Enhancement of light transmission through nanoslit in a circular metallic film flanked by parallel periodic gratings
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Enhancement of light transmission through nanoslit in a circular metallic film flanked by parallel periodic gratings Dinesh Kumar, and Kiyoshi Asakawa Center for Tsukuba Advanced Research Alliance (TARA), University of Tsukuba, 1-1-1 Tennodai, Ibaraki, Tsukuba, 305-8577, Japan ABSTRACT We have performed finite-difference time-domain (FDTD) analysis of optical transmission through a nanoslit surrounded by a finite array of gratings formed on a silver film, embedded in a high index dielectric medium. The investigation result shows that the transmission of the structure is strongly influenced by thickness of the dielectric film. In addition, we investigate the directional properties of radiated beam from the structure. It is demonstrated and explained that the directionality can be improved by tapering height of the grating elements on the exiting plane. We propose that the structure could be used as a plasmonic antenna for axial injection of light from single mode fiber to photonic crystal waveguide. INTRODUCTION The directional beaming property of subwavelength metallic apertures patterned with periodic corrugations make them interesting candidates for a variety of optical applications. Due to excitation of surface plasmons, the light flux per unit area through the apertures is 5-10 times greater than the flux per unit area of the incident beam [1-2]. However the total transmittance of the reported structure is poor. It is thus important to enhance their transmittance and improve directionality for efficient utilization in light emitting devices such as surface plasmon based optical antenna. The present research has been carried out with a motive to design a surface plasmon (SP) antenna which can efficiently couple light from a single mode (SM) fiber to an air bridge type two-dimensional photonic crystal waveguide (2DPC-WG) [3]. We study the transmission and directional properties of a plasmonic structure which consists of a nano slit surrounded by a finite array of gratings formed on a silver film, embedded in a high index dielectric medium. We performed FDTD analysis of its transmission properties; it was found that the transmission of the structure is strongly influenced by thickness of the dielectric film. High directionality is an important requirement of the plasmonic structure to work as an antenna for injecting light from SM fiber to PC-WG. We propose a novel method, tapering height of grating elements on the exiting plane for improving directionality and demonstrate its effectiveness by numerical calculations. This phenomenon has been explained by standard theory of antenna array. To our knowledge this is the first study on the effects of dielectric film dimension and grating taper on the transmission and directionality of such plasmonic structures. ANALYSIS AND SILUMATIONS Transmission properties We are interested in analyzing the transmission properties of the system depicted in figure 1a: a single slit flanked by surface gratings formed on a silver film, embedded in a high index dielectric m
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