Fluorescence enhancement by surface gratings

In our experimental geometry a layer of fluorescent material, either quantum dots or R6G has been spin-coated onto a PMMA grating. 40 nm thick PMMA nano-stripe gratings have been formed by E-beam lithography on top of two kinds of substrates: an ITO/Glass and an Au/Glass substrates. The thickness of the Au layer was about 50 nm. A typical periodicity of the PMMA stripe gratings was 500nm. The fluorescence coming from the PMMA gratings formed on top of the gold film was observed to be the brightest. Compared to the grating on the ITO substrate the fluorescence is enhanced by at least a factor of 10. In order to understand how the fluorescence signal is affected by the periodicity and angular rotation of the gratings, we made a sample with regions of different periodicity varying from 400nm to 1mum, and studied how fluorescence enhancement depends on the grating periodicity and sample rotation angle. It appears that the experimental conditions for maximum enhancement of fluorescence correspond to excitation of surface plasmon polaritons on the sample surface.