Structured light
Can a beam's twist be tuned smoothly instead of in whole steps?
Open access · cc by · source: Europe PMC
Two mirror-image gratings under a variable aperture create light beams whose average orbital angular momentum can be set to any rational value by how many phase singularities the aperture lets through.
Study at a glance
- Design
- Other — Lab experiment with simulation: chromium metagratings with different apertures generated vortex beams at 532 nm, characterised by interferometry and phase retrieval; the same phase was put on SLMs to measure quantum spiral spectra and coincidences of down-converted photon pairs.
- N
- No sample count; two groups of fabricated samples (q = 1–4 and q = 1.1–1.5) plus SLM-based quantum measurements.
- Population
- Binary chromium metagratings on quartz and SLM-encoded spiniform phase applied to entangled photon pairs
- Outcome
- Beam intensity and phase profiles, average OAM per photon, quantum spiral spectra and coincidence cross-talk
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Key findings
Measured intensity and retrieved phase patterns matched simulations, with the accumulated phase around the beam equal to 2π times q for integer cases and evolving smoothly for fractional ones. The average OAM per photon varied nonlinearly with q between 0 and 2 and roughly linearly beyond. In the quantum tests, coincidences peaked when the two photons' values were opposite, showing angular momentum conservation, and cross-talk between neighbouring states was −7.1 dB experimentally at unit spacing, improving to about −10.2 and −10.6 dB at spacings of 2 and 3.
Methodology
The authors designed a transmitter made of two tilted gratings that meet along a symmetry line, producing a row of equally spaced phase singularities; a circular aperture of varying diameter selects how many singularities pass. They fabricated samples in 100-nm chromium with integer (q = 1–4) and fractional (q = 1.1–1.5) singularity strengths and imaged the beams at 532 nm, reconstructing their phase by interference with a reference beam. They then placed the same phase on a spatial light modulator in one arm of a down-conversion source and measured photon coincidences.
Limitations
Truly continuous tuning with a single device and a smoothly adjustable aperture was not demonstrated; separate samples with fixed apertures were fabricated. The quantum experiments used a pixelated SLM rather than the metagrating, which the authors acknowledge cannot give rigorously continuous OAM and added noise. The binary gratings have a theoretical efficiency of only about 10%, and the largest device (about 480 μm) is too small for standard apertures.
How this study connects
Role on claims
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Closely spaced OAM charges leak into each other.
Crosstalk falls as charge spacing rises across very different systems: metagrating-derived OAM states improved from -7.1 dB at unit spacing to about -10.6 dB at spacing 3, OAM-multiplexed holograms degraded at spacing 1 versus 3, and a metasurface detector resolved simultaneous modes only at spacing 5 or more.
Evidence for the claim as stated.
Flat optics can create and measure twisted light.
Metasurfaces can both generate and read OAM: gratings under an aperture set average OAM to rational values, and a spin-decoupled metasurface identified charges 0 to 11 by focal-spot angle within +/-0.01 rad while sorting spin to opposite halves.
Evidence for the claim as stated.
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