Robust all-dielectric high Q-factor metasurface for sensing

. All-dielectric metasurfaces have seen a recent surge of interest as an alternative to plasmonic devices, due to low losses and desirable optical properties. High Q-factor quasi-bound state in the continuum resonances can be manufactured and manipulated via designed asymmetry in the nanostructures. The presented metasurface design, based on a slotted disk nanostructure, produces strong E-Field enhancement with good surface coverage external to the structure. The design transition from structure-in-air to structure-on-substrate in a water-based sensing medium is presented, along with the robust tunability and multiplexing potential of our fabricated resonances. Our structure maintains a high Q-factor and refractive index sensitivity over a wide wavelength range in the visible and near-IR.


Introduction
Plasmonic resonance-based sensors suffer from high ohmic losses which limits the quality factor of the resonances and impedes multiplexing (multiple resonance markers on the same spectrum) [1].These ohmic losses can also cause localised heating which can be a particular issue when used in biosensing, where the excess heat may damage or denature the analyte under detection.
All-dielectric nanostructures can be designed for perfect absorption, invisibility, directional emission, and high field enhancement; however, the regions of highest field enhancement are usually trapped inside the structure and inaccessible for sensing purposes [2].
The discovery of quasi-bound state in the continuum (qBIC) resonances in dielectric metasurfaces presents interesting possibilities for sensing applications.qBICs are sharp Fano resonant states with long optical lifetimes, manufactured and controlled by fabricating a small degree of asymmetry in the design of a nanoresonator [3].
A novel design for an all-dielectric nanoresonator [4] with qBIC resonances has been adapted to a Si3N4 platform, transferred onto a fused silica substrate and submerged in water as the sensing buffer medium.This results in a practical design which produces high Q factor and field enhancement external to the structure.
In this paper we confirm that the new practical design on a substrate maintains the qBIC resonance with minimal change in Q factor and E-field enhancement.

Design transfer
All design processes were performed with Lumerical FDTD simulations with an x-polarized light source.The Si3N4 metasurface is based on a slotted nanodisk geometry shown in figure 1, transferred onto a fused silica substrate, surrounded by a buffer sensing medium of water (RI = 1.333).The asymmetry factor,  = 1 − ( 1  0 ) ⁄ , is adjusted by asymmetrically changing the size of the gap, G, between the disk hemispheres, where  0 and  1 are the larger and smaller hemisphere areas, respectively.The smaller the asymmetry, the narrower the full-width half-max (FWHM) of the qBIC resonance, and therefore the larger the Q-factor.

qBIC robustness
Water was chosen as a buffer because it is an appropriate and easy to work with medium for sensing applications.In addition, the smaller refractive index difference between water and fused silica, as opposed to air and fused silica, facilitates an easier transition of the design to a substrate without compromising the qBIC resonance.
Figure 2 shows a comparison of the E-field mode and enhancement map for the original design without a substrate in air, versus the design transferred onto the substrate in water, both with a gap of 45 nm but with increased radius in the case with substrate.The Q-factors of the qBIC resonances remain equivalent (>10 5 ) and with only a small reduction of the E-field enhancement (~10% less).The resonance wavelength is also shifted from ~600 nm without substrate to ~750 nm with substrate.
The primary difference, however, is in the E-field mode.The original design has external hotspots confined to the gap due to the strong toroidal mode which becomes reduced and more laminar when the substrate is present.This mode change nevertheless produces considerable Efield enhancement external to the disk which can be well exploited for sensing purposes.The resonances of the qBIC metasurface on substrate were tested in the same fashion as is described in [5], and confirmed that they are indeed still qBIC in nature i.e.Q factor ∝  −2 [6].A bulk refractive index sweep was carried out for a refractive index range of 1.333 -1.38, in the same manner as [7], giving a sensitivity of 135 nm RIU -1 .

Geometrical tuning
The tunability of the qBIC resonance wavelength and FWHM were explored.The geometric variables are the disk radius, disk height, disk periodicity, and gap width.Figure 3 shows optical spectra for a structure with a target resonance wavelength of 650 nm and FWHM of 2 nm.
In this instance the disk height remains fixed at 100 nm, therefore the design reduces to a 2D geometry of radius, periodicity, and gap width, allowing for fabrication of multiple arrays on the same surface, each with a unique resonance wavelength and FWHM.This is very advantageous for multiplexing in sensing.With a large FWHM, the Q-Factor and E-field enhancement are much reduced, however these can be increased dramatically simply by reducing the gap width.

Fig. 1 .
Fig. 1.Si3N4 slotted disk array on fused silica substrate.(Inset) xy-view of individual slotted disk.y-min remains fixed while y-max is adjusted to alter A1 and thus the asymmetry.

Fig. 2 .
Fig. 2. E-field mode and vector maps for the qBIC resonances of the design in air (left), versus the design with substrate in water (right).