Many molecules, especially biomolecules, vibrate when excited with infrared light. The way the molecules vibrate, or their vibrational modes, is characteristic of the different bonds present in a sample. Being able to probe these modes can therefore be used to help elucidate the molecular structure of the sample, even if it is only present in small quantities.

Plasmonic resonances

Hatice Altug and colleagues, and indeed other research groups around the world, have previously shown that specifically engineered metallic nanoparticles can be made to resonate in the mid-infrared part of the electromagnetic spectrum. The nanoparticles interact strongly with light via localized surface plasmons (collective oscillations of electrons on a metal's surface) and so act as efficient optical nanoantennas that capture more light. Such plasmonic resonances in the infrared can be used as probes that can be tuned to and away from various vibrational modes in a molecule.

While such enhanced light absorption is an important property to have, one drawback is that only certain vibrational modes occurring in a narrow range of frequencies are enhanced. “This implies that only a few types of molecular bonds can be observed for a given resonating structure,” explained Altug. “Designing resonators that support multiple modes, and that would vibrate at a number of different frequencies, would thus allow us to overcome this problem.”

Multiple light-absorbing bands

Altug and team members Ronen Adato and Kai Chen at Boston decided to focus on this issue in their new work. Although they studied a polymer in their experiments, being able to observe multiple IR light-absorbing bands in biological molecules is very important too, as mentioned. “Multiple bands can provide information about different parts of the same molecule or particle as well as potentially distinguish between different molecules,” Adato told nanotechweb.org.

The team made a “dual-band perfect absorber” comprising two gold nanowires arranged in a cross shape separated in the vertical direction by just a couple of hundred nanometres from a gold film by a thin layer of magnesium fluoride. The particle/gold film system acts as a resonator that captures and absorbs more than 90% of the incident light falling on it – this is why it is called a “perfect” absorber.

Breaking symmetry

“Typically a single particle supports a single resonance and therefore a single narrow band of frequencies at which this perfect absorption occurs, but by breaking the symmetry of the cross structure, we can elicit multiple resonances from the single structure” said Altug.

The technique could be ideal for studying the structures of proteins, and other biological molecules like lipids and nucleic acids, as well as microorganisms such as bacteria, she adds.

The work is detailed in ACS Nano.