Research On Light Field Confinement In Metasurfaces And Its Applications

Metasurfaces are quasi two-dimensional（2D）structures consisting of scatterers with different sizes and shapes.By choosing the geometry and arrangement of scatterers,metasurfaces can modify the amplitude,phase,and polarization of electromagnetic waves.Compared with their bulk counterparts,the 2D nature of metasurfaces allows for more compact optical devices and reduced optical losses.With the development of the micro-nano fabrication technology,great progress has been made in metasurfaces.The ability of the light field confinement,which affects the performance of the device,is essential to the structure at the subwavelength scale.To overcome this problem,two different mechanisms,i.e.bound states in the continuum（BIC）and surface plasmon,are studied in this dissertation.Due to the different principles and mechanisms,these two mechanisms are applied to different materials:dielectric structures supporting BIC will achieve high-quality（Q）resonances,which can improve the light-matter interaction;surface plasmon may be introduced in the structure based on 2D materials.It can achieve strong field confinement,leading to the absorption enhancement in2D materials.The main contents are as follows.1.Excitation and regulation of BIC in silicon metasurfacesBy using the finite element method（COMSOL Multiphysics）,a periodic array consisting of silicon（Si）nanocubes was simulated.The symmetry structure supports the symmetry-protected BIC,and the Q factor of the mode is infinite.After introducing the symmetry-breaking defect,the BIC transforms into a“leakage”mode with a finite Q factor.To verify it,the metasurfaces based on Si nanocubes were fabricated.The experiment results show that the proposed structures achieve high-Q resonances,and the Q factor can be modulated by changing the defect size.This modulation may be adjusted by changing the dimensions of structures.The influences of non-ideal factors on the Q factor are also discussed,including the surface morphology,substrate,and array size.After optimizing the design scheme and fabrication process,the structure with an array size of 20×20 can achieve a Q factor more than 1000.The proposed strategy provides a new way to achieve high-Q resonances in Si metasurfaces,which constitutes a significant step toward the development of high-performance photonic and optoelectronic devices based on Si and further promotes the development of Si-based optoelectronic integrated circuits.2.Research on nonlinear optical effects in Si metasurfacesThe nonlinear optical effects,including second harmonic generation（SHG）and third harmonic generation（THG）,are investigated in Si metasurfaces.The pulsed pump laser is focused on the sample to excite the nonlinear responses.THG is dominated by the bulk nonlinear effect.With the excitation of an on-resonance laser,the THG intensity in the sample can be nearly 400 times larger than that in the Si substrate.As a centrosymmetric crystal,silicon is absent of bulk second-order optical nonlinearity.Theoretical analysis and simulation results show that after introducing the defect,SHG mainly originates from the surface effect.That is,the electric field enhancement at the defect interface improves the surface second-order nonlinear effect.Through the dimension optimization,the normalized conversion efficiency of SHG reaches 1.8×10^-4 W^-1.The proposed strategy can greatly improve the SHG efficiency in silicon and promotes the development of nonlinear devices based on Si,which also provides a new approach to develop second-order nonlinear optical effects and high-efficiency devices based on centrosymmetric materials.3.Research on BIC in thin lithium niobateBased on the mechanism of BIC,lithium niobate（LN）metasurfaces with/without etching are proposed.For the etching LN metasurface,the optical properties of the structure are studied by the finite element method（FEM）.The array of LN nanocubes also supports the symmetry-protected BIC.By changing the size of the introduced defect,the Q factor can be adjusted.Simulation results show that the Q factor is hardly affected by the etching process in the proposed structure.For the etchless LN metasurface,polymer nanocubes are placed on the thin LN film.The resonance mode will be adjusted by changing the dimensions of nanocubes.Simulation results show that the etchless structure supports multiple modes including the guided mode resonance and symmetry-protected BIC,which is confirmed by the experiment results.The nonlinear characterization demonstrates that the etchless structure improves the second-order nonlinear effect in LN.Both schemes can reduce/eliminate the negative effects caused by the etching process,which provides a new platform for the research of photonic nanodevices based on LN.4.Research on the surface plasmon in the nanocavity metasurface based on black phosphorusThe band gap of black phosphorus（BP）can be adjusted with its thickness.Due to this fact,BP nanocavity metasurfaces are proposed for visible light and terahertz bands.The absorption properties of the BP metasurface based on the nanocavity are studied by using the FEM.Simulation results show that the surface plasmon polaritons are produced at the interface between the silver porous layer and BP layer.They exhibit a strong ability of field confinement,which greatly enhances the absorption in BP.Under the illumination of the X-polarized light,the structure achieves broadband absorption（>80%）from 550 to 800 nm.By employing the finite difference time domain method（FDTD）,the absorption characteristics of the BP bowtie metasurface in the terahertz band are studied.Simulation results show that the absorption in BP can be improved due to the localized surface plasmon resonance（LSPR）.The absorption under the illumination of the Y-polarized light is 3 times larger than that for the X-polarized light,which may be potential for polarization detection applications in the terahertz band.The proposed strategy provides a new approach to improve the absorption in 2D materials,which further promotes the development of wearable and flexible devices based on 2D materials.