for broadband mid-infrared spectroscopy and dual-comb techniques.
Fully passive mode locking — without external modulation — remains experimentally difficult, largely due to the rapid gain dynamics and limited dispersion control. Nevertheless, ongoing advances in dispersion engineering, heterogeneous integration, and low-loss waveguide design continue to bring more robust mode-locked performance within reach, especially for applications requiring compact pulsed mid-infrared or terahertz sources.
Applications of Quantum Cascade Lasers
Perhaps the most important applications of quantum cascade lasers are in laser absorption spectroscopy of trace gases — for example, detecting extremely low concentrations of pollutants in air. Besides offering suitable wavelengths for many molecular absorbers, QCLs combine narrow linewidth with good tunability, making them ideal sources for precision spectroscopy.
Terahertz QCLs are also attractive for various imaging techniques; see also the article on terahertz radiation.
Another application area for THz QCLs is free-space optical communications. Although terahertz beams exhibit substantially higher beam divergence than optical beams, directed transmission over short distances in air is feasible.
A notable military application is their use in infrared countermeasures, where mid-infrared QCLs disrupt the sensors of heat-seeking missiles by directing tailored infrared radiation at them.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is a quantum cascade laser (QCL)?
A quantum cascade laser is a special type of semiconductor laser that produces light, typically in the mid-infrared or terahertz range. Its operation is based on electronic transitions within engineered quantum wells (intersubband transitions) rather than between the electronic bands of a bulk semiconductor.
How does the 'cascade' mechanism in a QCL work?
A QCL contains a series of many quantum well structures. An injected electron 'cascades' down this series, undergoing a laser transition and emitting a photon in each period, which allows for high optical gain and the generation of multiple photons per electron.
Why is the emission wavelength of QCLs so versatile?
Their emission wavelength is determined not by fixed material properties but by the physical design, particularly the layer thicknesses of the quantum wells. This allows the laser transition energy to be precisely engineered for wavelengths ranging from a few microns to the terahertz region.
What are the main applications of quantum cascade lasers?
The most important application is laser absorption spectroscopy for detecting trace gases. Other significant uses include terahertz imaging, free-space optical communications, and military infrared countermeasures to disrupt heat-seeking missiles.
Why can quantum cascade lasers be modulated at very high speeds?
QCLs have a very short carrier lifetime, limited by fast phonon scattering. This results in strongly damped relaxation oscillations, giving them a very high intrinsic modulation bandwidth of several tens of gigahertz.
What is the typical performance of a quantum cascade laser?
At room temperature, continuous-wave devices typically produce milliwatt-level output powers, though watt-level peak powers are possible in pulsed mode. With cryogenic cooling, multiple watts are achievable. The power conversion efficiency can be tens of percent.
Can QCLs be mode-locked?
While passive mode locking appears to be difficult, active and hybrid schemes have been used to obtain mode-locked operation, including the generation of frequency combs.
Suppliers
Sponsored content: The RP Photonics Buyer's Guide contains 31 suppliers for quantum cascade lasers. Among them:
Sacher Lasertechnik offers quantum cascade lasers with emission between 4 μm and 12 μm, suitable for applications like molecular spectroscopy.
Serving North America, RPMC Lasers offers quantum cascade lasers spanning MWIR-LWIR (≈4–17 µm) for gas sensing, spectroscopy, and defense. PowerMir delivers high-power, tunable output, while UniMir provides narrow linewidth and precision sensitivity.
Built on innovative InP- and InAs-based tech, they offer high efficiency and energy for CW or pulsed use, with excellent thermal/optical stability, precise DFB control, and integrated TEC cooling for reliability.
Customizable, compact, and lightweight, these HHL, OEM, or turnkey solutions feature hermetically sealed packages and fiber coupling for portable industrial and defense applications with long-term durability.
Let RPMC help you find the right QCL today!
Alpes Lasers designs and manufactures a wide range of QCLs with wavelengths from 4 to 14 μm and powers up to hundreds of milliwatts. This includes FP, DFB, THz, frequency comb and external cavity lasers in the mid-IR. Additionally, Alpes offers uniquely fast and widely tuneable lasers with our ET and XT product line.
Hamamatsu Photonics quantum cascade lasers are semiconductor lasers that offer peak emission in the mid-IR range (4 to 10 μm). These devices are an excellent light source for mid-IR applications, such as molecular gas analysis and absorption spectroscopy.
DRS Daylight Solutions offers a wide range of quantum cascade lasers:
- The MIRcat-QT™ is a very rapidly wavelength-tunable version with up to 30,000 cm−1/s, covering wavelengths beyond 13 μm.
- The Hedgehog™ models are wavelength-tunable laser for mid-IR spectroscopy with up to 0.5 W average power and 1 W peak power. Ultra-quiet, superior wavelength repeatability.
- The CW-MHF™ is the ultimate tool for high-resolution, mid-IR spectroscopy with high spectral resolution and phase-continuous tuning to avoid jumping over spectral lines.
- The Aries-2 series is a family of fixed-wavelength, narrowband mid-IR lasers with an average output power of up to 1 W.
- The H-Model mid-IR laser offers high-power, mid-IR OEM laser performance in a compact footprint. CW and pulsed operation are possible.
- We also have high-power multi-color laser systems (VIS, NIR, SWIR, MWIR, and LWIR) with best-in-class performance for aircraft protection.
Bibliography
| [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) |
| [2] | J. Faist et al., “Quantum cascade laser”, Science 264, 553 (1994); doi:10.1126/science.264.5158.553 |
| [3] | R. M. Williams et al., “Kilohertz linewidth from frequency-stabilized mid-infrared quantum cascade lasers”, Opt. Lett. 24 (24), 1844 (1999); doi:10.1364/OL.24.001844 |
| [4] | M. Beck et al., “Continuous wave operation of a mid-infrared semiconductor laser at room temperature”, Science 295, 301 (2002); doi:10.1126/science.1066408 |
| [5] | R. Köhler et al., “Terahertz semiconductor-heterostructure laser”, Nature 417, 156 (2002); doi:10.1038/417156a |
| [6] | B. S. Williams et al., “Operation of terahertz quantum-cascade lasers at 164 K in pulsed mode and at 117 K in continuous-wave mode”, Opt. Express 13 (9), 3331 (2005); doi:10.1364/OPEX.13.003331 |
| [7] | B. S. Williams, “Terahertz quantum-cascade lasers”, Nature Photon. 1, 517 (2007); doi:10.1038/nphoton.2007.166 |
| [8] | A. Kosterev et al., “Application of quantum cascade lasers to trace gas analysis”, Appl. Phys. B 90, 165 (2008); doi:10.1007/s00340-007-2846-9 |
| [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 |
| [18] | Y. Wang and A. Belyanin, “Active mode-locking of mid-infrared quantum cascade lasers with short gain recovery time”, Opt. Expr. 23 (4), 4173 (2015); doi:10.1364/oe.23.004173 |
| [19] | G. Villares and J. Faist, “Quantum cascade laser combs: effects of modulation and dispersion”, Opt. Expr. 23 (2), 1651 (2015); doi:10.1364/oe.23.001651 |
| [20] | {Revin 2016} D. G. Revin et al., “Active mode locking of quantum cascade lasers in an external ring cavity”, Nature Communications 7, 11440 (2016); doi:10.1038/ncomms11440 |
| [21] | P. Tzenov et al., “Passive and hybrid mode locking in multi-section terahertz quantum cascade lasers”, New Journal of Physics 20 (5), 053055 (2018); doi:10.1088/1367-2630/aac12a |
| [22] | {Beiser 2021} M. Beiser et al., “Engineering the spectral bandwidth of quantum cascade laser frequency combs”, Opt. Lett. 46 (14), 3416 (2021); doi:10.1364/OL.424164 |
| [23] | A. Khalatpour et al., “High-power portable terahertz laser systems”, Nature Photonics 15, 16 (2021); doi:10.1038/s41566-020-00707-5 |
| [24] | Yu Wu et al., “Tunable quantum-cascade VECSEL operating at 1.9 THz”, Opt. Express 29 (21), 34695 (2021); doi:10.1364/OE.438636 |
| [25] | P. Täschler et al., “Femtosecond pulses from a mid-infrared quantum cascade laser”, Nature Photonics 15, 919 (2021); doi:10.1038/s41566-021-00894-9 |
| [26] | C. Silvestri, X. Qi, T. Taimre and A. D. Rakić, “Harmonic active mode locking in terahertz quantum cascade lasers”, Phys. Rev. A 108 (1) (2023); doi:10.1103/physreva.108.013501 |
(Suggest additional literature!)
Questions and Comments from Users
2021-04-08
How does a multiple quantum well laser differ from a quantum cascade laser?
The author's answer:
Although both types of devices contain a sequence of thin layers of different semiconductor materials, they have very different principles of operation:
- In a multiple quantum well laser, the quantum wells do not substantially interact with each other. They may not even be electrically pumped. It is just that each one contributes some amount to the gain. Also, one uses optical transitions between conduction and valence bands.
- In a quantum cascade laser, the quantum wells are much closer to each other, so that carriers can tunnel from one well to the next one. Also, one uses intersubband transitions, which typically exhibit much longer wavelengths.
2021-01-21
Why is the injection region needed in addition to the active region in a quantum cascade laser?
The author's answer:
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 an entirely different electronic structure. Therefore, the only way is to use tunneling for the injection of carriers from one quantum well into the other.