Electromagnetic interactions of metallic reflection gratings with inner gap modification

In this work the electromagnetic interactions in metallic reflection gratings of subwavelength periods were investigated. The gratings were modified by inner air gaps introduced between the metal slats and conducting ground plane. The Fabry–Perot-like modes were excited within the grating slits (the cavities between adjacent metal slats), and depending on the gap width, the frequency locations as well as mode orders changed. Additional resonance modes in both evanescent and propagating types would be excited inside the introduced gap. They would interact with the fields inside the grating slits to form hybrid resonance modes in the gap-modified reflection gratings, i.e., mode coupling around the peripherals of the metal slabs. Such coupling interactions could produce a variety of modes and hybrid modes dependent on the sample’s dimensions and the incident wavelength. The unique electromagnetic functions of the gap-modified gratings were highlighted in the end for potential applications in devices.