A monopole antenna at optical frequencies: single-molecule near-field measurements

Abstract—We present a monopole antenna for optical frequen-cies ( 600 THz) and discuss near-field measurements with single fluorescent molecules as a technique to characterize such antennas. The similarities and differences between near-field antenna mea-surements at optical and radio frequencies are discussed in detail. Two typical antenna properties, polarization selectivity and reso-nances, are studied for the optical monopole by direct near-field measurements and finite integration technique calculations. The antenna is driven by the local field of a sub-wavelength aperture. This gives rise to a dependence of the antenna response on the ori-entation of the local field vector, in an analogous way to the polar-ization selectivity of linear wire antennas. The antenna resonances are studied by varying the antenna length. Typical monopole res-onances are demonstrated. The finite conductivity of metals at op-tical frequencies (in combination with the antenna radius) causes the wavelength of the surface charge density oscillation (surface plasmon polariton) along the antenna to be shortened in compar-ison to the free space wavelength. As a result, resonances for the optical monopole antenna occur at much shorter relative lengths than for conventional radio monopole antennas. Index Terms—Monopole antennas, near fields, optical antennas, optical resonance, plasmons, surface plasmon. I