Can a tiny laser change its beam's twist within picoseconds?
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.
Source
Ultrafast control of fractional orbital angular momentum of microlaser emissions
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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What they did
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.
What they found
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.
The limits
What it doesn't show
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.
Key terms
- Orbital angular momentum (OAM)
- Angular momentum carried by light whose phase twists around the beam axis; an integer charge l gives l twists per wavelength.
- Fractional OAM
- A non-integer average OAM per photon produced by superposing beams of different integer charges with chosen weights.
- Whispering gallery mode
- Light circulating around the rim of a ring or disk resonator, clockwise or counterclockwise.
- Non-Hermitian coupling
- Coupling in a system with gain and loss, which can be made one-directional, favouring one circulating mode over the other.
- Phase singularity
- A point in a beam where intensity is zero and the phase is undefined, seen as a fork in an interference pattern.
Flashcards
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Quiz yourself
What sets the weighting between the two chiral modes in time?
Common questions
How can OAM be fractional if each photon mode carries integer OAM?
The beam is a mix of integer-charge modes; the fractional value is the weighted average OAM per photon across that mixture.
Why does the delay between pulses matter?
The control pulse creates gain carriers that decay over about 263.15 ps, so the delay sets how much gain remains in the control arm when lasing happens, and thus how unbalanced the two circulating modes are.
Why is this relevant to communications?
Rapidly switchable fractional OAM states could encode more than two levels per symbol, increasing data rate within a given bandwidth, though this was not tested here.
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