M. Csete, A. Szalai, E. Tóth, A. Somogyi, J. Balázs, G. Szabó
University of Szeged, Hungary
pp. 263 - 267
Keywords: multilayer, grating, conical mounting, cross-coupling, bio-detection
Theoretical study was performed on rotated grating coupling phenomenon on multilayers comprising 416 nm periodic sinusoidal polymer gratings with 35 nm modulation amplitude aligned on a bimetal film made of gold and silver layers. The polar and azimuthal orientations were varied during 532 nm wavelength p-polarized light illumination. Double minima appear at two azimuthal orientations, namely at gamma=0° azimuthal angle (P-orientation) and at gamma~30° azimuthal angle (C-orientation). In P-orientation the first minimum is observable at fi=52° and the second minimum appear at fi=61°. At the first minimum the E-field is dominantly concentrated at the glass side with p/2 periodicity, while there is a weaker E-field on the polymer side as well, resulting in local enhancement at the edges of the valley (Fig 1aa, ab). At the second minimum enhanced E-field appears at both the glass and polymer side with p periodicity, however the E-field is strongly concentrated on the hills at the polymer side and weakly enhanced below the valleys at the glass side (Fig 1ba, bb). The field distribution is symmetrical in the longitudinal component both at the first and second minimum revealing that short range plasmons are coupled at both minima in P-orientation (Fig 1ac, ad and Fig. 1bc, bd ). In C-orientation the strong coupling between modes results in secondary and primary resonance minima at gamma=30° azimuthal orientation at fi=57° and fi=63° incidence angles. At both minima the E-field is concentrated with p periodicity at the polymer-side asymmetrically at the edge of the valleys (Fig 2aa, ab and Fig 2ba, bb). However, there is a well defined difference between the field distributions of the longitudinal component, which is vertically asymmetric/symmetric at the secondary/primary minimum proving the existence of a long-/short-range grating coupled modes (Fig 2ac, ad / Fig 2bc, bd). The dispersion characteristics of the reflectance curves indicate anti-crossing characteristics revealing that coupled modes coexist both in P- and C-orientations (Fig 1 and 2). The strongest coupling appears at the Brillouin zone boundary independently of azimuthal orientation. The mode appearing at the first minimum in P-orientation corresponds to the eigenmode of the multilayer, which propagates in the valley. This eigenmode appears due to the coupling of the incoming light on the grating in first order. Although, the normalized electric field is very similar at the secondary and primary minima in C-orientation, both the field distribution at and the origin of the secondary minimum is unique. The secondary minimum originates from a secondary grating scattering process on the wavelength-scaled grating, which is capable of coupling into a mode propagating along the valley and having a mode index similar to that of the eigenmode observable at the first minimum in P-orientation. However, due to the antisymmetric longitudinal component this mode does not suffer attenuation, has long propagation distance, accordingly can be considered as a long-range SPP. The secondary minimum in C-orientation is proposed for biosensing, since the biomolecules prefer to attach into the valleys, where the rotated grating coupling phenomenon results in E-field enhancement with unique distribution.