: ORIGINAL PAPER Dispersion Study of the Infrared Transmission Resonances of Freestanding Ni Microarrays Shaun M. Williams & James V. Coe Received: 24 February 2006 / Accepted: 10 March 2006 # Springer Science+Business Media, Inc. 2006 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 ex- traordinary 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 spec- trometer’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. Key words Surface plasmons in the infrared . Nickel bigrating . Subwavelength apertures . Extraordinary transmission in the infrared . Thinness splitting Introduction The study of the optical properties of subwavelength apertures has become Bone of the most exciting areas in optics research^ [1]. In pursuit of such studies, it is noteworthy that freestanding nickel microarrays can be purchased commercially that exhibit Ebbesen’s extraordinary transmission effect [2,3] in the infrared (IR) region of the electromagnetic spectrum [4,5]. A surface plasmon (SP)-mediated mechanism is general- ly cited as the means by which this increased trans- mission occurs. The biperiodic grating nature of the mesh couples with the incident light to produce an SP type of polariton [6] (oscillation of the metal’s conducting electrons at the surface of the metal). The SPs propagate along the surface of the mesh, perhaps on both sides, to another hole where they decouple, producing photons that emerge with the same direc- tion as the incident beam. Studies by other researchers have been made concerning ultrafast behavior [7], entangled photon [8], polarization effects [9], and SP propagation between separated microarrays [10]. These studies indicate the fundamental nature of the phenomena as a single-photon process [8] that can preserve phase [7] and polarization [11]. The present studies are interesting because the dielectric properties of metals are very different in the IR than in the visible region where much SP work occurs [9,12–15]. The effect on IR transmission of aperture shape and film thickness of metallic micro- arrays has also been studied [16–19]. The IR region has energies that match fundamental molecular vibrations, and IR absorption enhancements of more than 100 times have been reported for molecular surface species using these new phenomena [4,20–22]. Data recorded in this region can serve as a rigorous test of theories describing SP-mediated extraordinary transmission. The complex dielectric properties of metals can support some interesting surface states at the interface between the metal and a dielectric. The dielectric constant of a metal is complex " m ¼ " 0 m þ i" m 00   and SPs can be supported [6] on a semi-infinite, smooth air/ metal interface if " 0 m     >" m 00 . This condition is true for silver, gold, and copper at wavelengths longer than Plasmonics (2006) DOI 10.1007/s11468-005-9001-4 S. M. Williams : J. V. Coe (*) Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, OH 43210-1173, USA e-mail: coe.1@osu.edu NO59001; No. of Pages 7 Springer