Molecular Qualities Captured with Plasmons

Specific properties of small amounts of molecules can now be isolated with the use of graphene-metal film structures, thanks to the work of scientists in Russia and Spain.

Their work focuses on a plasmon, which is an electron oscillation that is coupled with an electromagnetic wave. By focusing these plasmons, they are converted into those with shorter wavelengths, called acoustic plasmons.

Probing the vibrational fingerprint of a molecule with a nanofocused acoustic graphene plasmon polariton. Courtesy of Kirill Voronin.

In typical spectroscopic experiments, a sample is lit up with a wave of different frequencies, with the reflected light caught by a detector. But when studying small amounts of molecules, the light wavelength is too wide to differentiate between specific elements of the sample.

“Typically, the standard far-field spectroscopy is diffraction-limited; that is, you need to illuminate the sample of a sufficiently large area (comparable at least with the wavelength of light) to get a signal,” said Alexey Nikitkin, a visiting professor at the Moscow Institute of Physics and Technology and a researcher at Donostia International Physics Center in Spain.

In their study, researchers devised a setup whereby acoustic graphene plasmon polaritons (AGPs) traveled along a graphene sheet above a metal substrate, with the distance between the graphene and metal decreasing, creating a taper. Then molecules are added with the AGP reflection from the end of the channel calculated. They noted that strong coupling of AGPs and molecular vibrational modes takes place as charges are sent along the sheet through holes along the sheet.

For purposes of the experiment, the team used CREB-binding protein (CBP), which can be found in many pharmaceuticals. This molecule’s absorption peak is at a wavelength of 6.9 µm. A layer of these molecules was put in the wedge setup, and the graphene was hit with a focused light beam. The plasmons were excited and interacted with the molecules.

“We had in mind a commercially available scattering-type scanning near-field microscope (S-SNOM), in which the laser source illuminates the needle (the tip),” Nikitkin said. “Then the tip launches plasmons along graphene.”

The researchers foresee the principles of their experiment being put to use in on-chip sensing at the nanoscale.

“In principle, the suggested technology can be implemented on chip without any need of the microscope,” Nikitkin said. “The plasmons can be excited by an optical antenna and the response can be read out by means of the generated photocurrent.”

The research was published in Nanophotonics (www.doi.org/10.1515/nanoph-2020-0164).