Can a beam's twist be tuned smoothly instead of in whole steps?
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.
Source
Spiniform phase-encoded metagratings entangling arbitrary rational-order orbital angular momentum
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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What they did
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.
What they found
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.
The limits
What it doesn't show
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.
Key terms
- Orbital angular momentum
- Angular momentum carried by a beam with a helical phase front.
- Phase singularity
- A point where the phase is undefined and the intensity is zero, around which phase winds by a multiple of 2π.
- Singularity strength q
- The aperture diameter divided by the spacing between singularities, controlling how many are enclosed.
- Quantum spiral spectrum
- The distribution of OAM components measured through photon coincidences.
- Cross-talk
- Unwanted signal from neighbouring states, here reported in decibels.
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Quiz yourself
What controls the average OAM in this scheme?
Common questions
How is this different from changing the charge of a Laguerre–Gaussian beam?
Instead of changing the winding of one central singularity, the device changes how many separated singularities are included, giving smooth control of the average OAM.
Why does the coincidence peak when the two photons have opposite values?
Down-conversion conserves angular momentum, so a pump with zero OAM produces signal and idler with opposite OAM.
Why were the quantum tests done with an SLM?
The fabricated metagratings were too small to select beams with an extra aperture in the quantum setup, so the phase was encoded on an SLM instead.
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