Fiber-Based Optical Nanoantennas for Single-Molecule Imaging and Sensing

We present the use of fiber-based resonant dipole and nanogap optical nanoantennas for extreme resolution optical microscopy. A typical optical dipole antenna is only $\sim$ 100-nm long, as the wavelength of light is typically a million times smaller than for radio waves. We show how by focused-ion-beam milling of metal-coated tapered optical fibers we overcome the challenge of fabricating resonant antennas at such small-length scales, with an accuracy of $\sim$ 10 nm. The optical fiber provides an ideal interface between the macro- and nanoscales, allowing the manipulation of such a tiny nanoantenna with nanometer precision relative to a sample surface. Imaging single fluorescent molecules and nanobeads, we achieve an optical resolution down to 40 nm (FWHM), far below the Abbe diffraction limit. The strongly localized antenna field results in an enhancement of fluorescence up to 100  $\times$ , while the vectorial nature of the local antenna field allows access to molecules of all orientations. Clearly, dedicated nanofabrication of fiber-based scanning optical antennas is a promising route to push the limits of optical nanoscopy.