quantum cascade lasers (original) (raw)

Acronym: QCL

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

Categories: optoelectronics, laser devices and laser physics

• solid-state lasers

  • semiconductor lasers
    * laser diodes
    * diode lasers
    * edge-emitting semiconductor lasers
    * surface-emitting semiconductor lasers
    * quantum cascade lasers

• 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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Contents

What is a Quantum Cascade Laser?

Typical Properties of Quantum Cascade Lasers

Output Wavelengths

Output Power and Efficiency

Dynamic Properties

Linewidth

Mode-locked Operation

Applications of Quantum Cascade Lasers

Frequently Asked Questions

Summary:

This article provides an introduction to quantum cascade lasers (QCLs). It explains their unique working principle based on intersubband transitions within a cascade of semiconductor quantum wells, which allows a single electron to generate multiple photons.

Key properties are discussed, including their wide wavelength coverage from the mid-infrared to the terahertz region, which can be engineered by design, and the potential for ultrashort pulse generation with different variants of mode locking. It also covers typical output powers, efficiencies, their excellent dynamic properties for high-speed modulation, and their narrow emission linewidth.

Finally, the article outlines major applications, with a focus on laser absorption spectroscopy for trace gas sensing, terahertz imaging, free-space communications, and military countermeasures.

(This summary was generated with AI based on the article content and has been reviewed by the article’s author.)

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: 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.

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.

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?

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 32 suppliers for quantum cascade lasers. Among them:

⚙ hardware

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!

⚙ hardware

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.

⚙ hardware

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.

⚙ hardware

Sacher Lasertechnik offers quantum cascade lasers with emission between 4 μm and 12 μm, suitable for applications like molecular spectroscopy.

⚙ hardware

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.

Bibliography

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