Plasmon-enhanced proton acceleration by extended planar resonant nanoantennas

Using fully kinetic three-dimensional particle-in-cell (EPOCH) simulations, we investigate resonant planar nanoantenna configurations supporting localized surface plasmon resonances in hydrogen-rich media under irradiation by 795 nm femtosecond laser pulses with intensities of 10^{17}-10^{19} W/cm^{2}. The study compares higher-order resonant dipoles with coupled planar nanoantenna configurations, including double-dipole and double-Yagi structures. We show that extended resonant nanoantennas support higher-order longitudinal modes with multiple field hot spots, markedly longer plasmon lifetimes, and enhanced absorptivity compared with fundamental λ/2 dipoles. Close-pair 3/2 λ double dipoles sustain resonance for almost the entire pulse duration, yielding proton energies of approximately 0.28 MeV. Among the planar configurations investigated, double-Yagi nanoantennas provide the strongest near-field localization and the highest proton energies, exceeding 0.5 MeV. Coupled nanoantenna configurations sustain plasmon resonances for a substantially larger fraction of the laser pulse, even under strong irradiation, increasing proton energies by approximately two to three orders of magnitude compared with undoped media. At laser intensities above approximately 10^{18} W/cm^{2}, rapid electron depletion suppresses the plasmon resonance and limits further ion-energy gain, highlighting the importance of plasmon longevity. These results identify coupled planar resonant nanoantenna configurations, particularly double-Yagi structures, as efficient platforms for enhanced nonthermal laser energy coupling and localized proton acceleration in dense media, with potential relevance for future fusion-oriented target concepts.