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采用时域有限差分法和麦克斯韦应力张量法, 系统研究了金薄膜衬底上介质-金属核壳结构所受的光学力. 研究结果表明: 由于核壳结构与衬底之间强的等离激元模式杂化效应, 其所受的光学力相较于单个核壳结构实现了一个数量级的增强; 同时, 通过改变激发波长, 实现了局域电场分布的调控, 以此观察到了核壳结构光学力方向的可控反转; 进一步, 详细分析了核壳结构所受光学力随其到衬底间距、内核介质的尺寸及折射率等的变化关系, 以此丰富了光学力大小、方向和峰值波长的调控方法. 研究结果可为精确控制颗粒/金属薄膜纳米腔的尺寸提供一种新的途径, 并为调控单分子级的光与物质相互作用、研发新型纳米光子器件提供有益参考.Manipulating the core-shell structure with the optical force has been extensively studied, giving birth to applications such as particle sorting, biomarkers and drug delivery. Tailoring the optical force exerted on the core-shell above the metallic film remains unexplored, despite the obvious benefits for both fundamental research and applications including strong coupling, surface enhanced spectroscopy, nanolaser, and nanoscale sensing. In this work, we systematically investigate the optical force exerted on a dielectric/metal core-shell above a gold film by utilizing the Maxwell stress tensor formalism. It is found that at the present gold substrate, the optical force on the core-shell can be one order of magnitude larger than that on the individual core-shell due to the strong coupling between the core-shell and the gold film. Interestingly, the direction of the optical force can be reversed from positive to negative by distributing the local field from the upside of core-shell to the structure gap through changing the excitation wavelength. Furthermore, we demonstrate that the magnitude and peak wavelength of the optical force can be well controlled by altering the structure gap, the size and refractive index of the core. More specifically, it is found that the coupling strength between the core-shell and the gold film decreases with the gap size increasing. As a result, we observe the blue shift of bonding mode and the decrease of local field in the gap, which leads the force peak wavelength to be blue-shifted and the force peak magnitude to decrease, respectively. Also, by increasing the radius and refractive index of the core, a red shift of force peak is accompanied with the red shift of the bonding mode. In addition, the force peak magnitude follows the same trend as the total local field enhancement factor when the radius and refractive index of the core change. We hope that our results open the way to control the cavity size of particle on film structure, which would be beneficial for tailoring the light matter interaction even down to single molecular level and promises to have the applications in novel functional photonic devices.
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Keywords:
- optical force /
- surface plasmons /
- core-shell structure /
- plasmon hybridization
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图 1 金膜衬底上介质/金属核壳结构的示意图
Fig. 1. Schematic diagram of a dielectric/metal core-shell placed above a gold film.
图 2 核壳-金薄膜结构的等离激元杂化示意图和散射光谱 (a)等离激元杂化示意图; (b)散射光谱, 插图为电场分量Ez在xy平面上的分布
Fig. 2. (a) Scheme of plasmon hybridization picture of the core-shell on gold film; (b) scattering spectrum of core-shell particles on gold film. The inset of panel (b) shows the z-component of the electric field in xy plane.
图 3 (a)核壳结构所受的纵向光学力Fz; 波长为(b) 540, (c) 670, (d) 790和(e) 830 nm时核壳结构周围电场强度增强因子EF的分布
Fig. 3. (a) Longitudinal optical force Fz exerted on the core-shell on gold film. The electric-field intensity enhancement factor map of the core-shell on gold film at wavelengths of (b) 540, (c) 670, (d) 790, and (e) 830 nm, respectively.
图 4 核壳-金膜结构的(a)散射光谱、(b)光学力谱和(c)纵向光学力幅值及间隙的平均电场强度增强因子随结构间隙h的变化
Fig. 4. (a) Scattering spectra of the core-shell on gold film; (b) longitudinal optical force spectra of the core-shell on gold film; (c) maximum longitudinal optical force and the average electric-field intensity enhancement factor as a function of gap size for the core-shell on gold film.
图 5 核壳-金膜结构的(a)散射光谱、(b)纵向光学力谱和(c)纵向光学力幅值以及间隙内平均电场强度增强因子随内核尺寸Rc的变化
Fig. 5. (a) Scattering spectra of the core-shell on gold film; (b) longitudinal optical force spectra of the core-shell on gold film; (c) maximum longitudinal optical force as well as the average electric-field intensity enhancement factor as a function of dielectric core radius for the core-shell on gold film.
图 6 核壳-金膜结构的(a)散射光谱、(b)光学力谱和(c)纵向光学力幅值及间隙的平均电场强度增强因子随内核折射率nc的变化
Fig. 6. (a) Scattering spectra of the core-shell on gold film; (b) longitudinal optical force spectra of the core-shell on gold film; (c) maximum longitudinal optical force and the average electric-field intensity enhancement factor as a function of index for the core-shell on gold film.
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[1] Ashkin A 1970 Phys. Rev. Lett. 24 156
Google Scholar
[2] Svoboda K, Block S M 1994 Opt. Lett. 19 930
Google Scholar
[3] Gong L P, Zhang X H, Gu B, Zhu Z Q, Rui G H, He J, Zhan Q W, Cui Y P 2019 Nanophotonics 8 1117
Google Scholar
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Google Scholar
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[6] Huang J, Yang Y 2015 Nanomaterials 5 1048
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[7] Lu J S, Yang H B, Zhou L N, Yang Y Q, Luo S, Li Q, Qiu M 2017 Phys. Rev. Lett. 118 043601
Google Scholar
[8] Bezryadina A S, Preece D C, Chen J C, Chen Z G 2016 Light-Sci. Appl. 5 e16158
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[9] Xin H B, Li Y C, Xu D K, Zhang Y L, Chen C H, Li B J 2017 Small 13 1603418
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Li Y M, Gong L, Li D, Liu W W, Zhong M C, Zhou Z H, Wang Z Q, Yao K 2015 Chin. J. Las. 42 0101001
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Tong L M, Xu H X 2012 Physics 41 582
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Google Scholar
[22] Demergis V, Florin E L 2012 Nano Lett. 12 5756
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Google Scholar
[24] Zhang Q, Xiao J J 2013 Opt. Lett. 38 4240
Google Scholar
[25] Simpson S H, Zemánek P, Maragò O M, Jones P H, Hanna S 2017 Nano Lett. 17 3485
Google Scholar
[26] Li G C, Zhang Y L, Lei D Y 2016 Nanoscale 8 7119
Google Scholar
[27] Xiao F J, Ren Y X, Shang W Y, Zhu W R, Han L, Lu H, Mei T, Premaratne M, Zhao J L 2018 Opt. Lett. 43 3413
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Google Scholar
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Google Scholar
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Google Scholar
[32] Oulton R F, Sorger V J, Zentgraf T, Ma R M, Gladden C, Dai L, Bartal G, Zhang X 2009 Nature 461 629
Google Scholar
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Google Scholar
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Google Scholar
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