Abstract
Parallel sorting of orbital angular momentum (OAM) and polarization has recently acquired paramount importance and interest in a wide range of fields ranging from telecommunications to high-dimensional quantum cryptography. Due to their inherently polarization-sensitive optical response, optical elements acting on the geometric phase prove to be useful for processing structured light beams with orthogonal polarization states by means of a single optical platform. In this work, we present the design, fabrication and test of a Pancharatnam-Berry optical element in silicon implementing a log-pol optical transformation at 1310 nm for the realization of an OAM sorter based on the conformal mapping between angular and linear momentum states. The metasurface is realized in the form of continuously variant subwavelength gratings, providing high-resolution in the definition of the phase pattern. A hybrid device is fabricated assembling the metasurface for the geometric-phase control with multi-level diffractive optics for the polarization-independent manipulation of the dynamic phase. The optical characterization confirms the capability to sort orbital angular momentum and circular polarization at the same time.
Highlights
During the last decades, different solutions have been introduced and engineered in order to carry many signals through the same optical link
We present the design, fabrication and test of a Pancharatnam-Berry optical element in silicon implementing a log-pol optical transformation at 1310 nm for the realization of an orbital angular momentum (OAM) sorter based on the conformal mapping between angular and linear momentum states
By patterning the silicon substrate with a sub-wavelength digital grating, an inhomogeneous form-birefringence was induced, with the local fast-axis orientation equal to half the local geometric phase that should be transferred into the input beam
Summary
Different solutions have been introduced and engineered in order to carry many signals through the same optical link. Other types of phase-modulating mechanisms could be employed for this purpose such as those that transfer a geometric phase [13] This is achievable by moving from a patterned isotropic substrate to a spatially-variant anisotropic medium, as in the so-called Pancharatnam-Berry optical elements (PBOE), which are able to transfer the desired phase pattern by locally manipulating the polarization state of the impinging light. The implementation of such a total optical angular momentum sorter was reported, with custom-made liquid-crystal devices as the required PBOEs [23,24,25], exploiting the birefringence properties of the photo-aligned polymers Those demonstrations were limited to the visible range, and the fabrication technique was limited in resolution and not compatible with massproduction techniques. The use of silicon will enable the integration of metasurface devices into existing technology platforms, due to the mainstream industrialization and mature development of silicon in electronics, on-chip photonics, and microelectromechanical systems
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