Dispersion Study of the Infrared Transmission Resonances of Freestanding Ni Microarrays

Abstract

Nickel films (several-micrometer thickness, with 5.2-mm square holes in a square lattice array with 12.7-mm hole-to-hole spacing) exhibit Ebbesen's extraordinary transmission effect in the infrared; that is, they transmit a higher fraction of incident infrared light than the fractional open area of the holes. The role of surface plasmons (SPs) in this phenomenon is much debated, so we have obtained a data set whereby this idea and others can be tested against empirically determined dispersion curves. Unpolarized, zero-order transmission spectra have been recorded by rotating the mesh (from j2-to 60-in 1-steps) relative to the spectrometer's incident beam about an axis along the mesh's nearest hole-to-hole spacing in order to create the dispersion diagram. The data are numerically analyzed for peak centers that are then projected outside of the light line (by SP momentum matching equations) to two SP dispersion curves that are spaced in frequency by a splitting. With this caveat, all of the observed structure is accounted for by a simple SP model.

Light passing through subwavelength apertures

“…The original observation was made on silver, gold, and chromium films at optical frequencies ͑Ebbesen et al, 1998͒. In keeping with the previous theoretical discussion, the intensity of the transmission peak decreases and its linewidth increases as the absorption, which is related to the imaginary part of the permittivity of the metal, increases ͑Grupp et al, 2000; Przybilla, Degiron, et al, 2006;Williams and Coe, 2006;Chang, Chuang, et al, 2007;Ctitis et al, 2007;Ekinci et al, 2007͒. Exhaustive studies of the dependence of EOT peaks on the type of metal have been conducted both experimentally ͑Przybilla, Degiron, et al, 2006͒ andtheoretically ͑Rodrigo et al, 2008͒. Figure 33͑a͒ shows the evolution of the experimental EOT peak heights for different metals as a function of the resonant wavelength.…”

Propagation lengths of surface plasmon polaritons on metal films with arrays of subwavelength holes by infrared imaging spectroscopy

“…At 697 cm −1 , the 1 / e propagation distance is 17.8 m, which is larger than the 7 m of metal between holes, the 12.6 m lattice parameter ͑hole-centerto-hole-center distance͒, and the 14.3 m wavelength of the incident light. The 697 cm −1 frequency is close to the strongest transmission resonance on this mesh, [10][11][12]29 which occurs at 749 cm −1 at perpendicular incidence ͑k x =0͒. The Cassegrain optical system of the FTIR microspectrometer does not transmit rays on the optical axis.…”

“…28 Some did not like the larger shifts of resonance positions from zero-order, smooth metal predictions, however, these are now understood as due to coupling of SPs between the front and back surfaces of the mesh through the holes. [10][11][12]29 One can also observe transmission resonances due to modes other than SPs for "bad" SP metals ͓such as Cr, 30 27,36 and cathode luminescence. 37 These and related techniques run into challenges as they approach the IR region.…”

Plasmonics under the Infrared Microscope: Sensing and Spectra of Single Particles Using Metal Film Hole Arrays

“…A unified approach to dispersion diagrams for plasmonic resonances is presented that links standard dispersion measurements in the typical symmetries of square lattices (ΓX and ΓM as explored by rotating mesh about an axis within the mesh) [1][2][3][4][5][6][7][8][9][10] to dispersion by rotation of the mesh in the focal plane of a microscope (rotation about the optical axis of the microscope).…”