Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - Topological-Plasmonics (Robust light manipulation in plasmonic nanostructures assisted by topological protection)

Teaser

Materials such as nobel metals or doped graphene are able to strongly interact with light and concentrate it at very short length scales. This is thanks to the excitation of surface plasmons, a combination of light and electrons oscillating at the material’s surface akin to...

Summary

Materials such as nobel metals or doped graphene are able to strongly interact with light and concentrate it at very short length scales. This is thanks to the excitation of surface plasmons, a combination of light and electrons oscillating at the material’s surface akin to ripples on the surface of a pond. Their nature combining waves and matter allows for remarkable properties: surface plasmons can squeeze light to the nanometer scale, at dimensions much smaller than its wavelength, contrary to what can be achieved with conventional optics. This makes plasmonics a key tool for guiding and focusing light in the nanoscale, which is promising for current photonic technologies that need to be ever more compact.

On the other hand, plasmonic platforms for guiding and focusing light are based on the nanoscale fabrication of the required structures. Importantly, fabrication imperfections and defects hinder the propagation of plasmons over long distances, a factor that adds to plasmon dissipation through material losses. In this context, the area of topological Physics is very promising to help in the design of plasmonic nanostructures capable of supporting robust surface plasmon modes that are not affected by imperfections. The framework of topology is currently a topic of great interest in various areas of Physics as it allows for designing protected states that are insensitive to material details and imperfections. Such states propagate along the edge between media characterized by different topological invariants and are said to be protected as their properties stem from the properties of the bulk structure and are not affected by the details on the edge. Thus, the possibility of light travelling along one-way routes at the nanoscale emerges by combining topology and plasmonics.

The overall objective of this project was to design and characterize topologically protected light modes confined at the nanoscale.

Work performed

Topological plasmonics was approached by considering a chain of plasmonic nanoparticles with alternating spacing. This system is a one dimensional topological insulator known as the SSH model and it served to characterize the main features of topological plasmonics, where radiative and retardation effects are relevant. It was shown that the nanoparticle chain is a one dimensional insulator with protected edge states localized at the edges of the chain [ACS Photonics 5 (6), 2271-2279 (2018)]. Furthermore, topological nanophotonics offers a flexible platform to study topological Physics without electronic counterpart. In particular, the topological nanophotonic systems studied in this project allowed us to study the effect of long-range interactions and finite sample sizes on the topological properties [Physical Review B (Rapid Communications) 96 (4), 041408 (2017)].

On the other hand, novel applications of plasmonic metasurfaces were envisioned through the application of the theory of transformation optics which considers spatial symmetries. This theory provides analytical insight on the design of plasmonic systems and was applied to plasmonic metasurfaces with novel designs [Physical Review B 95 (15), 155401 (2017)]. In particular, plasmonic metasurfaces based on graphene were demonstrated which are capable of absorbing half on the incident radiation on this atomically thin material and even perfect absorption [ EPJ Applied Metamaterials 4 (6) (2017)]. Furthermore, plasmonic metasurfaces comprising a periodic array of singularities in the form of sharp metal edges (or supressed conductivity on a graphene layer) were proposed [Science, 358(6365), 915-917 (2017)]. Interestingly, the optical response of these periodic two dimensional structures is characteristic of a bulk, that is, they present a continuous rather than discrete absorption spectrum. External radiation is trapped by the metasurface and it travels towards the singularities where it is compressed to the nanometer scale [Physical Review B 98, 125409 (2018)]. This is accompanied by very large field enhancements and a broad spectrum [ACS Nano, 12 (2), 1006-1013 (2018)].

Final results

The study of topological plasmonics opens up the possibility of robust light control in the nanoscale. In addition, it serves as a physical platform for the study of topological Physics with special characteristics such as long range interactions, non-hermiticity and finite size effects. The relevance of this lies on the fact that it allows to study properties of topological systems beyond what is possible with topological matter, expanding our knowledge on topological Physics and enabling novel effects.

On the other hand, this project has made use of the theoretical tool of transformation optics to design novel plasmonic metasurfaces. In particular, we have presented the concept of singular plasmonic metasurfaces which are able to trap and compress light in a metal surface with sharp edges or in a doped graphene layer with suppressed conductivity points. These results are promising for the design of absorbers capable of absorbing half of the incident light over a broad range of frequencies in the technologically relevant Terahertz region of the spectrum.