Colorimetric and Near-Absolute Polarization-Insensitive Refractive-Index Sensing in All-Dielectric Guided-Mode Resonance Based Metasurface

Colorimetric detection
of target molecules with insensitivity to
incident-light polarization has attracted considerable attention in
recent years. This resulted from the ability to provide rapid output
and reduced assay times as a result of color changes upon altering
the environment that are easily distinguishable by the naked eye.
In this paper, we propose a highly sensitive refractive-index sensor,
utilizing the excitation of guided modes of a novel two-dimensional
periodically modulated dielectric grating-waveguide structure. The
optimized nanosensor can numerically excite guided-mode resonances
with an ultranarrow linewidth (full width at half-maximum) of 0.58
nm. Sensitivity is numerically investigated by considering the deposition
of dielectric layers on the structure. For a layer thickness of 30
nm, the maximum sensitivity reached as high as 110 nm/refractive index
unit (RIU), resulting in a very high figure of merit of 190. The fabricated
devices with 30 nm aluminum oxide and zinc oxide coatings achieved
a maximum sensitivity of 235.2 nm/RIU with a linewidth of 19 nm. Colorimetric
detection with polarization insensitivity is confirmed practically
by a simple optical microscope. Samples with different coatings have
been observed to have clearly distinct colors, while the color of
each sample is nearly identical upon azimuthal rotation. Excellent
agreement is obtained between the numerical and experimental results
regarding the spectral position of the resonances and sensitivity.
The proposed device is, therefore, highly promising in efficient,
highly sensitive, almost lossless, and compact molecular diagnostics
in the field of biomedicine with personalized, label-free, early point-of-care
diagnosis and field analysis, drug detection, and environmental monitoring.