Fig. 1

a Schematic of a single unit cell of the gratings. The geometrical parameters are h₁ = 1 μm, h₂ = 225 nm, h₃ = 275 nm and d = 300 nm; b Simulated reflection spectra of the grating with ff = 0.47. The red and black curves present the cases for TM and TE polarizations, respectively; c Effect of ff on the ‘bluest’ reflection minimum of the gratings in the case of TM polarization