Plasmon Dynamics Under High Intensity Illumination

Surface plasmons have enabled great control over nanoscale electromagnetic fields. However, their behaviour in high intensity regimes is still not well explored. Plasmonic devices are usually simulated using linear models. While these models are effective at predicting fields at lower intensity, they are insufficient at higher intensities. When the field is comparable to inter-atomic binding forces and complex electron motion leads to nonlinear effects and a more detailed description is required. This limitation of commonly used simulation tools leads to open questions about the posibility of plasmon excitation and their dynamics in this regime.

In this work we study the excitation of plasmonic modes in a metallic (Au) nanostructure in the regime of extreme intensities (10¹² - 10¹⁶ W/cm²) using numerical particle in cell simulations. With this approach we trace motion of individual charged particles – electrons and ions - which includes the complex nonlinear behaviour. We treat the metal as a collection of free electrons around mostly stationary ions. As we apply an intense laser pulse (10 fs duration) the electrons begin to move creating density perturbations.

For all intensities we observe excitation of surface plasmon modes. We then study the dynamics of these plasmonic excitations and the effects they have on the system. First we examine how they impact direct atomic ionization inside the structure. Our simulations show higher ionization states appearing at plasmonic field intensity maxima. This leads us to the conclusion plasmon fields can substantially influence when and where ionization happens. At even higher intensities we observe significant number of electrons spilling out of the structure due to the strong electromagnetic field. These electrons respond to the surrounding electric fields produced by the mode leading to a complex evolution of the electron density. Our results show that even though the field and electron density configuration are modified at high intensities, plasmons can be excited and can influence the overall dynamics up-to intensities reaching 10¹⁶ W/cm².

Our results offer an exciting pathway towards utilizing the versatility of plasmonic modes at extremely high intensity. This capability has the potential to achieve a higher degree of control over high energy laser pulses as well as direct high field effects like ionization and particle acceleration.