Fig. 1: Ultrastrong coupling in the WS2 monolayer with a random multi-singular plasmonic metasurface.
a Schematic of a gold multi-singular metasurface with a dense array of nanometre-sized plasmonic gaps. Insets: left: schematic of a WS2 monolayer integrated with the multi-singular metasurface where the WS2 monolayer was mechanically exfoliated onto a PDMS tape and transferred onto the gold nanopattern using a dry transfer method; middle and right: two main paths for generating nanometre-sized gaps. Middle: the first path, a nanometre-sized crack generates in the fragment with saturated transferred stresses. Right: the second path, adjacent fragment domains are dragged infinitely close together (at nanometre-sized spacings) by biaxial mechanical loadings. b, c Transmission electron microscopy (TEM) images of a 20 nm-thick gold multi-singular metasurface showing a dense array of nanometre-sized plasmonic gaps. The darker grey areas correspond to gold, and the lighter ones to air. b refers to the first path, and (c) to the second. d Average number (\(\bar{N}\)) of nanometre-sized (~sub-3 nm in the scanning electron microscopy image) plasmonic gaps per \(1 {{{{{{\rm{\mu m}}}}}}}^{2}\) in the 20 nm-thick metasurface as a function of the second (2nd) elongation. Error bars are standard errors from multiple samples. e Scanning electron microscopy images of the 20 nm-thick metasurface at 80% 2nd elongation. Note that despite the random distribution and orientation of the singularities, the fabricated multi-singular metasurface retains ‘global uniformity’, which refers to the uniform distribution of singularities within the laser beam area or over the entire patterned gold film. This global uniformity can be precisely controlled during the cold-etching process (see Supplementary Fig. 6 for details). The white box corresponds to the simulation area in (f). f The simulated near field of the gap plasmons. g Dark-field scattering spectra of the WS2 monolayer on polymer (thermoplastic polyurethane, black) and the plasmonic metasurface uncoupled (green) and coupled (blue for SC and purple for USC) to WS2 excitons in arbitrary units (arb. units).
