Ultrafast control of fractional orbital angular momentum of microlaser emissions

Structured light: Fractional on-chip source An integrated vortex laser that is capable of emitting 1495 nm light with tunable fractional orbital angular momentum (OAM) could prove useful for next-generation optical communication systems. Zhifeng Zhang and co-workers from the US, Italy, and Spain fabricated an InGaAsP microring laser embedded in Si(3)N(4)using electron-beam lithography, inductively coupled plasma etching, and plasma-enhanced chemical vapour deposition. By spatially selective optical pumping with 140-fs duration, 800-nm wavelength laser pulses, an InGaAsP waveguide on the chip couples with the microring at three points and feeds it with dynamically varying optical gain to tune the state of OAM of the microring laser's output between a charge value of 0 and +2 within 100 ps. The tuning speed and range of achievable OAM values could likely be improved in the future, according to the researchers. On-chip integrated laser sources of structured light carrying fractional orbital angular momentum (FOAM) are highly desirable for the forefront development of optical communication and quantum information-processing technologies. While integrated vortex beam generators have been previously demonstrated in different optical settings, ultrafast control and sweep of FOAM light with low-power control, suitable for high-speed optical communication and computing, remains challenging. Here we demonstrate fast control of the FOAM from a vortex semiconductor microlaser based on fast transient mixing of integer laser vorticities induced by a control pulse. A continuous FOAM sweep between charge 0 and charge +2 is demonstrated in a 100 ps time window, with the ultimate speed limit being established by the carrier recombination time in the gain medium. Our results provide a new route to generating vortex microlasers carrying FOAM that are switchable at GHz frequencies by an ultrafast control pulse.