Enhanced Plasma Generation from Metal Nanostructures via Photoexcited Hot Electrons

We report hot electron-enhanced plasma generation by irradiating metal nanostructures with laser light. Here, a high-voltage nanosecond pulse is discharged across two electrodes interspersed with metal nanoparticles (e.g., Au and Pt) both with and without laser excitation. With laser excitation (532 nm in wavelength), we observe a 200-fold increase in the plasma emission intensity (i.e., plasma density) and a lower threshold for the onset of plasma discharge (i.e., lower voltage). This enhancement of plasma emission/discharge occurs for two reasons. First, the hot electrons photoexcited in these metals lower the effective work function that needs to be overcome for thermionic emission, thus initiating the plasma. Second, the metal nanostructures minimize the average distance the photoexcited carriers (i.e., hot electrons) have to travel to reach the surface. As such, the photoexcited hot carriers within the metal nanostructures can easily reach the surface before relaxing back to equilibrium. While these metal nanostructures have been shown to be strongly plasmonic (e.g., Au nanoparticles), we believe that the plasmon resonance does not play an important role in this plasma emission process. Plasma emission under 633 and 785 nm laser wavelength irradiation was also tested, but no enhancement in plasma emission was observed. We attribute this to the low photon energy (i.e., 1.9 eV), which lies below the threshold for interband transitions in Au and Pt.