“An optical microscope magnifies a tiny object so that it can be seen by the human eye. In the same way, an optical amplifier makes a molecule’s weak but useful Raman signal detectable,” explains team member Yu-Rong Zhen, who designed the new nanostructure. “Our amplifier, which exploits so-called Fano resonances, is 10 times more powerful than the best previous such devices and might be able to detect molecules too small to be seen until now.”

Analysing re-emitted photons

When light strikes a molecule, most of the photons bounce off or pass directly though it but a few manage to get absorbed and are re-emitted into another energy level. By measuring and analysing these re-emitted photons using Raman spectroscopy, researchers can determine which types of atom the molecule contains as well as how they are arranged.

Raman signals can be very weak – after all, only around one in a trillion photons in a sample are re-emitted. However, the good news is that these signals can be boosted. The Rice scientists, for their part, have used a two-coherent-laser technique called “coherent anti-Stokes Raman spectroscopy” (CARS) together with a light amplifier made of four nanosized gold discs arranged in a diamond shape.

The technique, dubbed surface-enhanced CARS (or SECARS) can detect single molecules in a powerful new way, says team leader Naomi Halas. It might even be used to detect molecules whose structure or composition is unknown, she adds. “This is not possible with current technology but our new technique certainly has that potential.”

Relying on a more complex multi-photon process

“The second laser in SECARS is important because it provides further amplification,” says team member Yu Zhang. “In a conventional single-laser set-up, photons are absorbed and re-emitted and the optical signatures of a sample are usually amplified by around 100 million to 10 billion times. By adding a second laser coherent with the first one, SECARS relies on a more complex multi-photon process.”

The new amplifier is about the same size as previous such devices but contains a relatively large gap of 15 nm (compared with just 1 nm previously). This means that it can be used to amplify the optical signal of a large number of molecules at once, which could be useful when it comes to measuring chemical reactions in situ, for example. The signals are dramatically amplified thanks to an optical effect called Fano resonance in which light is efficiently harvested and scattered by the special geometric arrangement of the four-disc nanostructure, or “quadrumer”.

“Our new technique, as a fundamental scientific tool, could come in handy for a variety of fields,” Zhen and Zhang tell nanotechweb.org. “It could be used in any application that calls for a sensitive chemical composition analysis – for example, non-invasively investigating archaeological materials, analysing environmental pollutants and detecting pharmaceutical ingredients and extremely low concentrations of drugs in saliva. It could instantly diagnose some disease biomarkers in just one drop of blood too and might accurately be able to detect some explosives very quickly (within a second).”

The Rice researchers, who report their work in Nature Communications, say they now plan to use their SECARS technique to monitor single-molecule reaction dynamics in situ. “We may also be looking at using it as a molecular computer chip or logic gate,” say Zhen and Zhang. “This way of computing would actually involve following some types of chemical reaction,” they explain. “We could easily distinguish the molecule types before and after the reactions and ‘count’ the number of these molecules by their quantized optical intensities. Such a molecular computing technique, relying on small molecules, might prove much more powerful – and cheaper – than today’s silicon-based computers.”