Optical antennas have the prospect to redirect light rays into engineered directions at the subwavelength scale. They offer new options for photodetection, color routing, fluorescence emission and sensing applications. Previously, metallic nanoantennas based on localized surface plasmon resonance (LSPR) have commonly been exploited to fulfill this purpose, e.g. the gold Yagi-Uda array and split-ring resonator. However, the intrinsic ohmic losses of metals are large especially in the visible range, hindering further efficiency improvement for practical applications. In addition, the interaction of the metallic nanoantennas with the magnetic component of light is relatively weak, adding to their lack of highly tunable scattering directionality. In this presentation, we will demonstrate our recent experimental progress on all-dielectric nanoantennas made of amorphous silicon with tunable scattering directionality. Our nanoantennas are designed as V-shaped single element and fabricated using electron-beam lithography followed by dry reactive ion etching. It is illustrated that the scattering cross section of the silicon nanoantenna can be considerably higher than that of comparable Au antennas. In addition, the extinction coefficient of amorphous silicon is adequately low in the considered wavelength range, resulting in minimal absorption losses and an enhanced scattering efficiency. More interestingly, compared to Au nanoantennas that exhibit light scattering in a single particular direction, by carefully engineering the geometry of the silicon nano-antennas, their scattering can be effectively tuned into two opposite directions within the visible range (Supporting figure). Over a spectral range of less than 100 nm, the scattering directionality gradually shifts from the leftward to the rightward. More simulation results based on the finite difference time domain (FDTD) methods are available to perfectly match and corroborate our experimental measurement. Initial analysis of the underlying physics for the tunable scattering directionality will also be discussed. Such unique optical properties make the silicon nanoantenna promising candidates for novel low-loss optical devices that can enable unprecedented control over the scattering directionality.
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Study of self-shadowing effect as a simple means to realize nanostructured thin films and layers with special attentions to birefringent obliquely deposited thin films and photo-luminescent porous silicon