具有尖锐吸收截面和显著增强电场的等离激元-光子混合谐振腔

Hybrid Plasmonic-photonic Resonators with Sharp Absorption Cross-section and Obvious Electric Field Enhancement

研究了由二维光子晶体微腔和金纳米天线组成的等离激元-光子混合谐振腔中光学谐振腔对等离激元谐振腔的额外贡献。研究结果表明:与纯等离激元谐振腔相比,混合体系的吸收截面和电场强度均明显增强。特别地,混合谐振腔的吸收截面谱呈现出类Fano线形和尖峰,可以通过改变金纳米天线的数量或金纳米天线与光子晶体微腔之间的共振波长失谐进行调制。此外,混合谐振腔的电场增强因子比金纳米天线高出3个数量级。可为光热探测和光谱增强研究提供新的平台。

In recent years,the plasmonic-photonic hybrid system has attracted extensive attention and research from scholars at home and abroad because of its combination of the advantages of the plasmonic and photonic resonators.In previous works,the plasmonic-photonic hybrid system has demonstrated obviously enhanced Purcell factor due to the deep subwavelength confinement of localized surface plasmon resonance and the additional contribution of the ultra-small mode volume.Different from previous works,in this paper,we are focused on the extra contribution of optical resonator on plasmonic resonator in a plasmonic-photonic hybrid resonator consisting of a two-dimensional photonic crystal microcavity and Au nano-antennas.Firstly,the absorption cross section spectrum and the distribution of electric field intensity of the hybrid resonators are calculated by using the finite difference time domain method.By comparing the coupling characteristics of singe-NA model,three-NAs model and five-NAs model,the physical mechanism of the enhanced absorption cross section and the law of the enhanced electric field intensity are analyzed.The results prove that compared with pure plasmon resonators,the hybrid system demonstrates obviously enhanced absorption cross-section and electric field intensity.In particular,the hybrid resonator exhibits Fano-like lineshape and sharp peak in its absorption cross-section spectrum which can be modulated by changing the numbers of Au nano-antennas or the resonance wavelength detuning between the Au nano-antenna and PC microcavity.With the increase of the number of Au-NAs,the absorption cross section of the hybrid system is continuously enhanced,but the response is different at 657 nm and 576 nm.Due to the increasing loss of the Au NAs,the Fano-like lineshape at 657 nm are getting more and more unconspicuous,while the peaks at 576 nm are becoming sharper,which is due to the superposition of each Au NA coupled with the PC microcavity's higher-order mode at 576 nm.In general,the five-NAs model has the best absorption cross section enhancement.Contrary to the enhancement law of the absorption cross section,the enhancement effect of the electric field intensity of the hybrid systems decrease with the increase of the number of Au-NAs.Among them,the single-NA model has the best electric field enhancement,especially when the resonance wavelength detuning isΔλ=−27 nm.At this time,the electric field intensity of the hybrid system is about 12 times that of the single Au NA and 80 times that of the bare photonic crystal microcavity.In particular,the electric field enhancement factor of the hybrid resonator is three order higher than that of the single Au NA,which can be widely used in the field of surface enhancement spectroscopy.Then,we establish the coupling coefficient formula to describe the coupling efficiency of the electric field intensity at 657 nm.The results show that the coupling coefficient is not the highest whenΔλ=0 nm.Actually,coupling coefficient will gradually increase asΔλincreases along the negative direction,which means a smaller Au NA corresponds to a higher coupling coefficient.We analyze that the above phenomenon are the results of the combined effect of the loss change of localized surface plasmon mode and the change of electric field intensity.For the three-NAs model and the five-NAs model,it is not difficult to make a conclusion that the electric field coupling coefficient is mainly determined by the PC microcavity's electric field intensity at the coupling position,that is,higher electric field intensity at the coupling position will result in larger electric field coupling coefficient.Finally,we also analyze the dependencies of linewidth and intensity of absorption cross section peaks at 657 nm and 576 nm on the resonance wavelength detuning.The results show that the relationship between them are almost linear,which is convenient for purposeful modulation in practical applications.In summary,this study provides a new platform for the study of photothermal detection and spectral enhancement.