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Timestamp: 2019-04-24 04:59:46+00:00

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The response of gold nanoparticle dimers is studied theoretically near and beyond the limit where the particles are touching. As the particles approach each other, a dominant dipole feature is observed that is pushed into the infrared due to interparticle coupling and that is associated with a large pileup of induced charge in the interparticle gap. The redshift becomes singular as the particle separation decreases. The response weakens for very small separation when the coupling across the interparticle gap becomes so strong that dipolar oscillations across the pair are inhibited. Lower-wavelength, higher-order modes show a similar separation dependence in nearly touching dimers. After touching, singular behavior is observed through the emergence of a new infrared absorption peak, also accompanied by huge charge pileup at the interparticle junction, if initial interparticle contact is made at a single point. This new mode is distinctly different from the lowest mode of the separated dimer. When the junction is made by contact between flat surfaces, charge at the junction is neutralized and mode evolution is continuous through contact. The calculated singular response explains recent experiments on metallic nanoparticle dimers and is relevant in the design of nanoparticle-based sensors and plasmon circuits.
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Fig. 1. (a) Wavelength dependence of the imaginary part of the polarizability of the dimer formed by two spherical gold particles (a=60 nm) for different distances between their surfaces d. The applied electric field is polarized along the interparticle axis. (b) Same as (a) for a gold particle consisting of two overlapping spheres (d<0). Dashed curves have been added to guide the eye through the evolution of spectral features with varying d.
Fig. 2. Contour plot showing the scattering cross section for two spherical gold particles as a function of the separation between their surfaces d (horizontal axis) and the light wavelength (vertical axis). Negative values of d correspond to overlapping spheres. The incoming electric field is polarized parallel to the line connecting the particle centers. The particle radius is a=60 nm. The cross section has been normalized to the projected area of one particle, πa 2. Solid curves are intended to guide the eye through the cross section maxima. The lower inset illustrates how unphysical modes become physical and dominant after touching (A-B-C curve).
Fig. 3. Near-field maps and the corresponding induced surface charge distributions for two neighboring gold spheres as a function of their separation d and wavelength λ corresponding to selected points of Fig. 2, as indicated by labels A-H for overlapping spheres and I-L for separated spherical particles. The sphere radius is a=60 nm. The contour plots show the squared electric field in a plane that contains both sphere centers and that is parallel to the incident light direction (the light is coming from the left) and to the incident electric field polarization. The induced surface charge distribution corresponding to the excited modes under consideration is represented in the accompanying 3D plots.
Fig. 4. Resonant wavelengths of standing and localized modes in gold dumbbell particles as a function of the length L of the central rod for two different values of the overlap distance d at L=0. All geometrical parameters are given in the insets.
Fig. 5. Wavelength dependence of the imaginary part of the polarizability of an overlapping dimer with a smooth junction for sphere radii a=60 nm and overlap parameter d=-5 nm. The junction is smoothed by a toroidal surface of inner radius s as shown in the inset. Various values of s have been considered.
Fig. 6. Wavelength dependence of the imaginary part of the polarizability of the dimer formed by two spherical KCl particles of radius a=2 µm for different distances between their surfaces d. The applied electric field is polarized along the interparticle axis. d<0 stands for overlapping particles. Dashed curves have been added to guide the eye through the evolution of spectral features with varying d.
(1) Λ = ε Au + 1 ε Au − 1 .

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