Structured light
Can a tiny laser change its beam's twist within picoseconds?
Open access · cc by · source: Europe PMC
By timing a weak control pulse against the main pump pulse, a microring laser's output beam could be tuned continuously between no twist and a double twist in about 100 picoseconds.
Study at a glance
- Design
- Other — Lab experiment on a fabricated InGaAsP microring vortex laser with control arms, pumped by a main and a delayed control femtosecond pulse; emission imaged through polarisation optics and self-interference, supported by coupled-mode modelling.
- N
- No sample N; one microlaser device characterised as a function of pump-control delay.
- Population
- InGaAsP multiple-quantum-well microring vortex laser (7-micrometre diameter) with non-Hermitian control arms
- Outcome
- Lasing chirality, spin-OAM component weights and fractional OAM charge versus pulse delay; gain carrier lifetime
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Key findings
The measured gain carrier lifetime was about 263.15 ps, which set the time window for control. Near zero delay one circulation direction dominated, and with cross-polarised components filtered out the beam became a clean doughnut with a charge of +2; at longer delays the weights evened out. This allowed a continuous sweep of the average OAM charge between 0 and +2 within 100 ps, and between 1.68 and 2 for the single-polarisation output.
Methodology
The researchers used a microring laser whose grating converts clockwise and counterclockwise circulating light into beams carrying different orbital angular momentum and polarisation. A main femtosecond pulse made the ring lase, while a weaker control pulse excited gain in one of two coupling arms; because the excited carriers decay over hundreds of picoseconds, the delay between pulses set how strongly one circulation direction was favoured. They imaged the emission through a quarter-wave plate and polariser and used self-interference patterns to read off phase singularities.
Limitations
Only one device was measured, and only the 0 to +2 range was demonstrated; the full −2 to +2 range is proposed but would need pumping both control arms. Switching speed is limited by the semiconductor carrier lifetime, so sub-picosecond control is speculative. The fractional charge is an average over mixed modes, and no actual optical communication link or data encoding was tested.
How this study connects
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