Conventional wave plates for polarization control are based on birefringent materials or nanostructures that assign real-valued phases [1,2]. Recently, a new regime of complex-valued birefringence was suggested , extending the notion of real-valued birefringence through the specially introduced polarization-sensitive loss or gain. However, this theoretical approach was based on a complicated metamaterial with loss and gain, which remains inaccessible for fabrication.
We develop and experimentally demonstrate a practical approach for optimal implementation of complex-birefringent wave plates with metasurfaces. We design dielectric metasurfaces incorporating pairs of nanoresonators, giving rise to engineered polarization-dependent diffraction, which effectively introduces only the minimally necessary amount of loss to achieve the desired unconventional polarization transformation.
We fabricate such metasurfaces, characterize the transmission of various polarization states, and demonstrate two representative applications. First, we show that a metasurface can transforms a pair of nearly identical polarization states into orthogonally polarized ones, which can be used for improving polarization detection sensitivity and quantum state discrimination. In contrast, real-valued birefringence cannot change the angle between pairs of polarization states. Second, we develop a metasurface implementing polarization coupling with an unconventional relative phase, which can strongly affect the quantum interference of photons and control their coherent attenuation.
The complex-birefringent metasurfaces can facilitate novel types of polarization manipulation and measurement with optical beams and quantum photon states.
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