Fig. 15.
(a) Schematic of the proposed tunable directional optical antenna. The subwavelength slit is supported with an array of periodic nanograting from the left-hand surface to confine and guide the confined energy of SPPs into one specific direction. εD, εM, and εSUM indicate the relative permittivity of surrounding medium, metal, and supporting substrate (glass slide), respectively. Directive radiation at an explicit angle θ can be reached by an appropriate choice of surrounding medium and operating wavelength λ. The directivity of the structure can be further improved by optimizing the illumination angle φ. (b) SEM of the fabricated device [50].