Near-field investigation of the effect of the array edge on the resonance of loop frequency selective surface elements at mid-infrared wavelengths

Mid-infrared scattering scanning near-field optical microscopy, in combination with far-field infrared spectroscopy, and simulations, was employed to investigate the effect of mutual-element coupling towards the edge of arrays of loop elements acting as frequency selective surfaces (FSSs). Two different square loop arrays on ZnS over a ground plane, resonant at 10.3 µm, were investigated. One array had elements that were closely spaced while the other array had elements with greater inter-element spacing. In addition to the dipolar resonance, we observed a new emergent resonance associated with the edge of the closely-spaced array as a finite size effect, due to the broken translational invariance.

Array truncation effects in infrared frequency selective surfaces

A metasurface consisting of an infinite array of square loops was designed for maximal absorptivity for s-polarized light at a wavelength of 10.6 µm and 60 degrees off-normal. We investigate the effects of array truncation in finite arrays of this design using far-field FTIR spectroscopy and scattering scanning near-field optical microscopy. The far-field spectra are observed to blue-shift with decreasing array size. The near-field images show a corresponding decrease in uniformity of the local electric field amplitude and phase spatial distributions. Simulations of the far-field absorption spectra and local electric field are consistent with the measured results.

Near- and far-field spectroscopic imaging investigation of resonant square-loop infrared metasurfaces

Optical metamaterials have unique properties which result from geometric confinement of the optical conductivity. We developed a series of infrared metasurfaces based on an array of metallic square loop antennas. The far-field absorption spectrum can be designed with resonances across the infrared by scaling the geometric dimensions. We measure the amplitude and phase of the resonant mode as standing wave patterns within the square loops using scattering-scanning near-field optical microscopy (s-SNOM). Further, using a broad-band synchrotron-based FTIR microscope and s-SNOM at the Advanced Light Source, we are able to correlate far-field spectra to near-field modes of the metasurface as the resonance is tuned between samples. The results highlight the importance of multi-modal imaging for the design and characterization of optical metamaterials.