Wideband Optical Filters with Small Gap Coupled Subwavelength Metal Structures

TL;DR

This paper demonstrates that small gap coupling in subwavelength metal structures significantly enhances the bandwidth of optical band-stop filters through plasmonic near-field interactions, with modeling limitations at very small gaps.

Contribution

It introduces a transmission line model to analyze bandwidth enhancement in subwavelength metal structures and explores the effects of gap size on spectral properties.

Findings

Bandwidth increases with smaller gaps due to near-field coupling.

The transmission line model accurately predicts spectra for larger gaps.

Model limitations occur when gap size approaches near-field decay length.

Abstract

In this letter, we show that the bandwidth of optical band-stop filters made of subwavelength metal structures can be significantly increased by the strong plasmonic near-field coupling through the corners of the periodic metal squares. The effect of small gap coupling on the spectral bandwidth is investigated by varying the gap size between the metal squares. An equivalent transmission line model is used to fit the transmission and reflection spectra of the metal filters. The transmission line model can characterize well the metal structures with the gap size larger than the near-field decay length. However, it fails to model the transmission and reflection spectra when the gap size reaches the decay range of the near-field in the small gaps.