Squeezing Light to One Atom

An artistic impression of the squeezed light in between the metal and graphene, separated by just one a one-atom thick dielectric. (Source: ICFO)

Most of all electronic devices in our daily lives consist of billions of tran­sistors. The transistor started out being as big as 1 cm, but thanks to advance­ment of tech­nology, it has reached an amazing size of 14 nano­meters, that is, 1000 times smaller than the diameter of a hair. At the same time, there has also been a race to further shrink devices that control and guide light. Light can function as an ultra-fast communi­cation channel, for example between different sections of a computer chip, but it can also be used for ultra-sensi­tive sensors or novel on-chip nanoscale lasers. 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 diffrac­tion limit, but more confine­ment would always come at the cost of more energy losses. This para­digm has now been shifted, by using graphene instead.

Now, researchers at the Insti­tute of Photonic Sciences ICFO in Barcelona have been able to reach the ultimate level of con­finement of light. That is, they have been able to confine light down to a space one atom thick in dimension, the smallest confine­ment ever possible. The work was led by Frank Koppens and carried out by David Alcaraz, Sebastien Nanot, Itai Epstein, Dmitri Efetov, Mark Lundeberg, Romain Parret, and Johann Osmond from ICFO, and performed in colla­boration with Univer­sity of Minho (Portugal) and MIT (USA). The team of researchers used hetero­structures of 2D materials, and built up a com­pletely new nano-optical device, as if it were atom-scale Lego. They took a graphene monolayer, and stacked onto it a hexagonal boron nitride (hBN) monolayer, and on top of this deposited an array of metallic rods. They used graphene because this material is capable of guiding light in the form of plasmons.

They sent infrared light through their devices and observed how the plasmons propa­gated in between the metal and the graphene. To reach the smallest space concei­vable, they decided to reduce as much as possible the gap between the metal and the graphene to see if the confinement of light remained efficient, e.g. without addi­tional energy losses. Stri­kingly enough, they saw that even when a monolayer of hBN was used as a spacer, the plasmons were still excited by the light, and could propagate freely while being confined to a channel of just on atom thick. They managed to switch this plasmon propa­gation on and off, simply by applying an electrical voltage, demon­strating the control of light guided in channels smaller than one nano­meter of height.

The results of this disco­very enable a completely new world of opto-elec­tronic devices that are just one nano­meter thick, such as ultra-small optical switches, detectors and sensors. Due to the paradigm shift in optical field con­finement, extreme light-matter inter­actions can now be explored that were not accessible before. What is really exciting is that the atom-scale lego-toolbox of 2D materials has now also proven to be appli­cable for many types of completely new materials devices where both light and electrons can be con­trolled even down to the scale of a nano­meter. (Source: ICFO)

Reference: D. Alcaraz Iranzo et al.: Probing the ultimate plasmon confinement limits with a van der Waals heterostructure, Science 360, 291 (2018); DOI: 10.1126/science.aar8438

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

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