Infrared spectroscopy is commonly used for testing the strength of various materials, to study archaeological finds, and in forensics. It’s a bit of a niche in medicine, partially due to the fact that the visible spectrum is itself so useful and equipment relying on it cheap to acquire. Researchers at the Agency for Science, Technology and Research (A*STAR) in Singapore have now developed an infrared sensing technique that relies on cheap visible spectrum detectors to do the job.
The researchers figured out how to split a laser beam into two lower energy, and therefore longer wavelength, beams. One beam is used for reference, while the other can interact with what it’s sensing and then merge back with the reference beam. The interaction between two beams as they come together can be detected using a visible light detector. “It’s a very simple setup, uses simple components, and is very compact, and we’ve hit a resolution comparable with conventional infrared systems,” said Leonid Krivitsky, one of the leads on the research.
Here’s some more details on the actual setup that was used, according to A*STAR:
The team fed laser light into a lithium niobate crystal that split some of the laser photons into two quantum-linked photons of lower energies, one in the infrared, and one in the visible parts of the spectrum, through a nonlinear process known as parametric down-conversion.
In a setup similar to a Michelson interferometer, the three beams were separated and were sent to mirrors that reflected them back into the crystal.
When the original laser beam re-entered the crystal, it created a new pair of down-converted beams that interfered with the light created in the first pass.
It was this interference that the team exploited: a sample placed in the infrared beam affected the interference between first-pass and second-pass beams, which could be detected in both the infrared and visible beams, because they are quantum linked.
Related papers: Measurement of infrared optical constants with visible photons…; Tunable optical coherence tomography in the infrared range using visible photons…