Demonstration of surface plasmons in metal island films and the effect of the surrounding medium - An undergraduate experiment

We present a demonstration of the surface plasmon phenomenon as it occurs i n thin metal island films. The metal films are deposited on glass microscop e slides. The effect of the surface plasmon resonance may be observed visua lly on the slide without further apparatus. Heating the film changes the sh ape of the islands and therefore the resonant frequency of the surface plas mon and changes the color of the film. Placing the film in a dielectric med ium changes the resonance condition for the surface plasmon again and chang es the color again. We show this by coating the slides with commercially av ailable liquids with different indices of refraction. We present a theoreti cal model that assumes the islands are oblate spheroids. There are enough d etails given so that the equations can be programed and the theoretical opt ical absorbance can be reproduced. We also present a modification to the th eory so that the shift in resonant frequency can be calculated when the sph eroids are immersed in the index fluids. We describe our apparatus for maki ng thin films and our optical spectrometer system. We then present optical absorbance measurements of thin films of both Ag and Au in air and in two l iquids with different indices of refraction. (C) 2000 American Association of Physics Teachers.