Lasers & photonics
it emits light that covers all of the wavelengths in the mid-infrared spectrum”.
Many biologically relevant chemicals, such as proteins, lipids and many organic molecules exhibit strong fundamental vibrational absorption bands in the mid-infrared. Although there are other sources of mid-infrared light, QCLs are highly tuneable and offer fast measurement speeds. This is why they have emerged as an attractive tool for mid-infrared spectroscopy for medical diagnostics as well as applications in industries such as environmental monitoring, manufacturing and agri-food, where they detect volatile organic compounds, toxic gases and microplastics, among other chemicals.
Diagnostic and therapeutic uses QCLs have high spectral brilliance, meaning they can concentrate massive optical power in a narrow wavelength range. Combined with the wide range of wavelengths that quantum cascade lasers can be tuned to, this makes them highly suitable for detecting many chemicals within seconds and with extremely high sensitivity. Moreover, since these lasers measure fundamental molecular vibrations, they capture spectral signatures of distinct molecules, making them more reliable than near-infrared or visible-light spectroscopy, which detects weaker interactions with light. Soon after the invention of QCLs, scientists demonstrated their potential in analysing metabolites in breath. The idea is that when a patient breathes into a chamber, a QCL beam that bounces back and forth can detect chemicals with a very high sensitivity. “You can get parts per billion sensitivity for many important chemicals and, in certain cases, even parts per trillion,” says Capasso. In industrial settings, QCLs are used to monitor concentrations of different gases. For example, carbon dioxide monitoring is critical from both a workplace safety perspective and for process optimisation in manufacturing where the gas is either produced or used up. Likewise, in a hospital setting, infrared sensors are used to monitor end- tidal carbon dioxide concentrations in patients’ exhaled breath under anaesthesia or intensive care. QCL-based devices could replace these sensors and provide greater sensitivity.
Another prominent application of QCLs in medical diagnostics has been non-invasive glucose monitoring. “The sensitivity [of glucose detection] is best in a mid-infrared frequency range which can only be accessed by a quantum cascade laser,” says Faist. QCLs emit high-power mid-IR light, and the light attenuated or scattered by the glucose molecules provides a measure of the blood glucose concentration. Minimally invasive continuous glucose monitors are often inaccurate and measure glucose levels in the interstitial fluid, which lag behind glucose levels. By eliminating the pain and
www.medicaldevice-developments.com Quantum cascade lasers market
Market size in $bn CAGR 4.4%
433.64 537.81
2025 Source: Mordor Intelligence
inconvenience of finger pricks, QCL-based glucose monitoring could improve compliance and lead to better insulin management.
Since QCLs can detect many metabolites, they have also been used in non-destructive tissue imaging. In a tissue section or cell culture, they can detect how different metabolites absorb mid-infrared light, thereby revealing the health or identity of the cells without any reagents or labels. Researchers are identifying new cancer biomarkers that could be detected with QCLs. This non-invasive imaging also makes them promising tools for guiding surgical interventions. For instance, mid-infrared QCL imaging has been explored for identifying tumour margins in breast tissue samples, demarcating healthy and malignant cells. If all malignant cells aren’t excised in a breast- conserving surgery, the patient might need to go through surgery again. “By defining the margin, you make sure you get everything in the tumour but not more,” says Capasso.
QCLs have also been proposed as a source for laser surgeries that ablate tissue. Presently, laser surgeries primarily use carbon dioxide and Nd:YAG lasers for tissue ablation with high precision. The tunability of QCLs allows specific targeting of the tissue to be excised, leading to cleaner cuts and fewer side effects in laser surgery. Therapeutic applications of QCLs, however, are still highly experimental. “No studies are really undisputed to the level of medical standards that would be required to prove such applications,” says Faist.
Technical advances in QCLs Terahertz QCLs can penetrate a few millimetres into tissues but are strongly attenuated by water content and are being investigated for many of the same applications as mid-infrared lasers. While the technology for these lags behind mid-infrared QCLs, they can also open new medical applications. For example, studies have shown they can be used to assess injury depth and oedema buildup in burn injuries. “We’re still struggling to understand exactly
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