Lasers & photonics
Mid-infrared QCL imaging has been explored for identifying tumour margins in breast tissue samples, which may have been flagged after a mammogram.
what are the things that we could measure well and very specifically with the terahertz,” says Faist. Manufacturing QCLs involves molecular beam
epitaxy, a process in which beams of atoms or molecules of specific elements are deposited on a surface. The process is extremely sensitive to material flaws, which limits production yields. Developing QCL-based devices therefore demands precise control over epitaxial growth, layer thickness and interface quality. This adds to the complexity of producing them and performing quality control, further increasing their costs.
The MILADO project, an international consortium of research institutions and companies in the field, is working to improve their manufacturability. Current QCL-based detection systems are bulky. The project aims to replace them with QCL arrays, which could be miniaturised, produced in large volumes and integrated like photonic chips. In the future, these could pack the diagnostic power of QCLs in a portable device or wearable, enabling commercial adoption in medical devices. QCLs can scan absorption spectra thousands of times per second, which allows them to take a very large number of measurements in that time. However, their spectral coverage is limited as individual QCLs typically sense absorption over a narrow wavelength range. Scientists are exploring ways to improve spectral coverage without sacrificing measurement speed. For instance, researchers at the Fraunhofer Institute for Applied Solid State Physics, one of the partners in the MILADO project, are developing multiplexed laser technology that couples spectrally adjacent modules together.
QCLs have been limited to laboratory-scale equipment because of their high power requirements and the large amounts of waste heat they produce. A push towards low-dissipation QCLs is critical to improving their portability. If these lasers can be made compact, energy-efficient and manufacturable
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at scale, says Faist, they could then be integrated into a range of health-monitoring systems. Another exciting development, he adds, is QCL frequency combs, which emit light spanning multiple discrete, equally spaced frequencies like a comb spectrum. “With frequency combs, we could do infrared spectroscopy with much [fewer] moving parts,” says Faist.
A killer application
While there are dozens of companies manufacturing quantum cascade lasers, many of them focus on industrial and research markets, but mass-market consumer devices using them have not yet materialised. “There is a market for them,” says Capasso, “but it’s not a high-volume market because no one has found a killer application yet.” Could that change in the coming years? There is a growing demand for highly sensitive technologies for medical diagnostics as well as industrial and environmental sensing, and QCLs are well placed to meet that demand. Advances in manufacturing QCLs, new semiconductor materials, miniaturisation and effective cooling solutions will increase their value proposition. Low-cost production of miniaturised QCL-based devices at scale could lead to wider adoption of the technology, including in the medical device sector.
It took a long time for QCL technology to mature to its current status. But for any technology, notes Faist, “It also takes a long time to develop the whole ecosystem around it, and for people to learn to use those correctly.” This part can be even more challenging when the intended application is medical diagnostics.
“A sensor contains a laser source, a detector, other optical elements, a sampling space and a data analysis chain,” says Faist. “You need to put all those things together and validate them for medical use, and that is a very complex thing.” ●
www.medicaldevice-developments.com
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