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Mesoscopic Plasmonics: Bridging Classical and Quantum Nano-Optics

Final Report Summary - MESOPLAS (Mesoscopic Plasmonics: Bridging Classical and Quantum Nano-Optics)

The boost experienced by nanophotonics research during the past decade has been driven by the ability of surface plasmons to collect and concentrate light into deeply sub-wavelength volumes. The hybrid nature of surface plasmons (which emerge from the coupling of photons to the collective oscillations of conduction electrons in metals) has allowed an unprecedented control of light at the nanoscale, a regime inaccessible to standard photonics technology. This scientific success has been possible due to two factors: the high precision of modern nanofabrication and characterization techniques, and the extraordinary predictive value of classical electrodynamics.

However, the miniaturization trend in experimental nano-optics is currently approaching dimensions comparable to the typical Coulomb screening length in noble metals (of the order of a few angstroms). A theoretical challenge arises in this spatial range for two reasons. On the one hand, at this sub-nanometre regime, macroscopic electromagnetism breaks down due to the emergence of quantum effects such as spatial non-locality. On the other hand, the enormous complexity of the full quantum numerical schemes available to describe the electron-ion dynamics in metals restricts their applicability to systems involving only a few hundreds of electrons.

The ultimate goal of MESOPLAS was to fill the gap between Maxwell’s equations and first principle condensed matter theory methods. It aims to devise a mesoscopic platform able to treat accurately and efficiently the interaction between light and matter in nanodevices which, presenting angstrom-sized geometric features, contain millions of electrons. This is a prominent fundamental problem with significant technological implications. MESOPLAS tackles it through 6 different and complementary lines (objectives):
O1. Development of a theoretical framework for mesoscopic plasmonics.
O2. General exploration of the quantum limit of plasmonic enhancement.
O3. Assessment of quantum effects in plasmon-assisted nonlinear phenomena.
O4. Refinement of the description of metal response in quantum plasmon-optics.
O5. Modelling hot electron generation in plasmonic nanoparticles.
O6. Design of setups for the experimental probing of quantum effects.

The advances along these 6 lines during the whole duration of MESOPLAS can be summarized as:
- Novel theoretical approaches were developed for the study of exciton-plasmon coupling (O1),
- A methodology to map classical and ab initio description of plasmonic rods was deployed (O1),
- Exciton-plasmon interactions in nanoparticle-on-a-mirror cavities were investigated (O1),
- The impact of light-forbidden transitions in plasmon-emitter coupling was revealed (O1),
- The limits of plasmon-assisted light-matter interactions at the nanoscale were clarified (O2),
- Spatial dispersion in metamaterials was exploited to achieve localization of spoof plasmons (O2),
- The violation on Planck´s law in far-field radiative heat transfer was proven (O2),
- The conditions yielding coherent exciton-plasmon coupling in gap cavities were established (O3),
- The electromagnetic boundary conditions for second harmonic generation were clarified (O3),
- The enhancement of photon correlations through plasmonic strong coupling was proven (O4),
- Quasichiral light-matter coupling in circularly polarized emitters was predicted (O4),
- Long-range exciton transfer between two different molecular species was demonstrated(O5),
- Three different collaborations with leading experimental groups were established (O6).

MESOPLAS has generated rise to 21 papers in high-impact journals, as well as 1 book chapter and 1 book. Another 8 scientific publications, closely related to MESOPLAS, were authored by the PI (fellow) during this period. Moreover, there are several works currently in preparation relevant for the project. These research advances have been presented in numerous international conferences and workshops, as well as seminars in prestigious institutions worldwide.