2, Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California, United States
Polarization is an important characteristic of electromagnetic waves that has a significant impact on number of applications such as molecular analysis, sensing, and quantum communications . Conventionally, the polarization state of an electromagnetic wave is tailored via propagating the wave through a birefringent crystal or polymer . As a result, the optical elements that perform polarization conversion are typically bulky. Alternatively, the polarization conversion can be achieved by using low-profile nanophotonic components based on metasurfaces. Metasurfaces are ultrathin nanophotonic structures composed of artificially designed arrays of optical scatterers, which introduce abrupt changes to the amplitude and phase of the scattered light within a subwavelength spatial region .
Here, we demonstrate that the polarization state of the reflected light can be actively controlled by using indium tin oxide (ITO)-based tunable metasurfaces. The proposed metasurfaces consist of an aluminum back reflector, a 20-nm-thick gate dielectric layer followed by a 5-nm thick ITO layer on which we fabricate an aluminum nano-antenna array. The period of the suggested metasurface is 400 nm while the operation wavelength is 1580 nm. When applying an electrical bias between the ITO layer and back reflector, the carrier concentration at the gate-dielectric/ITO interface is modulated, resulting in the change of the effective index of the ITO layer . The epsilon-near-zero (ENZ) mode, which is accessed under applied external DC bias, alters the interaction between the induced plasmonic modes (which corresponding to the orthogonal polarization components), leading to the modulation of polarization state of the reflected light. By suitably biasing the metasurface structure, the linearly-polarized incident light can be converted to a cross-polarized, circularly-polarized or elliptically-polarized light. This dynamic control of the amplitude, phase as well as the polarization state of the scattered beam provides prospects for various applications, such as dynamic wave plates, spatial light modulators, adaptive wavefront control, signal monitoring and detection.
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