The Study Of All-dielectric Nonlinear Metasurface Based On Bound States In The Continuum

Nonlinear optical effects have an important role in modern scientific research and technology,including laser light,frequency conversion and ultrafast all-optical switching.In the recent years,nonlinear metasurfaces provide a paradigm shift capability in nonlinear optics.They can manipulate the frequency conversion processes at nanoscale,also offering novel opportunities for applications in photonics,chemistry,materials science and biosensing.Frequency conversion processes such as second harmonic generation and third harmonic generation is usually realized by phase matching in macrostructures.The ultra-thin metasurfaces can substitute the bulk nonlinear material and reach better conversion efficiency without phase matching.In this thesis,we show that metasurfaces supports high quality factor(Q)governed by bound state in the continuums(BICs)for enhancing the nonlinear optical response.The main work is:Firstly,all-dielectric silicon metasurfaces with different unit molecular structures have been proposed in the optical communication band that support infinite Q value associated with symmetry-protected BIC at first Brillouin zone.The cross-like shaped silicon metasurface studied under the oblique incidence and the sharp electromagnetic response brought by the quasi-BIC is triggered.Under the condition of that the tilt angle is less than10°,the Q value can be achieved up to10~2.The electromagnetic field distribution in x-y plan of the eigenmode reveals the magnetic dipole feature of the BIC.Secondly,the rectangular hole metasurface with parallel electric field distributions magnetic dipole will be study.convex-like shape hole metasurface and concave-like shape hole metasurface can be got by introducing the same symmetry breaking in the plane inward,outward respectively.Since they are no longer protected symmetry,triggering the transition from BIC mode to quasi-BIC mode under normal excitations.And a sharp electromagnetic response yield.More importantly,their Q value will decrease with different amplitudes while the same symmetry breaking is changed.The Q value of the concave-like shaped metasurface is always bigger than the convex-like shaped one.When the symmetry breaking is more than90 nm,the Q value of the concave-like shaped metasurface remains above 10~6,and the Q value of the convex-like shaped metasurface is also maintained above 10~4.Finally,grating-like metasurface was introduced a periodic symmetry breaking between a grating strip and becomes the condition of the transition from BIC to quasi-BIC.The grating metasurface also supports an infinite Q value related to asymmetry-protected BIC at(38)point of the first Brillouin zone.Regardless of changes in the size of symmetry breaking,the Q value remain above 10~5 all the time under fixed period.Besides,there is maximum Q value more than 10~8related to a magnetic dipole with circular electric field distribution and an electric quadrupole with parallel electric field distribution in x-y plan respectively.The optical harmonic generation is one of the most promising applications of high-Q all-dielectric metasurface.The intensity of the third harmonic generation is calculated by simulation,and the high-Q metasurface that supports quasi-BIC shows a trend of magnitude enhancement at the resonance.