Effect of sub-micrometer polymer gratings generated by two-beam interference on surface plasmon resonance

Sub-micrometer periodic gratings were generated on thin poly-carbonate films spin-coated onto thin flat silver layer covered BK7 substrates by two-beam interference realized with the fourth harmonic of the Nd:Yag laser. The 416 nm period of the laserinduced grating was half of the applied master grating, the modulation depth was increased by the number of laser pulses at a given intensity. Attenuated total reflection measurements were performed in case of plasmon propagation directions parallel and perpendicular to the grooves, and on rotated gratings oriented at angles making the grating coupling effect detectable. Atomic force microscopy (AFM) was applied to determine the modulation depth of the gratings. It was shown that the periodically corrugated polymer surfaces resulted in the coupling of the surface plasmons already when the structure period was very close to the plasmon wavelength. The small shift of the resonance positions proved that the modulation depth cannot be explained by material removal, the material rearrangement plays an important role in the grating formation. These experimental results correlate with our temperature model calculations revealing that there is softening on the upper polymer surface and melting down to the silver layer at the lowest and highest applied laser fluences, respectively.