Researchers from Graphene Flagship partners ICFO, Spain, and Universidad do Minho, Portugal, together with researchers from France, Brazil and the US, have built the smallest optical cavity to date for infrared light. This was achieved by integrating metallic cubes nanometres in size over a graphene sheet, which enables miniaturised 'traps' for graphene plasmons. These results show promise for new miniaturised sensors that could be used in medicine, biotechnology, food inspection and security.
Miniaturisation has enabled many advances. Shrinking down electronic circuits has enabled technology like smartphones, wrist-worn health monitors, medical probes and nanosatellites – all of which were unthinkable just a few decades ago. In 60 years, transistors have shrunk from the size of your palm to just 14 nanometres in width: 1000 times smaller than the diameter of a human hair.
Miniaturisation has pushed technology into a new era of optical circuitry. But, in parallel, it has also unearthed new challenges and obstacles to overcome, such as controlling and guiding light at the nanometre scale. New techniques have been developed, aiming to confine light into extremely tiny spaces – millions of times smaller than current ones. Metals can compress light below the wavelength-scale, which is the so-called diffraction limit.
Graphene is capable of guiding light in the form of "plasmons", which are oscillations of electrons that strongly interact with light. Graphene – a single layer of carbon atoms – combines exceptional optical and electrical properties. These plasmons have a natural ability to confine light into very small spaces. However, until now, it was only possible to confine these plasmons in one direction, while the ability of light to interact with small particles, like atoms and molecules, resides in the volume that it can be compressed into. This type of confinement, in all three dimensions, is commonly regarded as an optical cavity.
Now, researchers from Graphene Flagship partners ICFO, Spain, and Universidad do Minho, Portugal, together with researchers from France, Brazil, and the US have constructed a new type of cavity for graphene plasmons, by integrating metallic cubes of nanometre sizes over a graphene sheet. Their approach enabled the smallest optical cavity to date for infrared light, which is based on these plasmons.
In their experiment, they used silver nanocubes 50 nanometres in size, sprinkled randomly onto graphene with no specific pattern or orientation. This allowed each nanocube, together with graphene, to act as a single cavity. They then passed infrared light through the device and observed how the plasmons propagated into the space between the metal nanocubes and graphene, being compressed into a very small volume.
Itai Epstein, from Graphene Flagship partner ICFO, comments: "The main obstacle we encountered in this experiment resided in the fact that the wavelength of light in the infrared range is very large and the cubes are very small – about 200 times smaller – so it is extremely difficult to make them interact with each other."
To overcome this, they took advantage of a special phenomenon – when the graphene plasmons interacted with the nanocubes, they were able to generate a special resonance, called a magnetic resonance. Epstein clarifies: "A unique property of the magnetic resonance is that it can act as a type of antenna that bridges the difference between the small dimensions of the nanocube and the large scale of the light." Thus, the generated resonance maintained the plasmons moving between the cube and graphene in a very small volume, which is ten billion times smaller than the volume of regular infrared light: something never achieved before in optical confinement. Even more so, they were able to see that the single graphene-cube cavity, when interacting with the light, acts as a new type of nano-antenna that can scatter the infrared light very efficiently.
The results of the study are promising for molecular and biological sensing, important for medicine, biotechnology, food inspection and even security, as this approach is capable of intensifying the optical field considerably and thus detecting molecular materials, which usually respond to infrared light.
Frank Koppens, Graphene Flagship Work Package Leader for Photonics and Optoelectronics and author of this study, says: "Such achievement is of great importance because it allows us to tune the volume of the plasmon mode to drive their interaction with small particles, like molecules or atoms, and be able to detect and study them. We know that the infrared and Terahertz ranges of the optical spectrum provide valuable information about vibrational resonances of molecules, opening the possibility to interact and detect molecular materials as well as use this as a promising sensing technology".
Marco Romagnoli, leader of the Electronics and Photonics Integration Division of the Graphene Flagship, adds: "Graphene plasmons in metallic nanocubes break the limit of the size of photonic integrated devices. Photons are much larger than electrons, and in this sense a photonic integrated circuit cannot be not comparable in size to an electronic integrated circuit. Nevertheless, graphene plasmons in nanocubes demonstrate that even photonic devices can be miniaturized, and this achievement opens new routes for nanomedicine, nanosensing, nano-wireless communications and other fascinating applications."
Andrea C. Ferrari, Graphene Flagship Science and Technology Officer and Chair of its Management panel, adds: "The interactions between metal nanoparticles and graphene have been at the centre of Graphene Flagship investigations since the beginning. The understanding of plasmons in graphene and in metals, and their interplay, has been spearheaded by the Flagship over the years. This paper pushes the limit of our knowledge to the next level and paves the way for a new generation of devices that were unthinkable a few years ago. It also proves, yet again, that graphene is an ideal ground for development in the areas of photonics and optoelectronics."
"Far-field Excitation of Single Graphene Plasmon Cavities with Ultra-compressed Mode-volumes." Itai Epstein, David Alcaraz, Zhiqin Huang, Varun-Varma Pusapati, Jean-Paul Hugonin, Avinash Kumar, Xander M. Deputy, Tymofiy Khodkov, Tatiana G. Rappoport, Jin-Yong Hong, Nuno M. R. Peres, Jing Kong, David R. Smith, and Frank H. L. Koppens, Science 368, 6496 (2020).