Miniaturisation has made unimaginable dreams come true. Shrinking down electronic circuits has allowed us to access technology such as smartphones, health watches, medical probes, nanosatellites—all unthinkable a couple of decades ago. Just imagine if in the course of 60 years the transistor had gone from being the size of your palm to 14 nanometres in diameter, 1,000 times smaller than the diameter of a hair.

Miniaturisation has pushed technology to a new era of optical circuitry. In parallel, it has also triggered new challenges and obstacles to overcome, for example, how to control and guide light at the nanometre scale. An increasing number of new techniques for confining light in extremely tiny spaces, millions of times smaller than those achieved up to now, have appeared. Scientists had already found that metals can compress light below the wavelength scale (diffraction limit).

Graphene—a material composed of a single layer of carbon atoms that has exceptional optical and electrical properties—is capable of guiding light in the form of plasmons, which are oscillations of electrons that interact strongly with light. These graphene plasmons have a natural ability to confine light to very small spaces. However, until now it was only possible to confine these plasmons in one direction, while the actual ability of light to interact with small particles such as 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.

In a recent study published in Science, researchers at ICFO –a research institute affiliated to the Universitat Politècnica de Catalunya · BarcelonaTech (UPC)–   Itai Epstein, David Alcaraz, Varum-Varma Pusapati, Avinash Kumar and Tymofiy Khodkow, led by ICREA Professor at the ICFO Frank Koppens, in collaboration with researchers from MIT, Duke University, Paris-Saclay University and the Universidade do Minho, have succeeded in building a new type of cavity for graphene plasmons by integrating metallic cubes of nanometric size over a graphene sheet. Their approach achieved the smallest optical cavity ever built for infrared light based on these plasmons.

In their experiment they used silver nanocubes 50 nanometres in size, which were sprinkled randomly on top of the graphene sheet with no specific pattern or orientation. This allowed each nanocube, together with graphene, to act as a single cavity. They then sent infrared light through the device and observed how the plasmons propagated into the space between the metal nanocube and the graphene, compressing themselves into this very small space.

As Itai Epstein, the first author of the study, comments, “The main obstacle that 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.”

In order to overcome this, they used a special phenomenon: when the graphene plasmons interacted with the nanocubes, they were able to generate a special resonance, called a magnetic resonance. As 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 resonance generated kept 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 acted as a new type of nano-antenna that is able to scatter the infrared light very efficiently when it interacted with the light.

The results of the study are extremely promising for the field of molecular and biological sensing and important for medicine, biotechnology, food inspection and even security, since this approach can intensify the optical field considerably and thus detect molecular materials, which usually respond to infrared light.

As Prof. Koppens states, “Such an achievement is of great importance because it allows us to tune the volume of the plasmons to drive their interaction with small particles such as  molecules and atoms and to be able to detect and study them. We know that the infrared and terahertz ranges of the optical spectrum provide valuable information about the vibrational resonance of molecules, opening up the possibility of interacting with and detecting molecular materials, as well as using this as a promising sensing technology”.

Reference article

• 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).