Ultrafast control of fractional orbital angular momentum of microlaser emissions

Zhifeng Zhang,Haoqi Zhao,Danilo Gomes Pires,Xingdu Qiao,Natalia M. Litchinitser,Stefano Longhi,Liang Feng,Zihe Gao,Josep M. Jornet

doi:10.1038/s41377-020-00415-3

Abstract

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.

Highlights

The vectorial nature of light empowers full control of topological features with spatially phase-variant fields, revealing vortex beams carrying orbital angular momentum (OAM) and spin angular momentum (SAM) in addition to the well-known linear momentum[1]
In integrated semiconductor nanophotonic circuits, the generation of vortex beams can rely on a different mechanism: robust selection of chiral resonant modes and their free space out-coupling with strategic phase matching conditions to convert the in-plane chiral modes into OAM vortex beams[26,27,28,31,32,33]
To demonstrate fractional orbital angular momentum (FOAM) control on the picosecond time scale, we consider our recently developed tuneable vortex microlaser[31] consisting of a microring resonator and an external coupling loop with two control arms, all made of 200-nm-thick InGaAsP multiple quantum wells and embedded in a Si3N4 substrate (Fig. 1)

Summary

Results

Integer OAM or FOAM beams are traditionally generated by the introduction of a phase variation into the transverse plane of an incident beam using carefully aligned optical elements, such as phase plates or metasurfaces. Temporal overlap of the laser action in the microring and the time-varying optical gain in the control arm, precisely controlled by the time delay between the two femtosecond pump pulses, facilitates ultrafast control of the weighting between the two chiral modes in the microring and the weighting between different mode-converted OAM components in Eq (4), yielding dynamic tuning of the FOAM of microlaser emissions with a picosecond resolution (Fig. 1). Ultrafast temporal control of the weighting between the two chiral modes and of the ratio between their associated powers (p↺/p↻), which is the key dynamic parameter to reconfigure the fraction of each spin–OAM component in the emitted vector beam [see Eq (4)], is achieved by tuning the temporal delay between the main pulse incident on the microring and the control pulse in the left control arm.

Discussion

Materials and methods

Design of transverse spin