Lasers & photonics


Advances in mid-infrared quantum cascade lasers and compact supercontinuum sources are unlocking high-sensitivity spectroscopic diagnostics and therapeutic possibilities for point-of-care (POC) optics. Sachin Rawat examines the clinical applications, miniaturisation, supply chain dynamics, and the pathway from lab prototype to commercial POC system and reimbursement, with insight from Professor Jérôme Faist of the Department of Physics at ETH Zurich and Professor Federico Capasso from the Harvard School of Engineering and Applied Sciences.


typically in the near-infrared range of around 1.3– 1.55µm, which transmits data across optical fibre networks. These lasers also find applications in detecting analytes via their interaction with light, typically at near-infrared wavelengths. However, these conventional semiconductor lasers require a different semiconductor material to produce each wavelength. This limits the wavelengths they can produce to those that have suitable semiconductor materials that emit them.


Guiding light S


emiconductor laser diodes make up the backbone of the global telecommunications network. These lasers produce coherent light,


quantum wells and barriers. As electrons cascade down the quantum wells, they emit photons that stimulate further electron transitions, generating the amplified light pulse.


In 1994, Jérôme Faist, Federico Capasso and colleagues at Bell Labs developed a semiconductor laser that could emit light across mid-infrared wavelengths using the same material system. Dubbed a quantum cascade laser (QCL), it consists of a stack of nanometre-thick semiconductor layers known as


“QCLs allow access to mid-infrared spectrum and terahertz spectrum frequency ranges, which are very difficult to access by other laser sources,” says Faist, now a professor of physics at ETH Zurich. Mid- infrared lasers generally produce light at wavelengths from about 3µm to 12µm, whereas terahertz lasers emit light at wavelengths of 50µm and beyond. An advantage of developing devices with quantum cascade lasers is that the fabrication technology used to make these lasers is similar to that used to make lasers for communications and so forth, says Capasso, now professor of applied physics at Harvard’s School of Engineering and Applied Sciences. Moreover, he adds, “you can design the underlying material in such a way that


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