Generation of Radial Polarized Lorentz Beam with Single Layer Metasurface

Abstract

as well as ultrafast phenomena sensing. [11] Practical applications for nondiffracted waves include light collimating, focusing, and shaping. [12][13][14] The Lorentz beam is a nondiffracted wave that is characterized by small size, long focal length, and extended transmission distance. [12] Typical techniques to produce Lorentz beams usually involve combinations of multiple optical elements. However, such systems are bulky, complicated, and inconvenient. To generate such novel beams simply, we employ a single layer metasurface device to modulate simultaneously the amplitude and polarization of the illuminating light. Metasurfaces are viewed as a kind of artificial material composed of periodic subwavelength metal or dielectric structures. When coupled resonantly to the electric and/or magnetic components of an incident electromagnetic field, they provide a spatially varying optical response (e.g., scattering amplitude, phase, and polarization). With the advantage of easy-etching, ultrathin as well as miniaturization, a broad range of applications of metasurfaces has been reported, including superlenses, [15,16] wave plates, [17][18][19] nonlinear optics, [20] and holograms. [21] However, these previous studies were focused on modulating one attribute of light beams, for example amplitude, phase or polarization. Several studies were reported to have modulated the phase [22] and polarization direction. [23] Only numerical simulations are carried out to demonstrate the ability of metasurface for multiple parameters modulation because double-layer structures [24] are difficult to fabricate. Traditionally, the concentric metal-ring wire grid is used to achieve radial polarization. [25][26][27] The structure is relatively simple but fails to generate amplitude modulation. Furthermore, circularly polarized light is required to generate radially polarized light; nevertheless, it comes accompanied by an unexpected vortex phase distribution. In this work, a designed metasurface is fabricated consisting of crosses of various sizes and orientations. The variation in size is employed to modulate the amplitude whereas variation in orientation is used to modulate the polarization distribution. With right/left circularly polarized (RCP/LCP) incidence, the output polarization may be radial or angular, whereas the amplitude obeys the Lorentz distribution. Nine cross structures were fabricated to achieve certain desired amplitude and polarization modulations. The polarization and amplitude distributions of the beam generated by the device are detected using a terahertz focal plane imaging system. A strong agreement between theoretical and experimental results was

Metasurfaces for Terahertz Wavefront Modulation: a Review

“…8 THz metasurfaces for special beams generation. a Designed metasurfaces for generating vortex beams with the topological number of l = 2 and l = 3 and the measured phase distributions [59], b layout of the designed metasurface for ring-Airy beam generating and the measured intensity distribution of the beam on the initial plane [60], and c scheme of the metasurface for generating the THz radially polarized Lorentz beam and the measured radially polarized Lorentz beam at 0.75 THz [61] antenna are employed to generate the desired wavefront of the ring-Airy beam in its initial plane. In the experiment, the THz ring-Airy beams with different parameters were generated, and the self-focusing of the ring-Airy beams was observed during the propagation process.…”

Terahertz Metasurfaces: Toward Multifunctional and Programmable Wave Manipulation

“…In contrast to traditional passive terahertz devices, metasurfaces possess superiorities like ultra-thin profile, remarkable lightness, easy fabrication, acceptable loss, and the tremendous potential to manipulate terahertz waves freely. Up to now, passive metasurfaces have been utilized to achieve beam deflection [17][18][19], beam forming [20][21][22], object cloaking [23], focusing [24][25][26], absorbing [27], and polarization converting [28][29][30], and so on. In this part, we focus on the most frequently used functions, for instance, beam deflection and beam forming.…”

“…For example, Lorentz beam can be used in a wide range of scenarios due to its non-diffraction characteristic, yet multiple optical elements are required to generate it. In 2017, Guo et al demonstrated a radially polarized Lorentz beam under circular polarized incidences in the terahertz spectrum utilizing a singlelayer metasurface composed of cross-shaped unit cells [22]. The schematic diagram of generating Lorentz beam is shown in Figure 3B, provides different phase responses, amplitude responses and polarization conversions by modulating typical sizes of the cross pairs and orientation angle θ .…”

Graphene-based tunable coding metasurfaces in terahertz band

“…Currently, more and more metasurfaces for control THz waves is presented, for example, wave plate [8], hologram [9], vector beam [10] and generation of non-linear harmonic wave [11]. However, most of these proposed terahertz metasurfaces with fixed performances after the design is completed, which limits their application.…”