Experimental verification of the rainbow trapping effect in adiabatic plasmonic gratings

Abstract

We report the experimental observation of a trapped rainbow in adiabatically graded metallic gratings, designed to validate theoretical predictions for this unique plasmonic structure. Onedimensional graded nanogratings were fabricated and their surface dispersion properties tailored by varying the grating groove depth, whose dimensions were confirmed by atomic force microscopy. Tunable plasmonic bandgaps were observed experimentally, and direct optical measurements on graded grating structures show that light of different wavelengths in the 500-700-nm region is "trapped" at different positions along the grating, consistent with computer simulations, thus verifying the "rainbow" trapping effect.slow light | surface dispersion engineering | surface plasmons S ince Ebbesen et al.'s report on extraordinary optical transmission through plasmonic hole arrays was published in 1998 (1), the study of plasmonics and metamaterials (2) has progressed at a rapid pace and led to the discovery of phenomena with unique optical properties. For example, recent theoretical investigations reported the "trapped rainbow" storage of terahertz waves in metamaterials (3) and plasmonic graded structures (4), and generated considerable interest for slow-light applications. It was predicted that tapered waveguides with a negative refractive index core (3) and graded metallic grating structures (4, 5) were capable of slowing a broadband rainbow to a standstill. By varying the nanotopology of metal surfaces, the optical properties of surface plasmon polaritons (SPPs) can be tailored via so-called surface dispersion engineering (6-8). Moreover, by scaling the feature size of the graded grating structures down to the nanometer scale, it was theoretically predicted that telecommunication waves and even visible waves can also be trapped (9, 10).The intrinsic slow-light properties of SPP modes in 1D metallic grating structures can be seen from their dispersion relations, where their group velocity v g is found to decrease significantly as the photonic band edge is approached. Our recent investigations of simple 1D metallic gratings demonstrated that the surface dispersion properties can be tuned by systematically varying the groove depth and grating period. The dispersion relations for adiabatically graded gratings vary monotonically with position, so that incoming waves at different wavelengths can be trapped or localized at different positions along the propagation direction of the grating.Advances in nanofabrication and characterization techniques now permit the experimental demonstration of this interesting class of structures. Rainbow trapping has not yet been unambiguously demonstrated in the visible regime for either metamaterials (3) or plasmonic structures (4, 5, 9), although in related studies photonic crystal nanocavities with graded hole size were recently shown to exhibit adiabatically reduced group velocities for photonic modes at telecommunication frequencies (11). Two preliminary efforts were recently reported to realize the trap...

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