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Molecule detection is essential to the monitoring of environmental pollution, the diagnosis of diseases, the identification of explosive substances, and other applications in which it is necessary to identify a hidden substance. Researchers from the Institute of Photonic Sciences (ICFO), an associate institute of the UPC-Barcelona Tech, working in collaboration with the Free University of Brussels, have discovered a detection system that uses the light generated by non-fluorescent molecules. With the new system, only 50-100 molecules and a low-powered (but high-quality) laser are needed to generate this light. The study was published as a highlighted article in the journal Nature Communications on 30 March.

Working in collaboration with the researcher Gregory Kozyreff of the Free University of Brussels, the ICFO research group, led by Prof. Jordi Martorell, who is also a researcher with the Department of Physics and Nuclear Engineering at the UPC-Barcelona Tech, was able to trap and analyze very small, non-fluorescent molecules (light-based detection is usually done using fluorescent molecules as markers) with a glass microsphere (spherical microresonator) 350 μm in diameter that acted as a detector, with the molecules becoming trapped on its surface. Molecules are detected by means of a process of generating nonlinear laser light in which a light wave of a particular optical frequency produces a new wave, generally of a much lower intensity and with double the frequency.

This process, also known as second-harmonic generation, was first observed in 1961, one year after the invention of the laser. The phenomenon has subsequently been used to obtain new optical frequencies from the limited number of frequencies available with lasers.

To use this type of light-generation to detect hidden substances, it is typically necessary to use a large number of molecules (on the order of one billion) and a very powerful laser. To generate a measurable second-harmonic wave with just a hundred molecules, the ICFO researchers applied the whispering-gallery modes, the name given to the propagation of light along the inside of a reflective circular or arched surface.
Whispering-gallery modes take their name from an acoustic phenomenon discovered in the dome of St Paul's Cathedral in London. Described for the first time by Lord Rayleigh in 1910, the whispering-gallery phenomenon explains why, if two people stand at opposite sides of the gallery, they can hear one another whisper thanks to the way in which the sound travels along the curved inner surface of the dome.

With the glass microsphere used in this new study, the whispering-gallery phenomenon is recreated using light waves. A light wave trapped in this way can travel many times around the inside of the microsphere and interact with a single molecule located on the perimeter of the sphere; this causes the nonlinear effects of the light to be multiplied.

The new device allows, among other things, for the highly sensitive detection of substances that may be potentially contaminant or hazardous to human health and safety. For instance, it can be used to identify the presence of PETN, a powerful explosive that is very difficult to detect given its low volatility and was used in the foiled terrorist attack of 2009 on a Northwest Airlines plane bound for Detroit. The technology can make this type of explosive “visible” as the molecule that forms the explosive contains four nitrate groups that have the combination required to generate second-harmonic light.

It can also be used to detect small contaminant molecules from an uncontrolled leak that may be potentially hazardous to human health.