Quantum cascade lasers are a type of semiconductor laser that typically emit in the mid-infrared region (~ 4 μm to 10 μm). They are somewhat unusual in comparison to other laser systems in terms of how they operate as, rather than relying on transitions between different electronic states in the lasing medium, a quantum cascade laser produces a ‘cascade’ of electrons in a multilayered semiconductor material causing photon emission from the different semiconductor layers.
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The output beam from a quantum cascade laser generally has a very narrow linewidth and excellent tunability of the central wavelength. The fundamental processes that contribute to the ultimate linewidth of the emission are complex, but the relatively long lifetime of the spontaneous emission process ultimately couples to the lasing mode that produces the observed laser output.
The broad energy tuneability of quantum cascade lasers has been a driving factor in many of their applications in gas sensing and as emissions sources for infrared imaging systems. The design of quantum cascade lasers with many stacked layers of semiconductor materials means that layer thicknesses can be varied to design systems that have different lasing ranges and some quantum cascade lasers can also be coupled to tunable external cavities for greater wavelength tunability as well.
Most quantum cascade lasers are relatively low power, with the typical output being in the milliwatt range. However, there are now devices that exceed 5 W of power output with the use of cryogenic cooling. Higher power lasers can be advantageous for more sensitive sensing applications as all detectors have an inherent noise limit, so a greater number of photons can help compensate for weak absorption signals due to very low sample concentrations.
Quantum cascade lasers are now a mature technology with many commercial options available. Many laser systems can be small, chip-based devices which makes them ideal for integration into onboard applications such as remote sensing with unmanned vehicles.
Applications
One of the most common applications for quantum cascade lasers is in sensing devices. The reason for this is most chemical species have ‘fingerprint’ absorption spectra in the mid-infrared region. These spectral features arise from rotations and vibrations in the molecule that are sensitive to both the identity of the atoms in the molecule as well as their local chemical environment. The high number of spectral features also helps improve the confidence in the qualitative identification of species, if the transition cross-sections are known, the intensity of the features can be used for quantitative chemical analysis as well.
The oil and gas industry has become particularly interested in recent years in the possibilities offered by miniature quantum cascade lasers for remote sensing applications. Gas pipes are often located in very remote and inaccessible regions and performing manual, visual inspections of leaks or routine inspections is very expensive.
Leaks of gases such as methane are not just an environmental problem due to the global warming potential of such gases, but also pose an explosion risk and are a waste of finite natural resources. Using compact laser systems that can be part of ‘lab on a chip’ instrumentation for sensing means that the entire instrument can be fitted on unmanned automated vehicles that can be sent to remote sites at significantly reduced costs with no additional risk to human health.
External cavity quantum cascade lasers are finding increasing use in very demanding explosive detection applications, where the concentrations of potential species are very low. The ability to sweep the cavity and tune the wavelength over large regions also makes it possible to design sensors that are capable of detecting a wider range of chemical species.
Terahertz
Some of the biggest areas of development for quantum cascade lasers are not in their typical mid-infrared regions but in the creation of new terahertz sources. Many of the properties that make quantum cascade lasers appealing for use in sensing make them ideal for use in quantum technologies to be used in fields such as communication and computation.
While a challenging region of the electromagnetic spectrum to work in for both light sources and detectors, the appeal of terahertz radiation is its transmissivity in many media that would be impenetrable to other types of electromagnetic radiation as well as a very good chemical sensitivity.
Being able to create tunable terahertz sources with very high emission powers, which is something that is potentially offered by quantum cascade lasers, would open up new possibilities for high-resolution terahertz spectroscopy. Terahertz radiation is often considered one of the most underdeveloped frequency ranges and so the possibilities of using this for sensing, material analysis, and biomarker detection are still largely unexplored, but something new quantum cascade lasers could change.
Quantum cascade lasers are now a common technology in atmospheric and gas sensing but with new frequency ranges opening up and new possibilities for generating high power, they may also become a cornerstone of novel quantum technologies.
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References and Further Reading
Yamanishi, M., Fellow, L., Edamura, T., Fujita, K., Akikusa, N., & Kan, H. (2008). Theory of the Intrinsic Linewidth of Quantum-Cascade Lasers : Hidden Reason for the Narrow Linewidth and Line-Broadening by Thermal Photons. IEEE Journal of Quantum Electronics, 44(1), 12–29. https://doi.org/10.1109/JQE.2007.907563
Wysocki, G., Curl, R. F., Tittel, F. K., Maulini, R., Bulliard, J. M., & Faist, J. (2005). Widely tunable mode-hop free external cavity quantum cascade laser for high resolution spectroscopic applications. Applied Physics B: Lasers and Optics, 777, 769–777. https://doi.org/10.1007/s00340-005-1965-4
Wang, F., Slivken, S., Wu, D. H., Lu, Q. Y., & Razeghi, M. (2020). Continuous wave quantum cascade lasers with 5 . 6 W output power at room temperature and 41 % wall-plug efficiency in cryogenic operation Continuous wave quantum cascade lasers with 5 . 6 W output power at room temperature and 41 % wall-plug efficiency in. AIP Advances, 10, 055120. https://doi.org/10.1063/5.0003318
Asadzadeh, S., Jos, W., Oliveira, D., Roberto, C., & Filho, D. S. (2022). UAV-based remote sensing for the petroleum industry and environmental monitoring : State-of-the-art and perspectives. Journal of Petroleum Science and Engineering, 208, 109633. https://doi.org/10.1016/j.petrol.2021.109633
Du, Z., Zhang, S., Li, J., & Gao, N. (2019). Mid-Infrared Tunable Laser-Based Broadband Fingerprint Absorption Spectroscopy for Trace Gas Sensing : A Review. Applied Sciences, 9, 338. https://doi.org/10.3390/app9020338
Narlagiri, L. M., Bharati, M. S. S., Beeram, R., Banerjee, D., & Soma, V. R. (2022). Trends in Analytical Chemistry Recent trends in laser-based standoff detection of hazardous molecules. Trends in Analytical Chemistry, 153, 116645. https://doi.org/10.1016/j.trac.2022.116645
Vitiello, M. S., & Natale, P. De. (2022). Terahertz Quantum Cascade Lasers as Enabling Quantum Technology. Advanced Quantum Technologies, 5, 210082. https://doi.org/10.1002/qute.202100082
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