Research And Applications Of Nonradiating Optical Modes In Resonant Metasurfaces

Nonradiating electromagnetic sources were initially proposed in quantum mechanics and astrophysics,and they did not receive much attention in the photonics community for a long time due to the lack of proper research systems.With the rapid development of metaphotonics over the last 20 years,metamaterials and metasurfaces have shown great ability to control electromagnetic waves.Also,with the proposal of a number of pioneering theories,nonradiating optical states have been discovered to possess a huge potential in trapping light,enhancing local fields,tuning far-field scattering,and manipulating nonlinear effects.Metasurfaces,especially the resonant metasurfaces with enhanced light-matter interactions,provide an ideal platform for the study of these exotic optical modes.The nonradiating optical modes in resonant metasurfaces have been used for the applications of laser,sensing,and nonlinear enhancement.This dissertation focuses on two major nonradiating optical modes in resonant metasurfaces:Anapole mode and bound states in the continuum(BICs).The physical mechanism of their origin,active and passive tuning methods,and the applications in terahertz modulators and photodetectors of these two modes are theoretically studied and experimentally demonstrated.The major results are summarized below:1.Theoretically designed and experimentally demonstrated an actively tunable Anapole metasurface.When the metasurface is illuminated by a normal plane wave with the electric field polarized along the split rings'gap direction,the surface current in the mirrored split ring resonators and the oscillating charges in the split gap respectively induce a toroidal dipole moment and an electric dipole moment with opposite direction.The interference between them eventually leads to an Anapole mode with suppressed far-field radiation and enhanced near-field.By adding an extra optical beam to pump different numbers of silicon patches under the gaps and hence change the photoconductivity of silicon,the left and right split ring resonators in a unit cell will be shorted.Eventually,the charge-current configurations of the whole structure changes,leading to the dynamic transition from nonradiating Anapole mode to radiating electric dipole mode or Fano resonance.The demonstrated actively tunable Anapole metasurface is also studied for the application of terahertz modulator.It shows a 201%high extinction modulation depth and 2 orders of near-field intensity change within an ultrafast modulation time of 554 ps.2.Theoretically proposed a"dark"BIC with tunable decaying features of radiative quality factor(Q_r)in the momentum space,and experimentally demonstrated it at terahertz frequencies.By introducing periodic perturbations,the Brillouin zone of the metasurface folds and the modes originally located at X point and below the light line are folded into the?point of the new Brillouin zone and become"dark"BICs or guided resonances(GR).Different with the common BIC,the Q_r's decaying feature of"dark"BIC in the momentum space is controllable.When the perturbation is close to zero,the Q_r of the"dark"BIC in the whole momentum space becomes infinite.By introducing different periodic perturbations,"dark"BICs swap with GRs.This is originated from the fact that the field distributions of the eigenmodes supported by the metasurface have different C₂(rotating the structure by 180°in the xy plane)symmetries regarding different high symmetry points in the structure.The angular-resolved transmission spectra of the fabricated all-dielectric terahertz metasurface clearly demonstrated the"dark"BIC and the tunable decaying features of its Q_r in the momentum space.The maximum Q factor of 258 is observed.By optical pumping the metasurface,the radiating feature of supported resonant modes is dynamically changed.3.Theoretically proposed a hot electrons photodetector using quasi-BIC and GRs in metasurface.By changing the angle of the incident light to control the radiative loss of quasi-BIC and matches with the structure's non-radiative loss,the perfect absorption is obtained at the critical coupling condition.Also,the Ag grating and cladding Ti O₂ layer forms a 3D Schottky barrier which efficiently enhances the generation and injection of hot electrons and leads to a high photoresponsivity of 9.12 m A/W.Due to the far-field topological properties of BIC,the perfect absorption induced by the quasi-BIC is robust against the change of structure's parameters.Furthermore,by changing the gap between the grating ridges to introduce periodic perturbation,the original guided mode located below the light cone is transformed into the light cone and becomes a GR.A narrow-band hot electron photodetector with a Q factor of 475 is obtained.Finally,after breaking the grating's C₂ symmetry,a three-band hot electrons photodetector based on quasi-BIC and GRs is demonstrated through simulation.