van der Waals Nanogap Loading of a \ce{TiO2} Nanobar Metasurface for Dual-Channel Fano Refractive Index Sensing

Abstract

Filling the gap of a \ce{TiO2} nanobar-pair metasurface with $α$-\ce{MoO3}, a biaxial orthorhombic crystal, produces two high-$Q$ Fano resonances with unequal quality factors: $Q_\mathrm{TE} = 87$ at 863.3~nm and $Q_\mathrm{TM} = 31$ at 960.1~nm, separated by 97~nm. The same device with amorphous \ce{Sb2S3}, which is isotropic, gives $Q_\mathrm{TE}=66$ and $Q_\mathrm{TM}=20$, a ratio of 3.35 against 2.85 for $α$-\ce{MoO3}, so the inequality of the two quality factors is not specific to the biaxial crystal and originates instead in the in-plane anisotropy of the resonator geometry. An unequal pair of quality factors is therefore not by itself evidence of material anisotropy. What the biaxial fill controls at fixed geometry is the partitioning of the response between the two channels: each polarization samples a different principal index, so the two resonance positions, the two linewidths and the ratio of the two sensitivities respond to a single material choice in different proportions. The TE channel delivers sensitivity $S=155.3$~nm~RIU$^{-1}$, figure of merit $15.71$~RIU$^{-1}$, and an ideal refractive index resolution of $6.44\times10^{-5}$~RIU; TM delivers $S=139.1$~nm~RIU$^{-1}$, $4.44$~RIU$^{-1}$, and $7.19\times10^{-5}$~RIU. Simultaneous readout produces a fixed polarization response with isotropic slope 0.896, which the gap material changes: the same geometry filled with amorphous \ce{Sb2S3} gives 0.620. All results reported here are numerical; no device was fabricated and no measurement was performed.