Sub-30 nm thick plasmonic films and structures with ultralow loss.

We report an alternative method of producing sub-30 nm thick silver films and structures with ultralow loss using gas cluster ion beam irradiation (GCIB). We have direct evidence showing that scattering from grain boundaries and voids rather than surface roughness are the main mechanisms for the increase in loss with reducing thickness. Using GCIB irradiation, we demonstrate the ability to reduce these scattering effects simultaneously through nanoscale surface smoothing, increase in grain width and lower percolation threshold. Significant improvement in electrical and optical properties by up to 4 times is obtained, before deviation from bulk silver properties starts to occur at 12 nm. We show that this is an enabling technology that can be applied post fabrication to metallic films or lithographically patterned nanostructures for enhanced plasmonic performance, especially in the ultrathin regime.