Small gaps between metallic nanostructures can strongly concentrate light and researchers have begun exploiting this effect in recent years in a relatively new branch of photonics known as plasmonics. The nanosized gaps absorb light by confining it into regions that are hundreds of times smaller than the wavelength of light (which is nearly a micron long) by exploiting plasmons – collective oscillations of electrons that occur at the surface of metals that interact strongly with light. Such a technique allows them to overcome the so-called diffraction limit and so image objects at the nanoscale and beyond.

There is a problem, however, in that it is a real challenge to fabricate such small gaps on sub-nanometre length scales. A team led by Jeremy Baumberg may now have come up with a solution to this problem thanks to graphene (a sheet of carbon just one atom thick). The researchers have found that they can use graphene as the thinnest possible spacer to separate gold nanocomponents by a distance of just one atom thick, so creating stable and sub-nanometre gaps in a nanosandwich structure. "Because of the extremely small distance between the separated gold surfaces, light is trapped in the gap and its field intensity greatly enhanced in these tiny cavities," explains Baumberg.

White light spectroscopy

Thanks to a technique called white light spectroscopy (which is used to characterise the optical properties of materials in the visible region of the electromagnetic spectrum), the Cambridge team, in collaboration with Nokia, studied which colours of light were scattered by the nanosandwich structure and how these colours could be changed. The researchers employed a hot light source emitting white light covering all visible wavelengths and illuminated the nanosandwiches with the light at large angles of incidence. Light that is reflected directly is blocked and does not reach the detector, explains Baumberg, and only scattered light sent in the other direction is collected. "We make sure that the collection area is small so that we can study the optical properties of a single nanoparticle spaced away from a gold surface by graphene," he says.

"When gold nanoparticles are irradiated with the hot white light, free electrons in the material can be resonantly excited," he told "Electrons move back and forth like a water wave in a swimming pool and this periodic electron motion (called a plasmon) leads to emission of light in directions other than that in which the illuminated light is directly reflected. The emitted light can therefore be collected by our detector."

Nanoparticle interacts with its reflection

"Here, we put a nanoparticle just above a gold mirror so that it ‘sees’ its reflection and interacts with it," he explains. "The combination of the particle and its image actually appears as two nanoparticles, and the gap between them is set by the thickness of the graphene sheet itself."

"For example, by using two layers of graphene, we can increase the space between the gold nanoparticles from one to two atomic thicknesses, which allows us to better study the coupling of the plasmons in the nanoparticles to their image in the mirror. In a spacer thickness measuring one atom across, the coupling is so strong that quantum mechanical effects become dominant in the structure. In this case, the coupled plasmonic resonance splits into two different energy values corresponding to the harmonics of localised electrons close to the gap. The quantum mechanical effects determine the exact position of these energy values."

Extremely sensitive photodetectors possible

According to the researchers, the set up could be used to make extremely sensitive photodetectors from graphene sandwiched between gold nanoparticles since light is so strongly confined in the spacer gap. Such devices might be used in transparent electronics and cameras that can be pasted on top of existing display screens.

The team is now busy looking at how to change the resistance of the graphene spacer sheets with an applied voltage. "This will change the properties of the carbon material and reversibly tune the colour of our plasmonic resonance," says Baumberg. "New sorts of optical switches and sensors will then be possible."

The present work is detailed in Nano Lett. DOI: 10.1021/nl4018463.

Further reading

Light-trapping technique helps solar cells thin down (Jul 2012)
Polariton coupling becomes stronger (Jun 2011)
Plasmonics ramp up nanocrystal photosensor performance (May 2013)