Article updated on 8th April 2020.
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Quantum cascade lasers (QCLs) are a specific type of semiconducting laser which work using different emission principles than other semiconducting lasers. In this article, we look at how these lasers work and how they are used in mid-IR spectroscopy applications.
What Are Quantum Cascade Lasers?
Quantum cascade lasers (QCL) are a type of semiconducting laser that emits within the mid to far infra-red section of the electromagnetic spectrum. There are many different types of semiconducting lasers, but QCLs work in a slightly different way to most. Most semiconducting lasers emit electromagnetic radiation from intraband emissions by combing electrons and holes across the band-gap of the material.
By comparison, QCLs are unipolar and emit a laser using intersubband emissions which arise from a population inversion within the conduction band when a series of quantum wells heterostructures are stacked together and an electrical voltage is passed. These lasers get their name because this process creates a cascade of quantum wells.
QCLs can come in many forms, and semiconducting materials of varying thickness can be stacked on top of each other to tailor the intersubband transitions and the laser emissions. The most common configurations utilized in QCLs are Fabry–Pérot (FP), distributed feedback (DFB) and external-cavity (EC) configurations. These configurations, and the layers of semiconducting material that make up the configurations, are often constructed using molecular beam epitaxy (MBE) or metalorganic vapour phase epitaxy (MOVPE).
Quantum Cascade Lasers in Mid-IR Spectroscopy
Because of the range that QCLs emit in, they have found their use in mid-IR spectroscopy applications, and QCL spectroscopy was the first mid-IR spectroscopy technique to be widely used. Here, we look at how QCLs possess some advantages over the lasers used in some spectroscopy methods and what applications it can be used for.
Advantages Over Other Spectroscopy Lasers
QCLs possess many advantages over other processes for use in spectroscopy applications. One of the major advantages is that it can perform continuous wave (CW) operations at room temperature without the need for liquid nitrogen. However, there are many other benefits such as the high spectral power density, single-mode emission and miniaturization/integration efficiencies, which are in more detail below.
The optical power density in current QCLs is 1 Wcm-2 and is roughly five times higher than the thermal emitters found in Fourier-transform infrared spectrometers. Additionally, QCL spectrometers have a single-mode emission which is colloquial to narrow spectral emission lines, and this is a pre-requisite for many high selectivity measurements, especially for gas measurements. Finally, because QCLs are semiconductor lasers, they can be easily miniaturized and integrated with many other technologies. This miniaturization and integration not only aids in increasing the robustness and stability when used in the field, it also helps to increase the wall plug efficiency of QCL spectrometers.
QCL Spectroscopy Applications
QCL spectroscopy can be used for a variety of applications across the different scientific disciplines. For the pharmaceutical sector, QCL spectroscopy can be used to determine the active pharmaceutical ingredient (API) in a drug formulation, where the API can be present at concentrations as low as 0.05%. For pharmaceutical ingredient identification, QCL spectroscopy has a comparable accuracy and precision as near-infrared spectroscopy, attenuated total reflection mid-infrared Fourier transform infrared spectroscopy and Raman spectroscopy.
QCL spectroscopy has also found its use across many areas of biomedical research, especially in the field of biomedical analysis; where a rapid, label-free and objective analysis is required (something which QCL mid-IR spectroscopy provides). QCL spectroscopy has also opened new ways of developing analytical methods that can quantify clinically relevant concentration levels and supporting methods for medical diagnostics. QCL spectroscopy can be used on a wide range of biological samples, including breath, urine, blood, interstitial fluid and biopsy samples.
QCL spectroscopy can also be used in atmospheric chemistry applications, particularly in the remote detection of trace gaseous elements and pollutants in the atmosphere. QCL is often used in this branch of chemistry to provide ground-based eddy covariance measurements, isotope measurements and airborne-platform atmospheric measurements. To date, QCL spectroscopy has been used to detect methane, nitrous oxide, carbon monoxide, carbon dioxide, formaldehyde, ozone, ammonia and acetylene.
Sources:
- “Applications of midinfrared quantum cascade lasers to spectroscopy”- Hancock G., et al, Optical Engineering, 2010, DOI: 10.1117/1.3498770
- “Applications of Quantum Cascade Laser Spectroscopy in the Analysis of Pharmaceutical Formulations”- Galán-Freyle N. J., et al, Applied Spectroscopy, 2016, DOI: 10.1177/0003702816662609
- “Quantum cascade lasers (QCLs) in biomedical spectroscopy”- Schwaighofer A., et al, Chemical Society Reviews¸ 2017, DOI: 10.1039/C7CS00403F
- “Quantum Cascade Laser Spectrometry Techniques: A New Trend in Atmospheric Chemistry”- Li J. S., et al, Applied Spectroscopy Reviews¸ 2013, DOI: 10.1080/05704928.2012.757232
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