Standing Wave Surface Plasmons

For a relatively thin (25 nm) AI2O3 Si substrate-Ag nanowire spacer layer they found that the optimum polarization for fluorescence enhancement was perpendicular to the long axis of the wires. They also found that there were two maxima in the fluorescence intensity versus nanowire period for this polarization the largest enhancement is at approximately a 0.65 Inaction of the wavelength of the incident light in air, or approximately a 0.8 fraction of the wavelength in solution, and the second, less intense maximum at approximately three times that period. 

The optimum period for arrays of square Ag nanopillars was smaller than for arrays of nanowires of the same height, approximately a 0.3 fraction of the incident light wavelength in air or a 0.4 fraction of the wavelength in solution. The maximum fluorescence enhancement ratio was approximately 4 times greater for the nanopillars, than that of the nanowire structures. 

Insight into the mechanism for the fluorescence enhancement from these arrays comes from comparison of the measured dependence on spatial period with that of the square of the local field. Guo et al. [24] carried out simulations of the latter using the finite-difference time domain (FDTD) method, illustrated in Fig. 7.6. 

A comparison of the measured fluorescence and calculated field intensity at the position of the fluorescent tags (8 nm above the surface) showed qualitative consistency, that both are larger for the polarization perpendicular to the nanowires. The FDTD calculations showed that the largest calculated field intensity coincided with the largest fluorescence enhancement this occurred for a spatial 

These observations indicate the excitation of plasmon modes consisting of standing waves at the surface of the nanowires. Such a standing wave consists of two surface plasmons traveling in opposite directions, each of which has a dispersion tu versus kx given by (see 1.3)  

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In the reflection mode, typically specular reflectance is measured on the electrode surface. It is anticipated that the variation of the surface structure (e.g., surface adsorption, phase transitions, etc.) will result in appreciable changes in the reflectivity properties. One can thus correlate the structural characterislics gleaned from spectroscopic measurements with electrochanical results. Figure 2.15 shows a cell assembly for internal reflection spectroelectrochemistry. Several spectroscopic techniques have been used, such as infrared, surface plasmon resonance, and X-ray based techniques (reflectivity, standing wave, etc.). Figure 2.16 depicts a cell setup for (A) infrared spectroelectrochemistry (IR-SEC) and (B) surface X-ray diffraction. 

Guo, S.-H., Heetderks, J. J., Kan, H-C., and Phaneuf, R. J. (2008) Enhanced fluorescence and near-field intensity for Ag nanowire/nanocolumn arrays Evidence for the role of surface plasmon standing waves. Opt Express, 16,18417-18425. 

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