Array truncation effects in infrared frequency selective surfaces

Abstract

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.

Spectral modification of array truncation effects in infrared frequency selective surfaces

“…As can be seen in the figure, the quasi-infinite array has an experimental resonance at 10.02 lm, while the unaltered 7 Â 7 arrays are resonant at 9.64 lm. This behavior of blue-shifting upon truncation is consistent with that which was observed in a previous study [12]. The 7 Â 7 arrays with the larger perimeter loops are seen to be resonant at 9.97 lm, while the 7 Â 7 arrays with a graded size variation are seen to be resonant at 9.93 lm.…”

“…It is noted that while the reported results are measured at the 60°design angle of incidence, it can be expected that these findings are applicable to other angles as well. This has been observed in both the experiments detailed here as well as a previous study [12], when comparing measurements at 60°off-normal to FTIR scans near normal incidence.…”

“…In the modified arrays, geometric alterations of certain unit cells were used to shift the peak wavelength of resonance. Due to a diminished mutual coupling between neighboring unit cell elements, the local effects of array truncation are most significant at the outermost elements in a finite array [12]. For this reason, the design modifications implemented here were concentrated toward the border elements of the 7 Â 7 arrays.…”

“…Most notably, a blue-shifting of the far-field resonant condition is observed due to reduced mutual coupling of the outermost elements in the truncated arrays [11,12]. The purpose of this current work is to compensate for the spectral shifting of the FSS resonance due to truncation.…”

“…Here, an infinite square-loop infrared FSS is designed with an absorptive resonance near 10 lm, when illuminated at 60°off-normal. The off-normal performance was of interest as an extension of a previous work, in which a near-field apparatus was used that required a 60°angle of illumination [12]. Geometric modifications are implemented in 7 Â 7 truncated arrays of the square-loop design in an attempt to mitigate the blue-shifting which occurs upon truncation.…”