Accessing the Optical Magnetic Near-Field through Babinet's Principle

Engineering the optical magnetic field with optical antennas or metamaterials extends the ways to control light-matter interaction. The slot antenna, as the electromagnetic dual of the linear rod antenna, provides the simplest form of a magnetic resonator tunable through its length. Using combined far-and near-field spectroscopy and imaging, and theory, we identify magnetic dipole and higher order bright and dark magnetic resonances at mid-infrared frequencies. From resonant length scaling and spatial near-field distribution, we confirm the applicability of Babinet's principle over the mid-infrared spectral region. Babinet's principle thus provides access to spatial and spectral magnetic field properties, leading to the targeted design of magnetic optical antennas.