Labyrinth Metasurface Absorber for Ultra‐High‐Sensitivity Terahertz Thin Film Sensing

Abstract

In this work, a labyrinth metasurface sensor operating at the low-frequency edge of the THz band is presented. Its intricate shape leads to a high electric field confinement on the surface of the structure, resulting in ultrasensitive performance, able to detect samples of the order of tens of nanometers at a wavelength of the order of millimeters (i.e., five orders of magnitude larger). The sensing capabilities of the labyrinth metasurface are evaluated numerically and experimentally by covering the metallic face with tin dioxide (SnO 2 ) thin films with thicknesses ranging from 24 to 345 nm. A redshift of the resonant frequency is observed as the analyte thickness increases, until reaching a thickness of 20 μm, where the response saturates. A maximum sensitivity of more than 800 and a figure of merit near 4500 nm À1 are achieved, allowing discriminating differences in the SnO 2 thickness of less than 25 nm, and improving previous works by a factor of 35. This result can open a new paradigm of ultrasensitive devices based on intricate metageometries overcoming the limitations of classical metasurface sensor designs based on periodic metaatoms.The THz band (ranging from 0.1 to 10 THz) has been historically known as the "THz gap" [1] due to the lack of efficient emitters and receivers operating in this regime at ambient temperature. However, a series of recent breakthroughs have helped to bridge this gap, to the point that nowadays it is a field of intense and multidisciplinary research with many applications coming into reality in numerous sectors, like medicine, security, communications, space, or sensing, among others. [2] Sensing at THz is of special relevance, because many substances exhibit molecular vibrations opening new routes for high performance detection platforms, allowing the identification of certain gases or solids that show absorption lines in this frequency range. Moreover, due to a larger wavelength and deeper penetration into many materials versus the visible or infrared ranges, THz waves are promising for examining optically opaque coatings. To date, several strategies have been followed to develop THz sensing devices such as plasmonic structures, [3] metamaterials, [4] and frequency selective surfaces, [5] among others.

Terahertz Sensing Based on Metasurfaces

“…[50] Copyright 2015, AIP Publishing. [57] for different analyte thicknesses (left). [52] (left); toroidal resonator used in ref.…”

“…c) Different types of asymmetric square SRRs used for exciting Fano resonances: SRR used in ref. [57] Copyright 2018, Wiley-VCH. [54] (center); SRR used in ref.…”

“…[57] by designing a labyrinth metasurface sensor working in absorption mode. A stronger interaction with the dielectric analyte and hence better performance can be possible by enhancing the electric field confinement in the surrounding of the metallic parts of the structure.…”

Multiresonant Split Ring Resonator with Meandered Strips

“…Fractal‐inspired cells were also considered for both a multiband operation and a bandwidth enhancement with application dedicated to ambient EM energy harvesting, nanoantenna, and EM wave absorbing, respectively. Other characteristic requirements can be achieved by such fractal structures, and a labyrinth metasurface showed a good angular robustness of the filtering behavior under oblique incidence . Finally, in the infrared wavelengths region, several resonances were experimentally evidenced in the reflection and transmission spectra measured for an array of SRRs and U‐shaped resonators .…”

“…Other characteristic requirements can be achieved by such fractal structures, and a labyrinth metasurface showed a good angular robustness of the filtering behavior under oblique incidence. [23] Finally, in the infrared wavelengths region, several resonances were experimentally evidenced in the reflection and transmission spectra measured for an array of SRRs [24] and U-shaped resonators. [25] Observed resonances were interpreted as a standing wave plasmonic resonance occurring when an integer number of half-wavelengths was equal to the length of the resonator.…”