Plasmonics
Can one nano-hole respond to twist one way and angle the other?
Open access · cc by · source: Europe PMC
A tiny helix-shaped hole in gold lets through one handedness of circularly polarised light from the front, but from the back it instead filters by linear polarisation angle, allowing two hidden images in one chip.
Study at a glance
- Design
- Other — Finite-element simulation plus focused-ion-beam fabrication and polarisation-resolved transmission measurements of helical nanoaperture arrays in gold, lit from front and back, then a Janus metasurface encoding two images.
- N
- No participant sample; results come from simulated unit cells and fabricated nanoaperture arrays, including one encoded metasurface built from 31 × 31 squares of 7 × 7 unit cells.
- Population
- Helical nanoapertures milled in a gold film on silica, in both mirror-image forms
- Outcome
- Forward circular dichroism in transmission, backward linear dichroism in transmission, and readability of the encoded QR-code and greyscale images
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Key findings
From the front, the measured circular dichroism in transmission peaked at 0.72 at 830 nm and exceeded 0.5 over roughly 680 to 900 nm, close to simulations. The effect came mainly from handedness-dependent reflection caused by mode coupling along the ramped groove, not from absorption, which is why a film thinner than a quarter wavelength sufficed. From the back, circular dichroism vanished, as reciprocity predicts, but linear dichroism reached up to about 0.87. The encoded chip showed a scannable QR code only under right-handed light from the front and a Bohr portrait only under x-polarised light from the back, with a working band of approximately 200 nm.
Methodology
The authors designed a helical nanoaperture: an arc-shaped hole joined to a groove whose depth ramps from zero to right through a 180 nm gold film, milled in one step with grayscale focused-ion-beam milling. They simulated and measured how much light passed for left- and right-handed circular polarisation from the front and for different linear polarisation angles from the back. They then built a metasurface where the choice of mirror-image form set a QR-code pixel and the rotation angle of each aperture set a greyscale pixel via Malus's law.
Limitations
Measured and simulated spectra differ somewhat, which the authors attribute to ion-beam fabrication tolerance, gold film quality and optical component imperfections. The paper reports two slightly different peak linear dichroism values (0.87 and 0.88) for the backward direction. Serial ion-beam milling took hours for one small metasurface, so it does not show a scalable manufacturing route, and the demonstration is limited to near-infrared wavelengths within a finite resonant band.
How this study connects
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