Full controls of OAM vortex beam and realization of retro and negative reflections at oblique incidence using dual-band 2-bit coding metasurface

This paper addresses a reflection-type dual-band 2-bit coding metasurface (CM) design to achieve the dual-band functionalities in two different operating bands, independently. We are particularly interested to control linearly polarized incident waves by encoding the propagation or dynamic phase in order to realize and fully control the orbital angular momentum (OAM) vortex beam (VB) for dual bands. In this regard, we perform digital addition operations to combine the coding sequences of traditional OAM (OAM that propagates in the normal direction) and phase-gradient to construct the proposed CM, which generates and steers OAM-VBs with different topological charges ($\pm l$) in both lower ($l_f$) and higher ($h_f$) bands. The proposed concept is further extended to realize multiple OAM-VB with various beam shaping by combining the coding sequence that generates OAM in the normal direction and the coding sequence of beam shaping, and we discuss three different scenarios. Firstly, split OAM-VB is generated in both lower and higher bands. Secondly, quad OAM-VB is realized to verify the proposed concept. Finally, yet importantly, we show that the reflected beam directions can be further independently controlled to achieve anomalous, negative and retro reflections at the two bands. Two CM samples are fabricated to experimentally validate the proposed OAM-VB generation with steering features and retro or negative reflections at oblique incidences, which are in good agreement with the simulation results. The proposed concept will open up possibilities of multifunctional meta-devices and will be useful in many applications such as optical spanners, quantum optics, spatial mode multiplexing for telecommunications, and astronomy etc. (paper)