The Enhancement And Manipulation Of Nonlinear Effects In Plasmonic Nanostructures And Ultra-broadband Achromatic Metalens

With the rapid development of micro/nano processing technology,a large number of subwavelength-scale artificial nanostructures have been fabricated,and then many novel optical characteristics brought with them have attracted the attention of researchers.As a special phenomenon existing in the metal nanostructures,surface plasmon resonances can produce a great enhancement of local electric field near the metal surface,which is very attractive to optical antennas,optical sensors,surface enhanced Raman scattering,nonlinear optics and other fields.At the same time,metasurface,assembled of artificial subwavelength-scale nanostructures(called metaatoms)specifically arranged on a 2D plane,can introduce arbitrary optical phase abrupt changes at the ultra-thin interfaces.With the advantages such as design flexibility and ultra-compact volume,the use of metasurfaces to achieve integrated optical devices with the functions of traditional optical elements have become a worldwide research hotspot.This thesis focuses on the two research hotspots of nonlinear optical effects and optical metasurfaces,and carries out corresponding researches on the two hotspots and their cross fields respectively.Nonlinear optics has important applications in all-optical regulation,optical switching,frequency and other fields,but its low conversion efficiency greatly hinders its development.The key point is still to enhance the generation of nonlinear signals,so we first explore the influence of double resonance on the intensity of third harmonic generation(THG)from plasmonic spherical nanocavity.As one of the most important properties of light,polarization is often controlled by the use of external optical components such as polarizers and wave plates in traditional nonlinear optics.To solve the problems of large volume,weight and cost in this optical system,we study the polarization manipulation of the second harmonic(TH)based on nonlinear plasmonic metasurfaces.Although metasurfaces show excellent optical functionalities and allow much more compact design than traditional optical components,severe chromatic aberration in metasurfaces strongly restrict their further applications,especially in the fields of color imaging devices.It is particularly important to elimainate chromatic aberration in the continuous wavelength region,so in the end we focus our research on the realization of the ultra-broadband achromatic metalenses working from visible range to near infrared.Specific research contents are arranged as following:1.The effect of double resonance in the core-shell spherical nanocavity on the enhancement of third harmonic generation.In dielectric-metal core-shell spherical nanostructures,we numerically study the effects of mode matching and field distribution symmetry of double resonance on THG intensity,and we find that when the two resonant cavity modes have the same angular momentum and same rotational symmetry field distributions THG obtains the optimal enhancement.The core-shell nanostructure supports various plasmon cavity modes from visible area to near infrared,and these multipole cavity modes have different symmetries and tunable resonance wavelengths.So the double resonance condition can be realized while the fundamental wave excites a multipole cavity mode and the THG signal excites another high-order cavity mode.The nonlinear numerical calculation of different double resonance conditions shows that the double resonance can enhance THG in comparison with the condition that only one cavity mode is excited at the fundamental or harmonic wavelength.When the local electric field distributions of the two cavity modes at the fundamental frequency and triple frequency have the same rotational symmetry,that is,the two modes have the same angular quantum number,the THG intensity has a very significant enhancement and the enhancement effect is far superior to other double resonance situations.The mechanism that the symmetric local fileds of double resonance enhance nonlinear effects is not limited to THG in the spherical nanocavity,but also brings new opportunities for the enhancement of other nonlinear effects in other micro-nano structural systems.2.Polarization generation and manipulation based on nonlinear plasmonic metasurfaces.Utilizing split-ring resonators(SRRs)and complementary split-ring resonators(CSRRs)to generate two orthogonal polarized SH basic vectors,we realize a nonlinear optical meta-device that integrates the functions of harmonic generation,beam splitting and arbitraty polarization manipulation.Under the same linear polarized fundamental wave CSRRs and SRRs rotated by 90 degress can generate orthogonal polarizations of SH components,and changing their geometric parameters can tailore the phase difference and amplitude ratio between the two SH components.By introducing spatial offset between adjacent basic supercells,we have another freedom to adjust phase difference between two orthogonal components and thus realize arbitraty polarization control of SH.We demonstrate a SH quarter-wave plate based on this metasurface.Two separated SH beams with orthogonal circular polarizations are achieved from a linearly polarized FW,and illuminated by circularly polarized FW,the SH quarter-wave plate can generate linearly polarized SH.Furthermore,several elliptical polarized SH are also obtained from properly designed nonlinear metasurfaces.In theory we can realize any polarization state of SH.Our design is of great significance for the realization of nonlinear integrated optical devices,especially miniaturized light sources for special polarization requirement.3.The explore of ultra-broadband achromatic metalens.Based on resonance phase and geometric phase control,we divide the phase designed for focusing into a basic phase and a chromatic aberration phase to separately control,and realize an ultra-broadband achromatic metalens working from 450-1500 nm.First we introduce the noticeable achievements over the past few years in the field of achromatic metalenses.Then,based on the principle of phase dividing,we use the resonance phase to control the wavelength-dependent chromatic aberration phase part and the geometric phase to control the wavelength-independent basic phase part respectively.And we successfully give a design solution for all-dielectric ultra-broadband achromatic metalenses working from 200-700 THz.As an experimental demonstration,we fabricate an ultra-broadband achromatic metalens with a diameter of 30 ?m and a focal length of 58 ?m.The numerical simulation and optical experiment results show that the circularly polarized light from 450-1500 nm is focused at the same focal position after passing through the metalens.The white light imaging in the transmission mode of this metalens also shows its good achromatic ability,and its resolution is 3.1 ?m.The realization of ultrabroadband achromatic metalens provides more possibilities for the development of meta-optical devices,especially integrated biological detection systems.