Fluorescence enhancement from a multilayered substrate fabricated with Ag and Al(2)O(3) was investigated using fluorescein, rhodamine B, Cy3, and Cy5 as fluorophores. The change in the fluorescence enhancement with Al(2)O(3) had two peaks and one valley in the range from 0 to 300 nm of Al(2)O(3) thickness, and such peaks and valley were found to appear periodically. Moreover, the reflection of the excitation light from the multilayered substrate was investigated. The reflection of the excitation light periodically changed depending on the Al(2)O(3) thickness as well, and the maximum reflection was observed near the Al(2)O(3) thickness of the peak fluorescence enhancement. It was found that the periodic changes of the fluorescence enhancement and the reflection of the excitation light could be explained, for the most part, with the integral multiples of the lambda/4 derived by a simple interference theory.
The fluorescence from a fluorophore on a multilayered substrate fabricated by a metal and a dielectric is known to be enhanced by more than 100-fold. In the course of this study, we prepared a multilayered substrate with Ag as the metal and Al(2)O(3) as the dielectric and then investigated the effects of the polarization of the excitation light on the enhancement of the multilayered substrate. It was found that the enhancement was attributed to an electric field oscillating parallel to the substrate. Maximum 200-fold enhancement could be achieved with 80 nm thick Al(2)O(3) when an unpolarized excitation light was used with an incident angle of 20 degrees.
When an alternating electric field of 10 kHz is applied to a suspension of micron-size polystyrene particles in an aqueous NaCl or KCl solution, four states appear: (a) homogeneous suspension, (b) pearl chains, (c) vortexlike turbulence and (d) 45° vortexes, i.e., clusters elongated at an inclination of ±45° to the electric field in which the particles are circulating. Conditions for the occurrence of the four states are determined by changing the salt concentration and the amplitude and frequency of the electric field. Experimental results confirm the hypothesis that the phase lag of particle polarization against the electric field plays an important role in producing the vortex states (c) and (d). The vortexlike turbulence (c) seems to be triggered by asymmetric boundary conditions existing in the experimental apparatus. Self-organization of the 45° vortexes (d) seems to be initiated by the formation of clusters due to particle density fluctuation.
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