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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - TOPONANOP (Topological nano-photonics)

Teaser

Summary

TOPONANOPâ€™s vision is to exploit the extraordinary topological properties of novel quantum materials in order to control light at the nanoscale in a radically new way. One of the main objectives is to generate non-reciprocal nanoscale optical fields (plasmons) that propagate in only one direction and implement topologically protected plasmons such that they move around defects and corners. At the same time, visualizing and controlling electromagnetic excitations will be used as a tool to unravel extraordinary phenomena in exotic quantum materials. Topological nano-photonics is a new paradigm for novel quantum materials and will enable novel future applications in miniaturized photonic isolators, diodes and logic circuits and could lead to completely new concepts for communication systems, optical transistors and optical information processing.

Work performed

Theoretical studies on the polaritons of Weyl semimetals:
The surface of a Weyl semimetal (WSM) displays Fermi arcs, i.e., disjoint segments of a two-dimensional Fermi contour. We presented a quantum-mechanical nonlocal theory of chiral Fermi arc plasmons in WSMs with broken time-reversal symmetry. These are collective excitations constructed from topological Fermi arc and bulk electron states and arising from electron-electron interactions, which are treated in the realm of the random phase approximation. Our theory includes quantum effects associated with the penetration of the Fermi arc surface states into the bulk and dissipation, which is intrinsically nonlocal in nature and arises from decay processes mainly involving bulk electron-hole pair excitations.
Hydrodynamic plasmons:
Hydrodynamic ow in two-dimensional electron systems has so far been probed only by dc transport and scanning gate microscopy measurements. In this work we discuss theoretically signatures of the hydrodynamic regime in near-eld optical microscopy. We analyze the dispersion of acoustic plasmon modes in two-dimensional electron liquids using a nonlocal conductivity that takes into account the effects of (momentumconserving) electron-electron collisions, (momentum-relaxing) electron-phonon and electron-impurity collisions, and many-body interactions beyond the celebrated random phase approximation. We derive the dispersion and, most importantly, the damping of acoustic plasmon modes and their coupling to a near-eld probe, identifying key experimental signatures of the crossover between collisionless and hydrodynamic regimes.
Hypercrystals in indirectly patterned hexagonal boron nitride

Atomically thin hexagonal boron nitride supports hyperbolic phonon polaritons, uncovnetional EM modes in the mid-IR, with a wavelength that can be orders of magnitude smaller than the vacuum wavelength. We employ a novel technique to indirectly pattern these materials (without damaging them) and form lattices with ~100nm periodicities. We experimentally observe, for the first time, the formation and modification of the bandstructure due to the nanoscale lattice, including the formation of unconventional defect states and bandgap-like features.
Valley Hall effect in gapped bilayer graphene
By excitation of gapped bilayer graphene with infrared light with circular polarization, we observed a Hall voltage, attributed to the Valley Hall effect. Detailed studies on the gap and temperature have been performed.

Final results

The first theoretical studies on non-local plasmons in Weyl semimetals and hydrodynamic plasmons have been performed. The first hypercrystals have been created and measured. The first optical Valley Hall measurements were performed on a low-gap 2D system.