Department of Experimental Physics, University of Pécs, Pécs, Hungary.
Phys Chem Chem Phys. 2011 Dec 14;13(46):20772-8. doi: 10.1039/c1cp21750j. Epub 2011 Oct 14.
Optical field distribution in micro-nano geometries of miniaturized optical devices is often significantly different from that in identical but macroscopic geometries. Plasmon effects and near-field diffraction can enhance the local field intensity, leading to enhanced cross section for light absorption and scattering, which can be utilized in substrate-enhanced spectroscopies for the detection of trace amounts of adsorbed chemicals. A specific problem is an ingenious but only empirically described way to enhance signal intensity in Raman spectroscopy by the use of a substrate patterned with gold coated micron size pyramidal pits. While Raman enhancement on nanostructured substrates is generally attributed to surface plasmons, here the micron size, and thus the sub-wavelength to near-wavelength dimensions suggest that resonant enhancement emanating from optical near-field diffraction might also play a role. To answer this question, light diffraction in a projection of the pyramidal pit: a V-groove, was modelled with a modified Neerhoff-Mur formalism suitable to calculate electromagnetic field distribution in sub-wavelength structures. Under the boundary conditions a perfect conductor screen was assumed, which excludes plasmon effects. The calculations show that interference in the cavity causes a modest resonant increase in local intensity and that near-field diffraction strongly influences the field distribution, which is explained with the electrodynamic edge effect. The magnitude of the resonant electric field on its own cannot account for the experimentally observed Raman enhancement. However, a resonant enhancement of a similar magnitude is expected for the emitted Stokes frequencies. In this case the geometry implements the conditions for the classical electromagnetic Raman enhancement, ~E(4), in a good agreement with experimental results.
微纳尺度的微型光学器件中的光场分布通常与相同但宏观的几何形状中的光场分布有很大的不同。等离子体效应和近场衍射可以增强局域场强度，从而提高光吸收和散射的截面，可以用于增强衬底光谱学来检测痕量吸附化学物质。一个具体的问题是一种巧妙但仅通过经验描述的方法，即通过使用涂有金的微米尺寸金字塔形坑图案化的衬底来增强拉曼光谱中的信号强度。虽然在纳米结构衬底上的拉曼增强通常归因于表面等离子体，但这里微米级的尺寸，即亚波长到近波长的尺寸表明，源自光学近场衍射的共振增强也可能起作用。为了回答这个问题，用改进的 Neerhoff-Mur 公式对金字塔形坑（V 形槽）的投影中的光衍射进行了建模，该公式适用于计算亚波长结构中的电磁场分布。在边界条件下，假设理想导体屏幕，排除等离子体效应。计算表明，腔内的干涉导致局部强度的适度共振增加，并且近场衍射强烈影响场分布，这可以用电磁动力学边缘效应来解释。单独的共振电场强度本身不能解释实验观察到的拉曼增强。然而，预计发射斯托克斯频率的共振增强幅度相似。在这种情况下，该几何形状实现了经典电磁拉曼增强的条件，~E(4)，与实验结果非常吻合。
