Study of the angular acceptance of surface plasmon Bragg mirrors

Surface plasmon based photonic devices are promising candidates for highly integrated optics. A surface plasmon (SP) is basically an electromagnetic wave confined in the interface between a metal and a dielectric, and is due to the interaction of the electromagnetic field with the surface bounded electron charges in the metal. A SP can propagate along the interface where it is confined (the propagation length being tens of micrometers in the visible range), but its associated electromagnetic field decreases exponentially in the perpendicular direction, in such a way that this vertical confinement makes SP very attractive for the design of optical devices in coplanar geometry. An important effort in the development of SP based photonic devices is necessarily dedicated to the design of systems allowing the control of SP propagation in two dimensions. Recently, it has been shown that Bragg mirrors consisting of gratings of metallic lines or indentations on a metallic surface are very efficient tools to perform this task. Up to now, the general performance of these systems has been characterized, but there are however important aspects in their behaviour that have not yet been analyzed. In this work we study, by means of leakage radiation (LR) measurements, the angular acceptance of these mirrors as a function of the angle of incidence. We have found that the angular acceptance decreases significantly when the angle for which the mirror is designed increases. To understand this fact, the dispersion relations of SP propagating on corrugated surfaces have been obtained by LR in the Fourier plane and compared to simulations based on the differential method. Our results show that the width of the bandgap is reduced for higher angles between the grating and the SP propagation direction.