Fig. 3: Anisotropic bright–dark contrast in a 3 μm Au disk antenna on sapphire.

a \({\theta }\)-dependent spectra of \({{S}}_{{\mathrm{{{bright}}}}}/{{S}}_{{\mathrm{{{dark}}}}}\). Spectral peaks emerge and disappear as \({\theta }\) rotates from \({0}^\circ\) to \({90}^\circ\). b A zoom-in comparison of \({{S}}_{{\mathrm{bright}}}/{{S}}_{{\mathrm{dark}}}\) at \({E}//{c}\) (red) and \({E}\perp {c}\) (blue) at different frequency ranges without vertical shift. c \({\theta }\)-dependent spectra of \({{\phi }}_{{\mathrm{bright}}}-{{\phi }}_{{\mathrm{dark}}}\). d A zoom-in comparison of \({{\phi }}_{{\mathrm{bright}}}-{{\phi }}_{{\mathrm{dark}}}\) at \({E}//{c}\) and \({E}\perp {c}\) at different frequency ranges without vertical shift. The black dashed lines in b and d indicate the LO phonon frequencies⁶⁸. For \({{E}}_{{u}}\) phonons (\({E}\perp {c}\)), \({{\omega }}_{{\mathrm{LO}}}={387.6},{481.7},\) and \({629}.{5}\,{\mathrm{cm}}^{-1}\). For \({{A}}_{{2u}}\) phonons (\({E}//{c}\)), \({{\omega }}_{{\mathrm{LO}}}={510.9}\,{\mathrm{cm}}^{-1}\). The spectra in a and c are vertically shifted for better display.