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 Babinetʼs 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.

Optical nanoantennas have been studied as a means to manipulate nanoscale fields, local field enhancements, radiative rates, and emissive directional control. However, a fundamental function of antennas, the transfer of power between a coupled load and far-field radiation, has seen limited development in optical antennas owing largely to the inherent challenges of extracting impedance parameters from fabricated designs. As the transitional element between radiating fields and loads, the impedance is the requisite information for describing, and designing optimally, both emissive (transmitting) and absorptive (receiving) nanoantennas. Here we present the first measurement of an optical nanoantenna input impedance, demonstrating impedance multiplication in folded dipoles at infrared frequencies. This quantification of optical antenna impedance provides the long sought enabling step for a systematic approach to improve collection efficiencies and control of the overall antenna response.

Sub-diffraction limited waveguides have been studied as a means to manipulate light into nanoscale regions. Hybrid waveguides are popular candidates in optical regimes for subwavelength confinement and long range propagation. However, advances in the mid-IR are lacking due to high propagation losses and limited confinement. Here we present the first analysis of hybrid phononic waveguide using a hyperbolic material h-BN to generate surface phonon polaritons. The strong coupling between the photonic cylinder and phononic surface enhances the confined field up to 10^-3 λ_o ² (λ_o is free-space wavelength) and enables propagation distances up to 100 λ_o. Our work is fully compatible with integrated polaritonic devices in the mid-IR and provides a systematic approach to design hybrid phononic waveguides.

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