Metasurface studies have demonstrated vast applications to control optical properties of light based on the ability to design unit cells with desired phase and reflectivity in 2D subwavelength periodic arrays. The simplified design strategy is only an approximation since the unit cells can be subject to near-field coupling effects due to influence from neighbor unit cells. In this work, we try to investigate this effect by numerically and experimentally studying the near-field response from gold nanobricks of varied length, fabricated in both quasi-periodic and periodic configuration on top of dielectric-coated (SiO2) layer and gold layer at telecommunication wavelength (1500 nm), which is the commonly used gap plasmon configuration for efficient metasurfaces. The experimental near-field investigation is performed using a phase-resolved scattering-type scanning near-field optical microscopy (s-SNOM) in the transmission mode. We demonstrate that near-field coupling becomes significant when edge-to-edge separation between GSP elements goes below ~200-250 nm. We also show that the reflection phase of any GSP element is approximately equal to its doubled near-field phase. Thus, our studies provide a direct explanation of a reduced performance of a densely-packed GSP metasurfaces. This technique can accurately predict the performance of different types of metasurfaces by observing their near-field response in different periodic configurations by considering factors ignored in the design stage, which include fabrication uncertainties, wrong design considerations along with near-field coupling effects.
Plasmonic terahertz (THz) resonators provide a promising route for exploring strong light-matter coupling phenomena. Double-metal resonator designs in particular enable strong enhancement of the THz field and provide well-defined field orientation and confinement within a sub-wavelength size volume. The strong field confinement however limits access to the internal fields essential for investigations of light-matter coupling. We propose and investigate a method for mapping and spectroscopic analysis of the internal fields in double-metal plasmonic THz resonators. We use aperture-type scanning near-field THz microscopy to access strongly confined fields with sub-wavelength spatial resolution of ~5 μm (~λ/100). Combined with the THz time-domain spectroscopy technique, the near-field method allows us to perform spectroscopic studies and investigate the field evolution inside the resonator. This experimental method opens doors to studies of strong light-matter coupling at THz frequencies in individual plasmonic resonators.