Beam-steering at optical frequencies using metal-grating antennas

摘要

The use of grating antennas to produce a directional radiation beam is well-established at millimetre wavelengths [1]. Directional emission at optical frequencies also has some history [2,3] and has recently come to the fore in transmission experiments using sub-wavelength apertures in metal films [4]. Although these experiments in the visible domain have been primarily interpreted using k-vector analysis of surface plasmons and photon waves, the link to antenna theory has been explicitly made by Oliner and Jackson [5]. In this work we present results on beam steering of red light using sub-wavelength metallic gratings and show that highly directional beams with controlled emission angle and divergence can be achieved. We adopt a simple dipole array model to describe the photon-plasmon interactions and show that this describes all of the main features seen in the experimental data. The classic k-vector matching condition for light to surface plasmons can also be derived from this antenna model with the assumption of dipole-dipole coupling via surface plasmon waves. A schematic and a top-view image of the experimental platform are shown in figure 1. A continuous 40nm thick Au film is evaporated onto a glass cover slip. Selected areas of this film have periodic corrugations with a 30nm step height produced by e-beam lithography. These are line gratings with 200nm wide features and of varying pitch. The glass is used as a multi-mode waveguide into which light from a diode laser (1 = 630 nm) is close-coupled. This arrangement uses the guided light (x-direction) to optically excite surface plasmon modes within the metal film and so study the effect of the periodic grating structure in producing out-coupled radiation from the top-surface. Although the basic physical interaction process is the same as in the experiment of Lezec et al [4] this experiment differs in that the optical excitation of the surface plasmons is over the whole grating rather than at the localised position of an aperture. When viewed from above there is strong emission from the gratings at a well-defined angle relative to the surface. By taking a series of images at increasing distance from the metal surface (z-direction) we have re-constructed the beam profile in the x-z plane. A number of highly directional beams are produced whose number and emission angle are controlled by the grating pitch. Examples for pitches of 450, 550 and 580 nm are shown in figure 2. The angle of emission relative to the surface, and the number of beams, increase as the grating pitch increases.