Longitudinal and transversal optical antenna plasmon resonance spectra from two-photon laser excitation

We record two-photon luminescence (TPL) emission spectra for two-arm nanoantennas of width 20 nm, height 30 nm and variable arm length of 25 to 65 nm, with a 20 nm gap between the arms, fabricated on an indium tin oxide (ITO) covered glass substrate using electron beam lithography and gold evaporation. Using a single excitation laser wavelength of 810 nm for all characterization experiments, the TPL emission is identified as a plasmon relaxation. To record the spectra, single structures are placed in the laser focus by making use of a raster scanning piezo stage. An oil immersion objective lens (100 ×, NA 1.46) is used both for the excitation and detection channel. A single-photon-counting avalanche photodiode detects the plasmon emission intensity, the response spectrum is observed with an electron multiplying CCD camera. The TPL nature of the excitation process is verified by recording emission versus excitation power curves [1].