Comparison of Active and Passive Approaches for Controlling the Near-Field Optical Path of Guided-Light Wave*

Both active and passive approaches are proposed and compared for controlling the optical path of $p$-polarized light wave guided through a surface-patterned metallic structure with sub-wavelength features. For active control, the dynamical role of photo-excited electrons in a slit-embedded atomic system with field-induced transparency (FIT) is demonstrated for modulating transmitted-light intensity in the near-field region. Additionally, the strong coupling between the optical transitions within slit-embedded FIT atoms and the surface-plasmon modes in a metallic slit array is found. For passive control, on the other hand, a geometrical effect is demonstrated for focused transmitted light passing through a Gaussian-shaped metallic lens embedded with an array of slits. This geometrical effect is further accompanied by a swing of the light-focusing pattern in the near-field region as the incident angle is increased, as well as by the reduction of an anomalous light refraction due to higher-order diffraction modes at longer wavelengths and larger incident angles.