Higher-Order Plasmonic Dipole Antennas for Enhanced Proton Acceleration

Laser–driven proton acceleration in hydrogen-rich media can be strongly modified by embedded plasmonic nanoantennas that localize electromagnetic energy on nanometer scales. Recent experiments prove that resonant nanoplasmonic dipoles parallel to the laser beam polarization increase the number of accelerated protons by an order of magnitude. Using kinetic three-dimensional particle-in-cell EPOCH simulations, we investigate nanoantennas operating at higher-order resonant dipole lengths (λ/2, 3/2λ, 5/2λ), under irradiation by 795-nm high intensity femtosecond laser pulses. We numerically study the effect of real laser pulses on hydrogen-rich matter embedded by higher-order dipoles and compare localized energy absorption and acceleration of protons. We show that elongated, multiple length dipoles sustain longer plasmon lifetimes and yield a higher number of energetic protons compared to λ/2 antennas. Prepulse simulations indicate that pre-plasma formation preserves the antenna core while forming a diffuse electron halo, indicating robustness under realistic conditions. The results identify higher-order dipoles and their close pairs as simple and scalable structures for enhanced laser energy coupling and localized proton acceleration in dense media.