Dielectric multi-momentum meta-transformer in the visible

Abstract

Metasurfaces as artificially nanostructured interfaces hold significant potential for multi-functionality, which may play a pivotal role in the next-generation compact nano-devices. The majority of multi-tasked metasurfaces encode or encrypt multi-information either into the carefully tailored metasurfaces or in pre-set complex incident beam arrays. Here, we propose and demonstrate a multi-momentum transformation metasurface (i.e., meta-transformer), by fully synergizing intrinsic properties of light, e.g., orbital angular momentum (OAM) and linear momentum (LM), with a fixed phase profile imparted by a metasurface. The OAM meta-transformer reconstructs different topologically charged beams into on-axis distinct patterns in the same plane. The LM meta-transformer converts red, green and blue illuminations to the on-axis images of "R", "G" and "B" as well as vivid color holograms, respectively. Thanks to the infinite states of light-metasurface phase combinations, such ultra-compact meta-transformer has potential in information storage, nanophotonics, optical integration and optical encryption.

Fig. 1

Comparative elaborations of multi-tasked metasurfaces (a–c) and our multi-momentum meta-transformer (d–h). a Schematic illustration of a spatially multiplexed metasurface^–. The spatial multiplexed metasurface reconstructed several distinguished field distribution ${\mathbf{U}}^{\left(\mathit{n}\right)}$ based on the spatial separations of metasurface $\left(S_0^{\left(n\right)}\right)$ or observed region (S⁽ⁿ⁾). The x₀ and y₀ (x, y, and z) are the coordinated variables of the metasurface plane S₀ (observation region S). b Illustration of a structurally multiplexed metasurface^–,,. The structurally multiplexed metasurface reconstructed several distinguished field distribution ${\mathbf{U}}^{\left(\mathit{n}\right)}$ based on the metasurface function ${\mathbf{U}}_{\mathbf{meta}}^{\left(\mathit{n}\right)}$. c Schematic illustration of a multi-beam meta-opener. The multi-beam meta-opener is formed by several kinds of “key arrays”, which is used to read out the distributions ${\mathbf{U}}^{\left(\mathit{n}\right)}$ carried by the preset incident beams. d Illustration of transmission-type multi-momentum meta-transformer. The multi-momentum meta-transformer decoder phase profile is implemented with TiO₂ nano-fin array with in-plane orientations on a quartz substrate. It controls multi-beams with different momenta (l⁽ⁿ⁾ and $k_0^{\left(n\right)}$) to reconstruct corresponding field distributions ${\mathbf{U}}^{\left(\mathit{n}\right)}$ Schematic of the OAM meta-transformer. Under the illumination of vortex beams with right circular polarization (RCP), the decoder can generate distinct images at the same plane with left circular polarization (LCP). f Schematic of the LM meta-transformer. Under the illumination of different LM beams with RCP, the meta-transformer can generate distinct LM-dependent field distributions at the same region on optical axis. g Top-view scanning electron microscopy (SEM) images of a partial region of the fabricated TiO₂ nano-fins arrays. Scale bar: 2 μm. h Oblique-view SEM image. Scale bar: 2 μm. Each TiO₂ nano-fin represents a phase pixel as defined in the meta-transformer

Fig. 2

Principle and demonstration of multi-momentum meta-transformer. a Geometry of the designed unit cell structure representing one pixel in the meta-transformer, with the periodicity of 325 nm. The TiO₂ nano-fin parameters are w = 80 nm, l = 250 nm, and h = 600 nm. The in-plane rotating angle φ of nano-fin will introduce the geometric phase of 2φ for the incident beam with RCP. b Measured conversion efficiency of the meta-transformer. The conversion efficiency is defined as the optical power of the transmitted light with opposite CP divided by the incident optical power. c Design principle of OAM meta-transformer. Under the illumination of vortex beam 1 (l⁽¹⁾ = −5) with RCP, the transmitted beam with opposite CP carries the total phase profile ${\psi }_{\mathrm{OAM}=-5}\left(x_0,y_0\right)+{\psi }_{\mathrm{meta}}\left(x_0,y_0\right)$ and reconstructs “apple” pattern in the observation plane. When using vortex beam 2 (l⁽²⁾ = 5) with RCP, the total phase profile of the transmitted beam is ${\psi }_{\mathrm{OAM}=5}\left(x_0,y_0\right)+{\psi }_{\mathrm{meta}}\left(x_0,y_0\right)$, which causes the reconstructed pattern change to a spider-shaped pattern. d Simulated (top) and measured (bottom) reconstructed patterns by vortex beam 1 (l⁽¹⁾ = −5) (left) and vortex beam 2 (l⁽²⁾ = 5) (right). Scale bar: 20 μm. e Design principle of LM meta-transformer. Under the illumination of right circularly polarized beam with $\mathrm{LM}=k_0^{\left(\mathrm{R}\right)}ħ$, the transmitted beam with opposite CP carry the dispersionless phase profile of metasurface ψ_meta (x₀, y₀). Due to the convolution of ${\mathbf{U}}_{\mathbf{inc}}\left(x_0,y_0\right)exp\left(i{\psi }_{\mathrm{meta}}\left(x_0,y_0\right)\right)$ and impulse response $h\left(x,y,z,k_0^{\left(\mathrm{R}\right)}\right)$, the transmitted beam reconstructs patterns at the observation plane. Because the impulse response h is k₀-dependent, by changing LM of incidence, the reconstructed images “R”, “G” and “B” components can be shifted to one identical plane (z = Z₀). f Simulated (top) and experimental (bottom) reconstruction of three-primary color holograms at the imaging plane. Scale bar: 20 μm. The original “spider” image was obtained from PNG image website

Fig. 3

OAM meta-transformer for the read-out of three OAM states. a Schematic of OAM meta-transformer designed on three OAM states. This meta-transformer is designed to reconstructed “0”, “3” and “6” patterns at l⁽ⁿ⁾ = 0, 3, and 6, respectively. b Simulated (top) and measured (bottom) reconstructed patterns. Scale bar: 20 μm

Fig. 4

Reconstruction of color images. a Schematic illustration of reconstruction of gradient color image. b Intensity profiles corresponding to red, green, and blue beams and the graded color holographic image. Scale bar: 30 μm. The original “merlion” image was obtained from Vecteezy.com