Oubre C, Nordlander P
Department of Physics, Rice Quantum Institute, M.S. 61, Rice University, Houston, Texas 77251-1892, USA.
J Phys Chem B. 2005 May 26;109(20):10042-51. doi: 10.1021/jp044382x.
The optical properties of metallic nanoshell dimers are investigated using the finite difference time domain (FDTD) method. We discuss issues of numerical convergence specific for the dimer system. We present results for both homodimers and heterodimers. The results show that retardation effects must be taken into account for an accurate description of realistic size nanoparticle dimers. The optical properties of the nanoshell dimer are found to be strongly polarization dependent. Maximal coupling between the nanoshells in a dimer occurs when the electric field of the incident pulse is aligned parallel to the dimer axis. The wavelengths of the peaks in the extinction cross section of the dimer are shown to vary by more than 100 nm, depending on the incident electric field polarization. The calculations show that electric field enhancements in the dimer junctions depend strongly on dimer separation. The maximum field enhancements occur in the dimer junction and at the expense of a reduced electric field enhancement in other regions of space. We investigate the usefulness of nanoshell dimers substrates for SERS by integrating the fourth power of the electric field enhancements around the surfaces of the nanoparticles as a function of dimer separation and wavelength. The SERS efficiency is shown to depend strongly on dimer separation but much weaker than the fourth power of the maximum electric field enhancement at a particular point. The SERS efficiency is also found to depend strongly on the wavelength of the incident light. Maximum SERS efficiency occurs for resonant excitation of the dimer plasmons.
采用时域有限差分（FDTD）方法研究了金属纳米壳二聚体的光学性质。我们讨论了二聚体系统特有的数值收敛问题。我们给出了同型二聚体和异型二聚体的结果。结果表明，对于实际尺寸的纳米颗粒二聚体，要进行准确描述，必须考虑延迟效应。发现纳米壳二聚体的光学性质强烈依赖于偏振。当入射脉冲的电场与二聚体轴平行排列时，二聚体中纳米壳之间的耦合最大。二聚体消光截面中峰值的波长显示会根据入射电场偏振而变化超过100nm。计算表明，二聚体结处的电场增强强烈依赖于二聚体间距。最大场增强发生在二聚体结处，代价是空间其他区域的电场增强降低。我们通过将纳米颗粒表面周围电场增强的四次方作为二聚体间距和波长的函数进行积分，研究了纳米壳二聚体基底用于表面增强拉曼光谱（SERS）的实用性。结果表明，SERS效率强烈依赖于二聚体间距，但比特定点处最大电场增强的四次方弱得多。还发现SERS效率强烈依赖于入射光的波长。二聚体等离子体激元的共振激发会产生最大SERS效率。
