Scaled-down MH-VSRRs with Fano resonances in the NIR and ultra-sensitive refractive index-sensing properties. (a) SEM images of the fabricated structures with parameters of h = 560 nm, l = 430 nm, w = 150 nm, ax = ay = 560 nm and px = py = 750 nm. Inset, the top view of an MH-VSRR. Scale bar: 500 nm. (b) Measured transmission spectra of the MH-VSRRs in air and oil with a refractive index of 1.300 ± 0.0002, respectively. Spectra are smoothed for clarity. The low-order and high-order Fano resonance dips exhibit spectral shifts with refractive index sensitivities of 2040 and 1400 nm RIU−1, respectively. (c) The measured transmission spectra of the MH-VSRRs in oil with refractive indices of 1.300 and 1.320, respectively. (d) The wavelengths of the low-order and high-order Fano resonance dips as a function of the refractive index of the surrounding medium. The solid lines are linear fits of the data; the corresponding slopes are indicated.