Abstract
In this paper, a novel method for generating a circularly polarized (CP) quasi-non-diffractive vortex wave carrying orbital angular momentum (OAM), based on the microwave holographic metasurface integrated with a monopole, is proposed. This method is the combination of the non-diffraction theory and the principle of waveguide-fed-based holography and is equivalent to a superposition of two scalar impedance modulation surfaces. To verify the proposed method, a holographic metasurface generating a left-handed circularly polarized (LHCP) quasi-non-diffractive vortex wave carrying −1 mode OAM at the normal direction, was simulated and analyzed. The metasurface consisted of inhomogeneous slot units on a grounded substrate and a monopole excitation. Moreover, the location distribution of slots was determined by a computed interferogram between the reference wave and the object wave with the non-diffractive feature. Compared with an ordinary vortex wave, the quasi-non-diffractive wave obtained by our proposed method possessed a smaller divergence radius and a stronger electric field strength in the 9 times wavelength range. It paved a new path for manipulating the non-diffractive vortex wave in medium distance without using an external feeding source, which holds great potential for the miniaturization devices applied in medium-distance high-capacity secure communication, high-resolution imaging and intelligent detection.
Highlights
The vortex waves with orbital angular momentum, known to have the potential for increasing the capacity of a channel largely due to their infinite species of orthogonal topological modes, have attracted tremendous interest from different fields in recent years.the vortex wave communication is highly limited in realistic applications due to its inherent character of divergence in the propagation path
We propose an innovative methodology for generating a circularly polarized (CP) quasi-nonIn this work, we propose an innovative methodology for generating a CP quasi-nondiffractive vortex wave with the desired orbital angular momentum (OAM) mode by combining non-diffracting theory diffractive vortex wave with the desired OAM mode by combining non-diffracting theory and the principle of waveguide-fed-based holography
By superposing x- and y-polarized holograms independently. Combining this method and the phase profile of the quasi-non-diffractive vortex wave obtained from Equation (1), the analytical expression of a required object wave PCPBOAM can be expressed as follows: PCPBOAM = Pobjx + Pobjy = A0 e jΦ( x,y) ( x + jŷ), (2)
Summary
The vortex waves with orbital angular momentum, known to have the potential for increasing the capacity of a channel largely due to their infinite species of orthogonal topological modes, have attracted tremendous interest from different fields in recent years. Lv et al proposed a low-profile transmitting metasurface using the dimension extension approach to achieve linearly polarized vortex waves in Ku-band [16], which facilitated the vortex wave-based communication applications These OAM modulators exhibited powerful control on the amplitude, phase and polarization of the spiral waves, the problems of the beam divergence were not considered and well addressed. The non-diffractive vortex-wave metasurfaces mentioned above provided profound insights into the generation methodologies; they were excited by the externally spatial feeding sources, which occupied large spaces and caused the spatial feed error To improve these issues, a tensor impedance modulation-based metasurface integrated with a feeding source was proposed by Meng et al [21], which reduced the space-occupied volume greatly and provided efficient control for the high-order Bessel vortex beams.
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