Evidence of Spatially Inhomogeneous Electron Temperature in a Resonantly Excited Array of Bow-Tie Nanoantennas

Abstract

We studied the excitation of large-area Au bow-tie nanoantenna arrays, which we have fabricated on indium-tin-oxide (ITO)-coated glass substrates using colloidal lithography with nanoscale polystyrene colloidal particles. Ultrashort (100 fs, 800 nm) laser pulses of a Ti-Sapphire laser resonantly excite electron emission from a few tens of nanometer wide-gap regions of the array. We investigated the near-field enhanced photoemission using time-of-flight momentum microscopy. The variation of the electron emission intensity as a function of kinetic energy, parallel momentum, power density, and polarization of the laser beam reveals two distinct emission mechanisms: a coherent multiphoton photoemission process from the optically heated electron gas and a field emission process resulting from the optical near-field enhancement at the nanoantenna tips. The analysis of the momentum-resolved kinetic energy spectra indicates a spatially inhomogeneous distribution of the electron gas temperature within the bow-tie resonators.