We report the first experimental observation of trapped rainbow1 in gradedmetallic gratings2-4, designed to validate theoretical predictions for this newclass of plasmonic structures. One-dimensional tapered gratings were fabricatedand their surface dispersion properties tailored by varying the grating periodand depth, whose dimensions were confirmed by atomic force microscopy. Reducedgroup velocities and the plasmonic bandgap were observed. Direct measurementson graded grating structures show that light of different wavelengths in the500-700nm region is "trapped" at different positions along the grating,consistent with computer simulations, thus verifying the "rainbow" trappingeffect. The trapped rainbow effect offers exciting pathways for opticalinformation storage and optical delays in photonic circuits at ambienttemperature.
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