Smallest Cavity for Light

Artistic illustration of the light compressed below the silver nanocubes randomly placed over the graphene-based heterostructure. (Source: M Ceccanti)

Miniaturization has pushed tech­nology to a new era of optical circuitry. But, in parallel, it has also triggered new challenges and obstacles to overcome, for example, on how to deal with controlling and guiding light at the nanometer scale. New techniques have been on the rise searching for ways to confine light into extremely tiny spaces, millions of times smaller than current ones. Researchers had earlier on found that metals can compress light below the wave­length-scale.

In that aspect, graphene is capable of guiding light in the form of plasmons, which are oscillations of electrons that are strongly inter­acting 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, 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, ICFO researchers Itai Epstein, David Alcaraz, Varum-Varma Pusa­pati, Avinash Kumar, Tymofiy Khodkow, led by Frank Koppens, in collaboration with researchers from MIT, Duke University, Université Paris-Saclay, and Univer­sidad do Minho, have succeeded to build a new type of cavity for graphene plasmons, by inte­grating metallic cubes of nano­meter sizes over a graphene sheet. Their approach enabled to realize the smallest optical cavity ever built for infrared light, which is based on these plasmons.

In their experiment they used silver nano­cubes of 50 nanometers 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. Then they sent infrared light through the device and observed how the plasmons propa­gated into the space between the metal nanocube and the graphene, being compressed only to that very small volume. As Itai Epstein 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 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 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, acted as a new type of nano-antenna that is able to scatter the infrared light very effi­ciently.

The results of the study are extremely promising for the field of molecular and biological sensing, important for medicine, bio­technology, food inspection or even security, since this approach is capable of intensifying the optical field consi­derably and thus detect molecular materials, which usually respond to infrared light. As Koppens states “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”. (Source: ICFO)

Reference: I. Epstein et al.: Far-field excitation of single graphene plasmon cavities with ultracompressed mode volumes, Science 368, 1219 (2020); DOI: 10.1126/science.abb1570

Link: Quantum Nano-Optoelectronics (F. Koppens), ICFO – Institut de Ciencies Fotoniques, Barcelona Institute of Science and Technology, Castelldefels, Spain

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