Remarkable blue shift modulation of the photonic bandgap in all-dielectric photonic crystals induced by He⁺ ion irradiation

This study investigates the influence of He + ion irradiation on the optical spectra of dielectric monolayers and photonic bandgap (PBG) of all-dielectric photonic crystals (PCs) through theoretical and experimental approaches. The refractive indices of the dielectric monolayers increased gradually with increasing He + ion irradiation fluence while their physical thicknesses decreased. The optical thickness of the monolayer diminished as the reduction in physical thickness surpassed the increase in refractive indices. Additionally, the refractive index ratios of the two monolayers at the central wavelength of the PBG decreased initially and increased subsequently. Increasing He + ion irradiation fluence causes a significant blue shift in the PBG of PCs composed of two distinct dielectrics. This shift is attributed to the reduction in optical thickness, while the narrowing of the PBG is mainly connected to the drastic reduction of the central wavelength rather than the change of the refractive index ratios between the two dielectrics. Theoretical analysis and experimental results, using refractive indices and thicknesses of the dielectric monolayers obtained under identical irradiation conditions, were in strong agreement. This work holds significant implications for the precise modulation of PBGs and the development of ion irradiation-tunable optical devices.