Single molecule detector: IR graphene plasmonics

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  • Published: Sep 1, 2016
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Single molecule detector: IR graphene plasmonics

Predefined microstructure

Design of the spaser with the graphene layer shown as a honeycomb lattice above the dielectric layer (blue). The spaser is optically pumped through the active (gain) medium shown in orange.

Graphene, the two-dimensional carbon allotrope could form the basis of a new plasmonic device capable of detecting single molecules of explosive materials, toxic chemicals, and other organic compounds using infrared, according to scientists at the Moscow Institute of Physics and Technology.

Quasiparticles known as plasmons have fascinated scientists and technologists for many years. The notion of a quantum of plasma oscillations being a phenomenon that might underpin new ways to investigate or manipulate matter. In the case of a solid body, plasmons are fundamentally the oscillations of free electrons in that material. But, of particular interest are the effects that arise from the surface interactions of electromagnetic waves with such plasmons, this is usually observed with metals or semimetals, because of their higher free electron density but can become manifest on other systems too.

Regardless, making use of these effects might be exploited in high-precision electronics and optics with one area of great potential being sub-wavelength light focusing. If this were achievable it would allow a device to address a single molecule, which is, of course, smaller than the wavelength of light and so way beyond the realm of classical optical devices. Using plasmons in metals is not going to be useful in this context because those plasmons tend to lose their energy rather too quickly because of resistance in the metal. They are therefore not self-sustained and must be repeatedly excited to perpetuate. One way of circumventing this issue is to preclude such rapid energy loss and researchers have turned to a material that has been touted as something of a wonder stuff in recent years, having as it does a predefined microstructure, and that is graphene.

Nobel work

Physicists Andre Geim and Konstantin Novoselov, both graduates of MIPT, were the first to isolate graphene in the well-known form, efforts for which they won the Nobel Prize in Physics in October 2010. Graphene is a semiconductor but has an extremely high charge carrier mobility. Moreover, its electrical conductivity is also exceptionally high, which has led to the concept of graphene-based transistors.

Spasers and SPEDs

In order to work with graphene in a plasmonic device, theoretical calculations on whether the relevant quantum mechanical equations would add up were needed. This has now been accomplished by a team of researchers at the Laboratory of Nanostructure Spectroscopy headed by Yurii Lozovik. Those researchers formulated and solved the requisite equations and have gone on to develop a quantum model that predicts plasmonic behaviour in graphene. Emerging from this study is the operation of a surface plasmon emitting diode (SPED) and the nanoplasmonic counterpart of the laser, known as a spaser, which could be built from graphene.

Spasers function like lasers but to produce its coherent radiation, it relies on optical transitions in the gain medium, and the particles emitted are surface plasmons, as opposed to photons produced by the laser. A laser is to a spaser what an LED is to a SPED the former being coherent sources of photons and plasmons respectively, the latter pair producing incoherent sources of photons and surface plasmons. However, it requires considerably lower pump power and because both could operate in the infrared region of the spectrum they would be useful for studying different organic molecules.

"The graphene spaser could be used to design compact spectral measurement devices capable of detecting even a single molecule of a substance, which is essential for many potential applications," says co-author Alexander Dorofeenko. "Such sensors could detect organic molecules based on their characteristic vibrational transitions [their spectral ‘fingerprints’], as the light emitted or absorbed falls into the medium infrared region, which is exactly where the graphene-based spaser operates,’ he adds.

Related Links

Phys Rev B 2016, 94, 035406: "Self-consistent description of graphene quantum amplifier"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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