Ohmic absorption of light is an indication of a light-matter interaction within metals, where many interesting phenomena and application potentials can be found. To realize the ohmic absorption of light at long wavelengths, where metals are highly reflective, one can use a metamaterial absorber design to concentrate the electromagnetic field within a thin metal film. This concept has enabled thinning of perfect absorbers from a quarter-wave thickness to several tens of nanometers, greatly improving the utility and efficiency of light-metal interactions. Further improvements on the performance are expected if the absorption can be additionally focused laterally, which is a possibility not yet explored. In this study, we report that nanoslot antennas can be a unique ohmic absorber of the low-frequency radiations, where it can incorporate 70% of incident light to ohmic absorption, focused laterally onto 1% of the unit cell area. The inductive field that drives both field enhancement and ohmic absorption is localized within a skin depth distance from the slots with amplitude being as large as 30% of the incident field. Mode-matching calculations and terahertz spectroscopy measurements confirm the inductive and localized nature of the absorption. The strong confinement of the inductive field and of the resulting ohmic absorption is expected to open a new venue in nanocalorimetry, optical nonlinearities of metals, and bolometer applications.
Keywords: Metamaterial absorber; nanoantenna; ohmic absorption; terahertz.