We propose an apparatus that allows for active control of the transmission phase up to 360° through subwavelength-spaced metallic slits partially filled with indium tin oxide (ITO). Incident light is coupled to the guided mode in the metallic slit at one side. After going through the slit with a certain length, light is coupled out to free space at the other side. The transmission phase is governed by the mode index and the slit length. By applying bias to the ITO in the metallic slit, it is possible to control the mode index, which in turn leads to tuning of the transmission phase. The judiciously designed slit configuration facilitates the individual control of the relative phase between the neighboring slit with a subwavelength distance. This phenomenon is different from resonance-based metasurface approaches that suffer from limited range of the phase change. It is believed that the devised configuration may open novel promising future applications such as hologram imaging with phase spatial light modulators, light-field infrared microscopy, and beam forming and steering devices.
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