Elsevier

Optical Materials

Volume 121, November 2021, 111574
Optical Materials

Research Article
Broadband transparent terahertz vortex beam generator based on thermally tunable geometric metasurface

https://doi.org/10.1016/j.optmat.2021.111574Get rights and content

Highlights

  • A reconfigurable InSb metasurface is proposed to generate vortex beams in terahertz region.

  • The operating frequency of InSb metasurface can be reconstructed by changing the external temperature.

  • Generated vortex beams have high mode purity over a broadband.

Abstract

Metasurfaces have been successfully employed for realization of orbital angular momentum (OAM) waves, however, current available vortex beam generators suffer from low efficiency and limited bandwidth. In this study, we propose a transparent geometric metasurface composed of InSb cylindroid to generate vortex beams in terahertz region. The operating frequency can achieve broadband coverage from 1.8 THz to 4.5 THz, with a fractional bandwidth of 85.7%, by changing the temperature of the InSb cylindroid from 220K to 340K. Based on geometric phase principle, the full 2π phase shift can be obtained at the operating frequency by rotating the cylindroid. Vortex beam generators with topological charges of +1 and −2 are designed over a broadband range by employing the InSb cylindroid with different temperature. The results indicate that the generated vortex beams have high mode purity at different frequencies. The proposed vortex beam generators can reconstruct the operating frequency by directly changing the external temperature without altering the size of device, which show tremendous potential in wireless communication system.

Introduction

Vortex beams have been gained tremendous interests in the past decades owing to its intriguing possibilities to alleviate the contradiction between the spectrum shortage and increasing demand of wireless service. The transmission capacity of the communication link using the traditional plane electromagnetic (EM) wave has approached Shannon limit [1]. Vortex waves carried orbital angular momentum (OAM) have helical phase profile, and different OAM modes are theoretically orthogonal. Therefore, multiplexing information can be transmitted over different modes of OAM at the same frequency, which provides new freedom for information transmission and more possibilities of high communication capacity [[2], [3], [4]]. Acquirement of effective vortex beams with various OAM modes is an important task in practical application. Nowadays, various methods have been reported to generate vortex beams, such as antenna array [5], spiral phase plate [6,7], subwavelength gratings and computer-generated holograms [8]. However, these approaches suffer from the issues of low efficiency, complex configuration and huge cost, which limit their practical application. The emergence of metasurfaces provide a new approach for generating vortex beams. Metasurface, which is the 2D counterpart of metamaterial, consisting of arrays of subwavelength elements, possess unique capability to manipulate the amplitude, polarization and phase of EM wave [[9], [10], [11]]. Metasurface with a thickness of subwavelength is capable of tailoring EM wavefront discretionarily by introducing discontinuous abrupt phases at the interface, replacing the traditional method of phase accumulation in space. In 2011, Yu et al., firstly proposed a metasurface composed of diverse V-shaped gold nanoantennas to deflect the flow of reflected and refracted EM waves, rewriting the traditional Snell's law. In addition, they designed a plasmonic interface to generate a vortex beam with topological charge equal to 1 by spirally arranging the eight constituent nanoantennas [12]. Thereafter, various metasurfaces able to generate phase gradient have been proposed, which are used for the focusing of lenses [[13], [14], [15]], high-resolution holograms [16] and other functions [17,18] besides vortex beam generation.

In general, there are two kinds of design schemes of metasurfaces for wavefront manipulation. One is transmission phase metasurface, which introduces the local phase shift by changing the size of the unit cell and achieves the coverage of full 2π phase at a specific frequency [[19], [20], [21]]. The other is geometric metasurface, also called Pancharatnam-Berry (P-B) metasurface, which has the capabilities of controlling the phase and polarization of circularly polarized (CP) waves by rotating the subwavelength resonators [[22], [23], [24], [25], [26], [27]]. Compared with transmissive phase metasurfaces, geometric phase metasurfaces are widely concerned since it's non-dispersive, polarization dependent and easy to fabricate. Actually, the transmissive geometric metasurfaces with single-layered metal structures can achieve full 2π range phase control at a wide band, however, their efficiencies are less than 25% due to the astricts of single-layered metal structure [28]. To improve the efficiency, transmissive metasurfaces with multilayer metal structures and reflective geometric metasurfaces are widely studied [[29], [30], [31], [32], [33]]. Li et al. designed a terahertz vortex beam generator based on reflective geometric metasurface, which generated OAM having high mode purity from 0.3 THz to 0.45 THz, with a fractional bandwidth of 40% [29]. Gao et al. proposed a transmissive geometric phase metasurface to generate high performance vortex beams working from 9.3 GHz to 12 GHz [30]. Tang et al. experimentally demonstrated a high-efficiency transmissive vortex beam generator based on geometric metasurface, which exhibited a maximum efficiency of 87% at 10.6 GHz [31]. These researches indicate that the transmissive geometric metasurfaces have a narrower bandwidth than the reflective ones. In addition, the operating frequencies and functionalities of these metasurfaces are fixed once their physical sizes are decided. To achieve wavefront manipulation of more freedom, various novel devices based on tunable and reconfigurable metasurfaces have been proposed [[34], [35], [36], [37]]. Kim et al. experimentally demonstrated a gated-graphene metasurface to dynamically modulate the efficiency of transmitted wave [34]. Liu et al. proposed a VO2 integrated metasurface to realize active meta-holograms by changing the temperature of VO2 [35]. In addition, as a typical semiconductor with high electron mobility, InSb exhibits excellent electron transport performance under the excitation of electric field and is strongly dependent on temperature. Consequently, it has become a hot material of THz tunable devices recently. Yang et al. proposed a thermally tunable THz metasurface absorber based on InSb film [38]. Tan et al. proposed an active geometric metasurface by combining InSb and P-B elements [39]. However, these active metasurfaces are widely used for meta-lenses, absorbers and meta-holograms, but rarely for vortex beam generators in terahertz region.

In this paper, a thermally tunable vortex beam generator is proposed in terahertz region. It is composed of InSb cylindroid with different rotation angle adhered on the polyimide substrate. Based on geometric phase principle, the complete 2π range phase coverage can be achieved by rotating the cylindroid. Thus, the incident CP plane wave can be converted into the desired vortex beams by properly arranging InSb elliptical cylinders. In addition, the operating frequency will move from 1.8 THz to 4.5 THz, with a fractional bandwidth of 85.7%, when the temperature changes from 220K to 340K. The phase, the intensity, the far-field patterns and mode purities of vortex beams with different OAM modes (topological charges of +1 and −2) at five sampling frequencies were obtained through numerical simulation. All these results show that high quality OAM can be created at a broadband just by changing the external temperature.

Section snippets

Structure design and theoretical analysis

Fig. 1(a) shows the functional schematic diagram of the proposed vortex beam generator, which consists of an array of unit cell shown in Fig. 1(b). It can be expected that the normal incident left-handed circularly polarized (LCP) plane wave can be transformed to the transmitted right-handed circularly polarized (RCP) vortex wave in terahertz region. As seen in Fig. 1(b), the unit cell consists of InSb cylindroid and polyimide substrate (with relative permittivity of ε = 3.5). It is noted that

Results and discussion

A vortex beam carrying OAM with a topological charge m has a phase distribution of eimϕ in the transverse plane, where ϕ is the azimuthal angle. To create the spiral phase profile, the unit cells with a similar varying pattern are arranged in xoy plane, and the required phase distribution of each unit cell at position (x, y) as follows:ϕm(x,y)=mtan1(yx)

To simplify the design, the device is divided into N triangular regions and the phase distribution of each region can be rewritten as [29]:ϕm(x

Conclusion

In summary, a thermally tunable vortex beam generator based on geometric phase metasurface has been proposed in terahertz region. The metasurface is composed of InSb cylindroid and polyimide substrate, and the complete 2π phase coverage of transmitted cross CP wave can be obtained by rotating the InSb cylindroid. Vortex beam generators with topological charge m = +1 and m = −2 have been designed by arranging the InSb cylindroid properly. The simulated results indicate that the operating

CRediT authorship contribution statement

Qili Yang: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing – original draft, Formal analysis. Yan Wang: Methodology, Formal analysis, Writing – review & editing, Visualization. Lanju Liang: Funding acquisition, Project administration. Maosheng Yang: Data curation, Resources.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by National Natural Science Foundation of China (61701434), Natural Science Foundation of Shandong Province (ZR2020KF008). Youth Innovation Technology Project of Higher School in Shandong Province (2019KJN001).

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      As shown in Table 1, most of the components in the previously published reports operating in reflection mode. Moreover, the vortex beam generator shown in the reference is mainly concerned with the active control of the focal length [32,42–45], while our work not only has dynamic modulation characteristics, but also makes the generated vortex beam have vectorial characteristics. In addition, the proposed design also has the switchable property of inhomogeneous polarization states in the longitudinal direction compared to references, further enhancing its application in meta-optical systems.

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