Optical Extinction Properties of Perforated Gold-Silica-Gold Multilayer Nanoshells

Abstract

Symmetry breaking in gold nanoshell (or multilayer nanoshells) can supply many interesting optical properties, which has been studied in gold nanostrucutres such as nanocup, nanoegg, and core offset gold-silica-gold multilayer nanoshells. In this work, the optical extinction properties of the perforated gold-silica-gold multilayer nanoshells are studied by the discrete dipole approximation method simulations and plasmon hybridization theory. The extinction spectra of these particles are sensitive to the orientation of the particle with respect to polarization of the light due to the symmetry breaking. Because of the coupling of the plasmon resonance modes between the inner gold sphere and the outer nanocup structure, the perforated gold-silica-gold nanoshell provides the additional plasmon resonance peak and an even greater spectral tunability comparing with the nanocup of similar dimensions. By changing the geometry of the particles, the extinction peaks of the particles can be easily tuned into the near-infrared region, which is favorable for biological applications. The local refractive index sensitivity of the particles is also investigated, and the multiple extinction peaks simultaneous shift is found as surrounding medium is altered. The perforated gold-silica-gold multilayer nanoshells may provide various applications ranging from angularly selective filters to biological sensors.

Dual Symmetry Breaking in Gold-Silica-Gold Multilayer Nanoshells

“…When the angle of the polarization of the incident light is changed by 90°, the spectrum dip about 680 nm in the spectrum of transverse coupling is almost changed to the quadrupolar plasmon resonance peak, whereas the spectrum dip about 820 nm maintains a minimum. The blue shift of the |ω ⊥ − 〉 1 mode and the almost no shift of the |ω ⊥ + 〉 1 mode during the rotation of 90°are consistent with the previous results of the perforated Au-silica-Au multilayer nanoshells [23]. In the 'a' type of DSMNS, the two transparency windows (marked by the dashed boxes in Fig.…”

“…One of them is usually called transverse mode, in which the polarization of the incident light is perpendicular to the axis of symmetry; the other is usually called the axial mode, in which the polarization of the incident light is parallel to the axis of symmetry. For the perforated Au-silica-Au multilayer nanoshells without Au core offsetting, as we discussed in previous work [23], the plasmon hybridization only occurs between Au sphere mode and nanocup mode (transverse mode or axial mode) of the same order. When the Au core is offsetting, the selection rules for interaction are relaxed, allowing modes of different orders to mix [29,36].…”

“…This interaction results in the two new resonances: the lower energy antisymmetric coupling mode (for the transverse mode |ω ⊥ − 〉 1 and for the axial mode |ω // − 〉 1 ) and the high energy symmetric coupling mode (for the transverse mode |ω ⊥ + 〉 1 and for the axial mode |ω // + 〉 1 ). This coupling is similar to plasmon interaction in the perforated Au-silica-Au multilayer nanoshells [23]. Moreover, the quadrupolar mode of the sphere |ω i 〉 2 can now interact with all multipole of the nanocup (the transverse mode |ω ⊥ o 〉 l and the axial mode |ω // o 〉 l , l=1,2,3,⋯,∞), resulting in the quadrupolar antisymmetric coupling mode (for the transverse mode |ω ⊥ − 〉 2 and for the axial mode |ω // − 〉 2 ).…”

“…One is the shell cutting, which is generated by excising a part of the outer Au shell; the other is the core offsetting, which means the nanostructures have a nonconcentric core. The nanostructures of shell cutting include the nanocap [19], the nanocup [20][21][22], and the perforated Ausilica-Au multilayer nanoshells [23], and so on. The shellcutting nanostructures can be fabricated by anisotropic etching [24,25] or template deposition [19].…”

“…Thus, the nanoshells and multilayer nanoshells with more complicated asymmetry are expected to be studied. In our previous study, the optical properties of the perforated Au-silica-Au multilayer nanoshells [23] and the Au nanoshell with two holes [32] have been studied. The great tunability of the plasmon resonances and the flexibly angular selectivity of optical properties have been found in these nanostructures.…”