Generating Focused 3D Perfect Vortex Beams By Plasmonic Metasurfaces

Abstract

from the Fourier transformation of the Bessel-Gauss (BG) beam, [13,14] by utilizing a series of free-space bulky optical components including axicon, spiral phase plate, Fourier transform lens, and spatial light modulator. This fact greatly increases the optical system complexity and also hinders the device integration in OAM-based photonic circuits.Recently, plasmonic metasurfaces made of subwavelength nanoantenna arrays in thin metallic films have provided a powerful and efficient platform for manipulating the intensity, wavefront, and polarization of light beams beyond the diffraction limit. [26,27] In contrast to the bulky optical components with wavelengthdependent phase shift based on optical path difference, in the Pancharatnam-Berry phase optical elements (PBOEs), the PB phase in broadband operation originating from the geometric phase and accompanied polarization conversion is introduced. [28][29][30][31][32][33][34] The PBOE-based metasurfaces have been widely designed for wavefront shaping applications in making on-chip structured beam generators, [35][36][37][38][39][40][41][42][43][44][45] flat lenses, [46][47][48][49] ultrathin wave plates, [50][51][52] and holograms. [53][54][55][56][57] In this work, we present PBOE-based plasmonic metasurfaces made of rectangular-hole nanoantenna arrays as integrated optical beam converters for the direct generation of the focused 3D PV beams, without using bulky optical components, such as axicon, lens, and spatial light modulator. The PB phase profiles encoded on metasurfaces are uniquely designed with the combined phase distributions of axicon, spiral phase plate, and Fourier transform lens. The single ultrathin plasmonic metasurface device with compact area of 50 µm × 50 µm can create 3D PV beam in a long propagation distance and operate in a broad wavelength range from 600 to 1000 nm, which is useful in wavelength-multiplexed OAM-based optical fiber communication. For comparison, the previously demonstrated PB phase element system used to produce PV beams contains three in-sequence metasurface plates with efficient diameters ≥6 mm fabricated in glass boards, with the single operating wavelength at 632.8 nm. [13] It is shown that the obtained PV beams have the almost constant vortex ring radius and the same beam divergence for different TCs. We also demonstrate that the PV beam structures can be easily adjusted by varying several control parameters in the metasurface design including axicon period, lens focal length, and operation wavelength. Furthermore, multiple PV beams with Perfect vortex (PV) beams possessing annular intensity profiles independent of topological charges promise significant advances in particle manipulation, fiber communication, and quantum optics. The PV beam is typically generated from the Fourier transformation of the Bessel-Gauss beam. However, the conventional method to produce PV beams requires a series of bulky optical components, which greatly increases the system complexity and also hinders the photonic device integration. Here, ...

Optical Metasurfaces for Generation and Superposition of Optical Ring Vortex Beams

“…[22] axicon (x, y) = −2 √ x 2 + y 2 ∕d is the phase profile for the axicon, where d is the axicon period to control the radius of the generated ring OAM beam. [26,28] The relationship between d and an axicon angle can be expressed as d ⋅ sin = , where is the wavelength. Details of explanation are Figure 2.…”

“…A perfect vortex beam is the Fourier transformation of a Bessel Gaussian beam, which can keep the size of intensity pattern. [19] To generate perfect vortex beams like those in the references, [19,26] a phase profile of spherical wave for a Fourier transform lens with a focal length is necessary. By contrast, the phase term for Fourier transform lens is not needed in our design.…”

“…Because there is no lens, the resulting ring diameters change with the observation plane at different distances, so that the observation plane far enough away is suitable to see the ring with the naked eyes. Unlike conventional or perfect vortex beams, [26][27][28] the radius of the ring is only dependent on the axicon period [28,29] and the ring OAM beams in this work can thus be designed to stay constant with different TCs, enabling them to meet with each other and generate superposition for far field observation. As an application example, the superposition of ring OAM beams is used for polarization measurement.…”

Twofold Polarization‐Selective All‐Dielectric Trifoci Metalens for Linearly Polarized Visible Light

“…For instance, recent decades have witnessed immense research activities in an optical vortex (OV) beam which carries an orbital angular momentum and exhibits a helical wavefront with an azimuthal phase term exp(ilΦ), where Φ stands for the azimuthal angle and l represents the topological charge that determines the ring radius of the vortex intensity profile. [47] In general, a number of bulky optical components are usually engaged to generate an OV beam exhibiting a specific topological charge and one has to ceaselessly change some of the components for producing the OV beam with a different topological charge. Based on the proposed method, we embody a single metasurface which can provide three on-and off-axis focused optical vortex (FOV) beams carrying different topology charges.…”

Terahertz Angle‐Multiplexed Metasurface for Multi‐Dimensional Multiplexing of Spatial and Frequency Domains

“…Therefore, metasurfaces are believed to be a more efficient way to generate OAM beams, compared with the conventional methods, such as the spiral phase plate, [5] antenna array, [6] q-plate, [7] and fiber grating. [8] However, most of the reported research activities aimed at generating an OAM beam [9][10][11][12][13] or simultaneously multiple orthogonal OAM beams carrying the same information, [14][15][16][17][18] based on a single metasurface. It is found to be extremely challenging to enhance the information capacity with these methods.…”