Can one flat metasurface shape both a beam's wavefront and polarization?
By giving each tiny antenna a fully tailored polarization response, a flat metasurface can produce light beams with designed wavefronts and position-dependent polarization, including on-surface plasmon vortices.
Source
Efficient generation of complex vectorial optical fields with metasurfaces
Study at a glance
- Design
- Other — Theory plus fabrication and optical measurement of three metal-insulator-metal metasurface devices, compared with Green's-function calculations and numerical simulations.
- N
- Not applicable: three fabricated meta-devices (plus characterisation samples of individual meta-atoms) are measured; no sample-size statistic.
- Population
- Gold/SiO2/gold metal-insulator-metal metasurfaces working at near-infrared wavelengths (1550 nm and 1064 nm)
- Outcome
- Reflection angle and efficiency, polarization conversion ratio, orbital angular momentum (spiral interference patterns), local polarization maps and surface plasmon coupling efficiency and propagation length
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What they did
The authors showed that a lossless reflective element has three free parameters in its Jones matrix and mapped these onto three geometric knobs of a gold cross-shaped antenna: two bar lengths and a rotation angle. They built a library of such meta-atoms and used it to design and fabricate three devices: a reflector that bends light to a chosen angle while rotating its polarization, a device that turns linearly polarized light into a vortex beam with spatially varying elliptical polarization, and a coupler that converts circularly polarized light into a radially polarized vortex surface plasmon wave. They measured each device with angle-resolved spectroscopy, polarizer-filtered imaging, interferometry and leakage-radiation microscopy.
What they found
The anomalous-reflection wave plate sent most light into the designed angle with 85% absolute efficiency at 1550 nm and converted polarization with over 90% ratio across a very broad band from roughly 1000 to 2000 nm. The far-field device produced a beam whose interference pattern showed a first-order spiral (orbital angular momentum of one) and whose polarization changed from linear to elliptical around the beam as designed. The near-field coupler launched a radially polarized plasmon vortex with a propagation length of 22 to 28 micrometres and a measured coupling efficiency of about 34%, below the simulated maximum of 61.4%.
The limits
What it doesn't show
The design theory assumes lossless materials and plane-wave (paraxial) illumination; real gold absorbs, so measured efficiencies, especially for the plasmon coupler, fall short of simulation, which the authors attribute to fabrication imperfections and non-ideal input beams. The method cannot control the local amplitude of reflected light, only phase and polarization. Some meta-atom properties (the resonance phase) could not be measured and were taken from simulation, and designs with rapidly varying polarization lose fidelity through interference between neighbouring regions.
Key terms
- Metasurface
- An ultrathin layer of sub-wavelength structures (meta-atoms) patterned to control the phase, amplitude or polarization of light.
- Jones matrix
- A two-by-two matrix describing how an optical element transforms the two polarization components of a light wave.
- Vectorial optical field
- A light beam whose polarization varies from point to point across the beam, rather than being uniform.
- Orbital angular momentum (OAM)
- A property of beams with a helical (corkscrew) wavefront; the topological charge counts how many twists occur per wavelength.
- Surface plasmon polariton
- A light wave bound to a metal-dielectric interface, coupled to oscillations of the metal's electrons.
- Poincaré sphere
- A sphere on which every possible polarization state is a point; latitude sets ellipticity and longitude sets orientation.
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Quiz yourself
What makes these metasurfaces able to control both wavefront and polarization?
Common questions
Why does the meta-atom need three adjustable parameters?
Energy conservation and time-reversal symmetry leave only three free quantities in a lossless reflective Jones matrix, so three geometric knobs (two bar lengths and a rotation) are enough to reach any target phase and polarization.
Why did the plasmon vortex show a charge of two rather than one in the interference test?
To remove directly transmitted light they filtered out the left-circular component; the remaining right-circular part carries a charge of two, and combining it with the removed part would give the designed total of one.
How could the efficiency be improved?
Using low-loss dielectric meta-atoms instead of gold would reduce absorption, which the authors identify as the main loss.
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