Corner-Promoted Focus Enhancement of Light in Conical Holes for Extraordinary Optical Transmission

Extraordinary optical transmission (EOT) is generated when light transmits through an array of subwavelength holes on a metallic sheet. Most studies have focused on the EOT effect with ideal cylindrical holes. Subsequently, it has been recognized that imperfection in hole fabrication could alter light-matter interaction. Later, adiabatic taper is reported to promote light nano-focusing effect in the optical and near infrared wavelength regime. Due to geometrical and fabrication constraints, it is difficult to fabricate adiabatic taper for mid-infrared responsive nanohole array. In this work, we report the construction of nanohole arrays with varying near-adiabatic taper holes. We demonstrate that when the positive (inverted conic) angle increases from 10° to 20°, the peak full width half maximum (FWHM) improves by 14% from 800 to 700nm, respectively. Furthermore, inverted conical hole is shown to be superior to negative conic angle. Unlike cylindrical hole array, conical hole array maintains high transmission throughput for any given hole dimension while achieving better FWHM. The betterment in performance for conical over cylindrical holes is attributed to a localized plasmonic effect at the terminal exit. Subsequently, the device is demonstrated as a CO2 low flow sensor. The highest sensor sensitivity is determined to be between 1-4sccm.