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Molding free-space light with guided wave–driven metasurfaces

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Science Advances  17 Jul 2020:
Vol. 6, no. 29, eabb4142
DOI: 10.1126/sciadv.abb4142
  • Fig. 1 Working principle of guided wave–driven metasurfaces.

    (A) Schematic of a guided wave–driven metasurface. The phase of the extracted light from a guided wave by each meta-atom can be tuned individually. An array of meta-atoms on the waveguide work collaboratively to form certain wavefronts and fulfill different functions, such as beam deflection and focusing. (B) Illustration of the wavefront formation of the extracted wave. The total phase shift of the extracted wave at coordinate x is contributed from two parts: the phase accumulation βx from the guided wave propagation and the abrupt phase change Δϕ(x) induced by the meta-atom. As a result, the phase of the extracted wave can be expressed as ϕ0 + βx + Δϕ(x), where ϕ0 is the initial phase of the incidence.

  • Fig. 2 Design of meta-atoms for controlling the phase (and amplitude) of free-space waves extracted from guided ones.

    (A) Schematic of a metal/dielectric/metal sandwich-structured meta-atom on top of a photonic integrated waveguide. The bottom left inset shows the simulated electric field distribution of the TE00-guided mode propagating inside the waveguide. The bottom right inset is the simulated magnetic field distribution of the sandwich-structured nanoantenna, which indicates an effective magnetic dipole. (B) Pseudocolor map of the simulated abrupt phase shifts in a parameter space spanned by the meta-atom width (lx) and length (ly). A thickness of 30 nm was used for each layer. The meta-atom was placed on top of a silicon ridge waveguide (height, 220 nm). The three white stars indicate the meta-atom designs covering 2π phase range with an even interval. We also ensured that the extracted waves from the chosen meta-atoms have roughly the same amplitude of 1.5 × 105 V/m. (C) Simulated electric field distribution (Ey) of the extracted waves from the three selected meta-atoms, showing abrupt phase shifts of 2π/3, 0, and −2π/3, respectively. (D) Electric field distribution of the extracted light from a phase-gradient metasurface driven by forward-propagating (left) and backward-propagating (right) guided waves. The metasurface consists of an array of meta-atoms that form a phase gradient ∂Δϕ(x)/∂x (which is along the –x direction in this example). The extracted light from a forward-propagating guided wave carries a transverse wave vector kx = β + ∂Δϕ(x)/x, where β is the propagation constant of the guided wave. It is launched into free space with a well-defined angle θ = sin−1(kx/k0). In contrast, light extracted from the backward-propagating wave gains a transverse wave vector so large that it exceeds the maximum supportable wave number in free space, and therefore, it bounds to the metasurface and eventually dies out due to ohmic loss from the materials. a.u., arbitrary unit.

  • Fig. 3 Demonstration of off-chip beam deflection with guided wave–driven metasurfaces.

    (A) Field emission scanning electron microscope (FESEM) images of a guided wave–driven metasurface on a silicon waveguide (220 nm thick and 600 nm wide). Each supercell consists of three meta-atoms as depicted in Fig. 2B. (B) Output beam angle versus the incident guided wave wavelength with supercell size Λ = 575 nm measured by our Fourier-space imaging system (fig. S4). The blue dots and the red dashed line depict the experimentally measured and the simulated data, respectively. Three typical Fourier-space images of the extracted free-space light corresponding to the circled data points are shown on the right. The horizontal and vertical axes represent kx and ky, respectively. An objective with NA of 0.95 was used in the measurements. (C) Output beam angle versus the supercell size at 1550-nm wavelength. The blue dots and the red dashed line depict the experimentally measured and the simulated data, respectively. Similar to (B), three typical Fourier-space images are shown on the right.

  • Fig. 4 Demonstration of off-chip light focusing with a guided wave–driven metalens.

    (A) Simulated electric field distribution above a guided wave–driven metalens on a silicon waveguide (500 nm thick and 1.5 μm wide). The extracted light converged at the designed focal point (5 μm above the waveguide) at 1550-nm wavelength. (B) Experimentally measured intensity profile of the focusing effect of a fabricated device. The inset shows an FESEM image of the metasurface region. The designed focal length is 225 μm.

  • Fig. 5 Photonic integrated OAM laser based on the guided wave–driven metasurface incorporated on an InGaAsP/InP micoring resonator.

    (A) Schematic of a microring OAM laser enabled by the guided wave–driven metasurface. Unidirectional phase modulation provided by the metasurface breaks the degeneracy of the CCW and CW WGMs inside the microring resonator, leading to a selective OAM radiation. (B) FESEM images of a fabricated device. The diameter of the microring is 9 μm and the width is 1.1 μm, and it consists of a 500-nm InGaAsP MQW layer and a 1-μm InP layer. A supercell of the metasurface consists of four Au/Si/Au meta-atoms, which provides the extracted wave with abrupt phase shifts from 0 to 2π. The total number of supercells on the microring is N = 58. (C) Light-light curve of the microring laser (top row), which shows a lasing threshold of about 0.47 GW/cm2 at 1555-nm wavelength. Three emission spectra corresponding to different stages—photoluminescence, amplified spontaneous emission, and lasing—of the laser are shown from the second to the last row. (D) Far-field intensity distribution of the OAM laser radiation captured by an infrared camera (right), which matches well with the simulated one (left). Both figures show an annular shape. (E and F) Calculated (left) and measured (right) self-interference patterns of OAM laser radiation. The calculation only took into account the interference between a plane wave and an OAM beam; therefore, it shows one set of fork in the interference pattern. The double fork (E) and triple fork (F) in the fringe patterns confirmed that the resulting OAM emission has a topological charge of +1 (E) and +2 (F), respectively.

Supplementary Materials

  • Supplementary Materials

    Molding free-space light with guided wave–driven metasurfaces

    Xuexue Guo, Yimin Ding, Xi Chen, Yao Duan, Xingjie Ni

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