Structured light
Multiplying and dividing the twist of a light beam
Open access · cc by · source: Europe PMC
A pair of patterned optical surfaces on a single glass chip can double or triple the orbital angular momentum of a light beam, or split it into beams carrying a half or a third of it.
Study at a glance
- Design
- Other — Theory and Fresnel simulations of circular-sector conformal transformations, followed by e-beam-lithography fabrication of diffractive multipliers/dividers and interferometric testing with SLM-generated OAM beams.
- N
- No sample; devices for twofold and threefold multiplication and division were each tested over a range of input OAM values.
- Population
- Laser beams (632.8 nm HeNe) carrying integer orbital angular momentum
- Outcome
- Output OAM value (counted from spiral interferogram arms) and beam intensity profiles
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Key findings
The twofold multiplier doubled input OAM values from −4 to +4 and the threefold multiplier tripled values from −3 to +3, as shown by the number of spiral arms. The twofold divider split beams with even OAM from −8 to +8 into two beams each carrying half the OAM, and the threefold divider split inputs from −9 to +9 into three beams each carrying a third. Simulations indicated conversion efficiency close to perfect, dropping slowly at higher input OAM because the beam's twisted wavefront distorts the output ring.
Methodology
The authors derived phase patterns for an optical transformation that maps a full circle onto a circular sector, and superposed several such transformations to multiply or divide a beam's azimuthal phase gradient. They simulated the beam propagation, then fabricated the transformer and its phase corrector side-by-side on a single substrate using electron-beam lithography, with a mirror folding the beam back onto the corrector. They tested the devices with OAM beams from a spatial light modulator and read the output OAM from the spiral arms in interferograms.
Limitations
Only twofold and threefold operations and modest OAM values were demonstrated experimentally; performance at high OAM or in cascades is proposed, not shown. The paper reports efficiency mainly from simulation rather than a measured experimental efficiency, and outputs show slight asymmetric distortion. Division here produces equal output modes only, and applications like OAM computing or single-photon use are speculative.
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