  We theoretically demonstrated and experimentally verified high-order radial
spoof localized surface plasmon resonances supported by textured metal
particles. Through an effective medium theory and exact numerical simulations,
we show the emergence of these geometrically-originated electromagnetic modes
at microwave frequencies. The occurrence of high-order radial spoof plasmon
resonances is experimentally verified in ultrathin disks. Their spectral and
near-field properties are characterized experimentally, showing an excellent
agreement with theoretical predictions. Our findings shed light into the nature
of spoof localized surface plasmons, and open the way to the design of
broadband plasmonic devices able to operate at very different frequency
regimes.
