Abstract
Wavefront manipulation is of great importance in the development of science and technology, and the generation of vortex beam is one of the most effective methods to improve the channel capacity and communication accuracy. However, current available vortex beam generators suffer from complex structure, large size, especially narrow bandwidth. To solve these issues, we propose a general strategy to enhance the bandwidth of the vortex beam generator by using double-layered reflective metasurface. The well-optimized double-layered reflective element possesses dual resonance, which realizes complete phase coverage (larger than 360 degrees) and a flat slope of the reflection phase, resulting in a broad bandwidth (8.5-11.5GHz). A compact and low-profile vortex-beam generator is proposed by combining the parabolic phase and the spiral phase plate phase. Both near field and far field experiments are performed to demonstrate the predesigned effects. A pure vortex beam is observed clearly with a topological charge of m=1 in a wide frequency window of 3 GHz (8.5-11.5GHz). More importantly, the working efficiency of the vortex beam is better than 90% at center frequency 10 GHz. The findings in this paper motivate the realization of high-performance reflective metasurfaces and other functional metadevices.
Highlights
Vortex beam has raised great attention recently on account of its unique electromagnetic characteristics and has been applied widely in optics, atomic physics and communications.[1,2] vortex beam refers to a kind of beam carrying phase factor eilφ in optics, whose wavefront transmits forward with a helical property along the direction of propagation, and the center intensity is zero.[3,4] Since vortex beams with different topological charges are orthogonal to each other,[5] each mode can be employed to load signal independently and conveniently.[6]
Vortex beam is considered as an effective method to improve beam channel capacity[2,7] and the basis to achieve multiplexing orbital angular momentums (OAMs).[8]
The Vivaldi antenna is fixed and supported by the screws and foams, which have no influence on the performance of the device
Summary
Vortex beam has raised great attention recently on account of its unique electromagnetic characteristics and has been applied widely in optics, atomic physics and communications.[1,2] vortex beam refers to a kind of beam carrying phase factor eilφ in optics, whose wavefront transmits forward with a helical property along the direction of propagation, and the center intensity is zero.[3,4] Since vortex beams with different topological charges are orthogonal to each other,[5] each mode can be employed to load signal independently and conveniently.[6]. In 2011, Yu proposed the generalized Snell’s law by using the phase gradient metasurfaces (PGMSs) which can arbitrarily control the amplitude, phase and polarization of the electromagnetic waves.[16] PGMSs introduce a design freedom of phase gradient in wavefront manipulation and provide a new avenue to generate the vortex-beam. Despite of the great successes so far achieved, we note that vortex beams with wide bandwidth and high efficiency are far less explored, which limit the further applications of them To solve these issues, of this paper, we propose a new strategy to enhance the working bandwidth and working efficiency of vortex beams by using a double-layered reflective metasurface.
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