Research Of Fano Resonances In All-dielectric Metamaterials

Metamaterials have attracted great attention because of their unique response to the electromagnetic wave,and have been a hot field in Physics,Chemistry,Biology and Medicine.The Fano resonance in metamaterials originates from the destructive interference between the superradiant bright mode and the subradiant dark mode.Because of the excitation of the dark mode,the radiation loss of the system is effectively suppressed,leading to high quality factor(Q-factor)resonance and large enhancement of the near field.Hence,the application performance of the Fano resonance is improved in the fields of biosensing,surface enhanced Raman scattering and nonlinear optics.So far,most of metamaterials are made up of metallic nanostructures with large ohmic loss.The Q-factor and spectral contrast ratio of Fano resonances are very low,and localized heating is induced.These disadvantages have severely impeded many applications of metallic metamaterials.The all-dielectric metamaterials made of high-refractive-index dielectric(Si,Ge,and Ga As)particles provide an effective platform to circumvent the issues,which are based on Mie multipole resonances.Due to their low intrinsic loss in visible and near-infrared wavelengths,Q-factor and spectral contrast ratio of the Fano resonance are very large.And the Fano resonance is easy to be modulated.The Fano resonances in all-dielectric metamaterials have a good potential application prospect in many fields.Therefore,the study of Fano resonance in all-dielectric metamaterials has attracted more and more attention in recent years.The main results are as follows:1.The Q-factor of Fano resonance is proportional to the net dipole moment of the unit cell in metamaterials.The effects of meta-atom interactions on the Fano resonance in all-dielectric metamaterials are investigated by introducing alternately flipped configurations.In alternately flipped asymmetric paired bars(APBs)and split asymmetric paired bars(SAPBs),the Q-factor of the Fano resonance is significantly enhanced up to one order of magnitude,and the electric field is strengthened by more than twice.Moreover,the Q-factor is markedly improved with the decrease of the period,because the destructive interaction among the neighboring dipole moments is enhanced.Abnormally,the Q-factor increases with gap size in the alternately flipped SAPBs due to the offset of the y-component of the electric dipole moment.Furthermore,the less the absorption loss is,the more the Q-factor of FRs can be enhanced.2.The excitation of multiple Fano resonances needs several dark subradiant modes supported in nanostructures.Generally,such nanostructures are relatively complex.And it is a great challenge to get an independent and precise tailoring of the multiple Fano resonances.An all-dielectric metamaterial composed of a single silicon disk with two asymmetric slots is proposed,which can support double high-Q Fano resonances originating from different mechanisms.One originates from the hybrid dark mode between the electric quadrupole of two slots and the magnetic dipole of the disk,the other the hybrid dark mode between the electric quadrupole and two inphase magnetic dipoles of the disk.The positions of both Fano resonances can be tailored independently or simultaneously by tuning the geometric parameters.Because the dark modes suppress the radiation loss,both Fano resonances show high Q-factor and large field enhancement.Our proposed metamaterial contributes to flexible design of optical devices with excellent performance.The surface-enhanced Raman scattering(SERS)enhancement factor of the double-resonance substrate is calculated to be up to 7.0×1015,which would allow single-molecule SERS detection.3.We demonstrate that high Q-factor Fano resonance and strong near-field enhancements around and inside of dielectric nanostructures are simultaneously realized in split nanocuboid arrays.The simulated scattering spectrum and the distribution of the electromagnetic field prove that the single silicon nanocuboid supports the anapole mode,but its Q-factor is low.By modulating the near field coupling between the nanoparticles in the nanocuboid array,the radiation loss is reduced,leading to the increase of the Q-factor of the anapole mode and the near field enhancement.But the electromagnetic field is mainly localized in dielectric particles.By introducing the splitting gap in the cuboid,the coupling of two coherent modes can be modulated.The high Q-factor Fano resonance is excited,and the quality factor is greatly increased with the increase of the gap width.The Q-factor is up to 4×106.Meanwhile,strong near-field enhancements around and inside of the split cuboids are realized.It shows excellent performance in refractive index sensing.