Tunable color switch and sensor using a cross-shaped metasurface

Abstract

An innovative tunable colour switch and sensor based on a cross-shaped metasurface (CSM) on a polydimethylsiloxane (PDMS) substrate is created. This structure comprises periodic arrays, each unit consisting of a cross-shaped hole etched on a silver (Ag) layer topped by a suspended cross-shaped Ag cover layer. The three-dimensional finite-difference time-domain (3D FDTD) simulation results show that by tuning the geometric parameters, the CSM enables tunable optical responses, including the period (P) of the CSM array, and the separation (h) between the etched hole layer and the suspended cover layer. By increasing P at the simulation level, the resonances of the CSM show a redshift in the wavelength range from 545 nm to 758 nm, spanning most of the visible spectrum. By tuning h, the CSM enables the real-time adjustment of reflection intensity, while the resonance remains nearly unchanged. The reflection intensity can be changed from 3.60% to 86.37%. In addition, by changing the thickness (t) of the Ag layer, the quality factor (Q-factor) and the full width at half maximum (FWHM) of the response can be adjusted. Through intentional selection of t = 50 nm, the CSM depresses the high-order mode while delivering a high optical performance (FWHM = 8.98, Q-factor = 78.15). Moreover, the CSM exhibited a high sensitivity that is dependent upon the refractive index (n), and exhibited three resonances that can be applied to high-precision dynamic detection for environmental sensing. The highest sensitivity and figure of merit (FOM) are 585.86 nm RIU⁻¹ and 72.58 RIU⁻¹, respectively, for n increasing from 1.0 to 2.0. These results indicate that there is significant potential for the CSM to be used in dynamic color displays and biomedical monitoring applications.