Multipolar Optical Resonance And Enhanced Second Harmonic Generation In Hybrid Nanoantennas

Surface plasmon resonance can boost optical properties,both linear and nonlinear,due to their capability of concentrating light into deep subwavelength volume,which makes it be widely applied in physics,chemistry as well as biomedicine.And surface plasmon resonance is mainly contributed by electric and magnetic origins.Specifically,both electric and magnetic modes are contributed by many multipolar components.Perfect mode matching may improve the performance of the applications that based on surface plasmon resonance.The multipolar resonances can be generated and tuned by plasmon hybridization.In this article,we mainly concern the multipolar resonance that stemms from plasmon hybridization and reveal their optical properties.Besides,we also study the enhancement of nonlinear optical effects,such as second harmonic generation,via mode matching.The details of our results are given below.Firstly,we theoretically study the effects of the refractive index of the surrounding media on the gain threshold in gain assisted double-phyramid-like gold nanoshell.We take advantage of "tip effect" of double-phyramid-like gold nanoshell,which is very sensitive to the surrounding media.The gain threshold will drop when reducing the refractive index of the surrounding media.Specifically,we find that the gain threshold can drop by 26.6%when the refractive index of surrounding medium decreases from 1.69 to 1.33,which could offer an effective method to lower the gain threshold of the nanosystem.Our work may contribute to the designing of low gain threshold nanodevice.Secondly,we study the properties of gain threshold of multipolar resonant modes in multimer.We find that modes with lower radiative losses due to the cancellation of dipole moment usually have lower gain threshold.High order plasmonic resonances,including electric quadrupole and magnetic dipole,can be achieved by the reduction of symmetry in multimer.Meanwhile,the dipole moment can be reduced greatly in these high-order modes,and consequently,the radiative losses decrease efficiently.The low-loss modes can lead to lower gain threshold in the gain-assisted nanosystem.Specifically,compared with the electric dipolar mode in single nanoshell,the gain threshold of the electric quadrupolar and magnetic dipolar modes in multimer can drop by 57.66%and 59.22%,respectively.On the other hand,the gain threshold can reflect the extent of optical losses of the plasmonic mode in a nanosystem.These findings may have potential applications in the design of nanolaser,plasmon waveguide and photo-thermal device.Thirdly,we theoretically study the gain-assisted double plasmonic resonances to enhance second harmonic generation in a centrosymmetric multilayered silver-dielectric-gold-dielectric nanoshell.Introducing gain medium into the dielectric layers can not only compensate the dissipation and lead to giant amplification of surface plasmon,but also excite local quadrupolar plasmon which can boost second harmonic generation by mode matching at second harmonic frequency.Second harmonic generation is sharply enhanced at fundamental and second harmonic frequencies by coupling with the local plasmon.Besides,the resonant modes at fundamental and second harmonic frequencies can couple with the gain media respectively or simultaneously,which give a deep insight into the contributions to enhancing second harmonic generation from the local plasmon at fundamental and second harmonic frequencies.We have designed a high-efficiency nanostructure to enhance the second harmonic generation and revealed a significant approach to take advantage of the mode-matched plasmon at SH frequency for second harmonic generation enhancement in the centrosymmetric nanostructures.Fourthly.We show that pure high-order magnetic resonance can be achieved via Fano resonances in metal and high refractive index media hybrid-nanoantennas.In noble metal(silver or gold)and high refractive index dielectric(AlGaAs)hybrid-nanoantennas,high-order Fano resonances can be observed,which lead to the suppressions of electric multipoles and magnetic dipole.When the resonant peak of magnetic quadrupole is tuned to the wavelength of Fano dips,ideal high-order magnetic scattering is achieved.With the energy transfer,magnetic quadrupole resonance can be greatly enhanced at the Fano dips.And the strong magnetic quadrupole resonance exhibits great capability of boosting the nonlinear processes,such as second harmonic generation.The findings achieved in subwavelength geometries have important implications for functional metamaterial developments.