Basis and Lattice Polarization Mechanisms for Light Transmission through Nanohole Arrays in a Metal Film

Abstract

The extraordinary light transmission through double-hole and elliptical nanohole arrays in a thin gold film is investigated for different orientations of the holes relative to the lattice. Even though these bases have similar symmetry characteristics, the polarization follows the orientation of the basis for the ellipse but remains fixed along a lattice vector for the double holes. Furthermore, the maximum transmitted intensity for linearly polarized light is constant for the ellipse, but decreases for the double holes as they are rotated away from being aligned with the lattice. Finite-difference time-domain simulations agree well with the experimental findings. These experiments show how the basis determines both the coupling into the surface plasmon waves and the evanescent transmission through the nanoholes. Both of these effects need to be considered when designing nanophotonic devices using the extraordinary transmission phenomenon.Light transmission through subwavelength holes can be increased by several orders of magnitude with respect to Bethe's theory 1 when the holes are periodically arranged in a thin metal film. 2 This extraordinary transmission is the result of resonant coupling to surface plasmon (SP) waves at the metal-dielectric interface via the array of holes. 3,4,5,6,7 As with other periodic systems, the optical response of the hole array is determined by both the lattice and the basis. The SP waves are longitudinal, so the polarization is collinear with their direction of propagation. 8 This means that the polarization direction of the incident field selects which resonance of the lattice is excited. Recent works have shown that changing the basis shape from circular holes to elliptical 9,10 and rectangular 11 holes has a strong influence on the polarization and resonance properties; as the aspect ratio is increased, the polarization perpendicular to the broad edge of the hole has enhanced transmission and the resonance peaks shift in wavelength. Those works focused on the situation in which the elongated basis was aligned with the array's lattice vector. As a result, they did not separate the contributions of the basis and the lattice. Furthermore, both the ellipses and the rectangles considered in those works have a strongly polarized (evanescent) transmission mode through the holes, which was not distinguishable from the other polarization effects.In this work, we investigate the effect of the array basis and its orientation on the transmission. By comparing the behavior of ellipses to double-hole arrays, the contributions of the lattice and basis are distinguished. When the ellipses and the double holes are aligned with the lattice, they both show enhanced transmission for light polarized perpendicular to their long axis. Upon rotation of the basis, however, the transmission of the ellipses and double holes have different polarization behaviors. These experimental results are in good agreement with theoretical simulations based on finitedifference time-domain (FDTD) calculations....

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