We analyze the effective opening of finite bands of inhibited transmission in realistic systems excited by actual out-of-plane sources. We first observe how the excitation of surface plasmon polaritons in one-dimensional arrays of metal slits depends on the angle of incidence of the source field. Then, the well-known grating-coupling equation is revised in order to find an asymmetric structure with equivalent parameters which, under perfectly normal excitation, is able to exhibit surface plasmon polariton modes at the same wavelengths of the original structure which undergoes a nonorthogonal incidence of the light. In this way we demonstrate through finite-element simulations that a realistic system, probed by a source beam in a finite light-cone, can be effectively decomposed in several equivalent systems with different physical and geometrical parameters, with results in the enlargement of the theoretically expected punctual minimum of transmission. 1. Introduction In the last years, the excitation of collective oscillations of free electrons over metal interfaces has been deeply investigated in literature since it allows the successful control of light in terms of localization and enhancement of electromagnetic fields. As already known, the free-electron oscillations can assume several forms, ranging from localized surface dielectric oscillations on metal nanoparticles (localized surface plasmons, LSPs) to freely propagating electron density waves on metal surfaces (surface plasmon polaritons, SPPs). Surface plasmons show unique properties when they propagate over metal gratings and interact with free waves in the dielectric surroundings. The results of the fundamental research in the area of plasmonics have demonstrated that SPPs and LSPs can be exploited for many potential applications residing in sensing [1–5], light manipulation [6, 7] and emission [8–10], energy conversion [11–13], and optical nanoantennas [14, 15], just to mention a few. Many researchers have proved that under particular conditions many effects of enhanced and inhibited transmission emerge. For instance, subwavelength apertures carved on metal layers provide extraordinary optical transmission (EOT) caused by the resonant excitation of SPP modes over the input and the output interfaces between the metal and the dielectric [16, 17]. The in-phase cooperation among modes allows light beams impinging far from the apertures to assist the spectral response with results in normalized-to-area transmissions higher than one. Moreover, a second mechanism of resonance starts arising when
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