Near-field driving of a optical monopole antenna
Nanosized optical antennas have the potential to confine and enhance optical electromagnetic fields, making nano-antennas essential tools for applications in integrated nano-optical devices and high-resolution microscopy. The size, shape and material of the nano-antenna, together with the optical frequency, determine the antenna response and its resonances. Here, we discuss a ?/4 long optical nano-antenna, analogous to the radio frequency monopole antenna. The antenna is fabricated at the end of a near-field aperture-type fibre probe by focused-ion-beam milling in two sequential steps. Illumination through the fibre creates a localized evanescent excitation source, with the advantage of a lower background compared to 'apertureless' techniques, which require far-field excitation. Previously, we have studied the field localization, antenna excitation conditions and antenna resonances, both in experiment, by near-field single-molecule detection experiments, and in theory, by finite integration technique simulations. In this study we investigate the importance of both polarization conditions and antenna position in creating an efficient local driving field for the monopole antenna. It is shown that the antenna is driven by the field component along the antenna axis. Next we show the advantage of the antenna over the aperture: upon reduction of the diameter the antenna gains local field intensity, while the aperture field decreases rapidly. Finally, the highly localized field near the antenna apex is probed by single molecules and detected molecular emission features below 30 nm FWHM are presented.