Abstract text (incl. figure legends and references)

Surface Plasmons (SPs) are collective charge oscillations in solids which arise at interfaces of media with opposite sign of the dielectric functions. By confining SP to plasmonic nanoparticles, nanoantennas, etc., the electromagnetic fields of the plasmon oscillations are localized both spatially and spectrally including a strong field amplification, which is exploited in various applications, such as Surface Enhanced Raman Spectroscopy or energy harvesting.

To further enhance localization and field amplification, disorder-induced localization (induced by Anderson localization mechanism) have been proposed and controversially discussed previously [1,2]. To further elucidate this localization mechanism, we study recently discovered ultrathin randomly disordered gold webs [3] (see lower right image in Fig. 1 b) by using Electron Energy Loss Spectroscopy (EELS) in combination with Scanning Transmission Electron Microscopy (STEM). This technique allows probing both, the spatial and spectral localization of the SPs at highest resolution (Δx~1nm and ΔE~60meV). The experimental data is compared to simulations employing Babinet"s principle equating holes of the web with thin oblate nanodiscs, which are mutually interacting through their dipole moments. This trick (i.e. simulation of inverse structure as coupled discrete dipoles) allows to handle large webs of several microns diameter, which are not accessible by conventional boundary element or FTDT methods. Within the discrete dipole model localization of resonant modes is quantified, among others, by so-called inverse participation number, which corresponds to the average number of active hotspots within resonant modes that are experimentally obtained by analyzing the number of hotspots within a narrow energy band.

Both, the experimental and simulated participation numbers decrease with increasing excitation energy until ~1.7eV. At this frequency, maximal spatial and spectral localization of dipole coupled modes is observable, leading to very high quality factors (strong field enhancement) close to the theoretical maximum. Beyond 1.7eV dipole coupled localized SPs modes are completely suppressed. Our simulations show that the threshold may be deliberately tuned by material and geometry parameters of the nets, which opens interesting prospects for strong light absorption and energy conversion by those ultrathin networks.

[1] V. A. Markel, J. Phys. Condens. Matter 18, (2006).

[2] S. Grésillon et al., Phys. Rev. Lett. 82, (1999).

[3] K. Hiekel et al., Angew. Chem. Int. Ed. 59, (2020).

[4] This project was funded by the European Research Council (ERC) under the Horizon 2020 research and innovation program of the European Union (grant agreement no. 715620) and the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany's Excellence Strategy through Würzburg‐Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter ‐ ct.qmat (EXC 2147, project‐id 390858490).

Fig. 1: a) EEL spectra collected at different spatial subsets, revealing numerous LSP modes. b) STEM image of the investigated map collected with the High-angle annular dark-field detector (lower right) and spatially resolved loss probability maps at different loss energies. The loss probability maps show increasing localization of the field hot spots with increasing energy.

Fig. 2: Experimental and simulated participation number, which reflects the increasing localization observable in Fig. 1.