Recently, improving the sensing performance of refractive index sensors by using of the weak far-field radiation and strong local field enhancement properties of toroidal dipole resonances has been intensively studied. Transmission/reflection spectra with significant narrow linewidth resonance has a vital effect on improving the sensing performance. However, narrower linewidth always leads to smaller modulation depth of the resonance which hinders the sensing performance to be improved for experiments. In this paper, we design an ultrathin all-dielectric asymmetric X-type metasurface array where an extremely narrow linewidth and high modulation depth of transmission resonance in the near-infrared has been demonstrated with Mie lattice resonance formed by the coupling of the toroidal dipole with Rayleigh Anomalous diffraction. With optimized structure parameters, a transmission dip with a full width at half-maximum as narrow as 0.061 nm and a modulation depth as high as 99.24% is achieved at the wavelength of 943.33 nm with a corresponding Q factor of 15464. According to the analysis of the displacement current distributions and the scattered powers in far field at the resonant and non-resonant wavelengths, it is confirmed that the narrow linewidth resonance is originated from the coupling of the toroidal dipole with Rayleigh Anomalous diffraction. A sensitivity and a figure of merit of 321 nm/RIU and 5262 RIU-1 are numerically demonstrated respectively for a refractive index sensor based on the all-dielectric asymmetric X-type metasurface array.
Metasurface refractive index sensor has become an important research direction in the field of optical biosensors owing to its capabilities of real time monitoring, compact device size and high sensitivity to the refractive index change of the external environment. During the last decades, the sensitivity of the sensors has been dramatically improved. Improving the figure of merit is another strategy to enhance the sensing performance. In this paper, we propose an asymmetric double semicircular all dielectric metasurface structure. With the interference between excited toroidal dipole and Rayleigh anomaly in the metasurface array, ultra-high figure of merit has been achieved. According to the simulation results, with optimized structure parameters, a Q-factor of 14559 has been obtained with a full width at half maximum (FWHM) of the resonance of 0.06 nm, while the sensitivity is 320 nm/RIU and the figure of merit is 5333 RIU -1 .
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