Vortex beam from microring resonator

By carefully tweaking the complex refractive-index structure of microring resonators, researchers from the University of Buffalo (USA) and Politecnico di Milano (Italy) created CMOS-friendly vortex microlasers 9 µm in diameter. [Image: University of Buffalo]

For years, the orbital angular momentum (OAM) in structured helical light beams—so-called twisted light—has tantalized scientists and engineers as a potential channel for carrying information, and thus for easing the looming “capacity crunch” in optical communications. But integrating OAM-carrying light, which commonly is created using bulk optics, into the modern, hyper-miniaturized world of silicon chips has represented a big challenge.

A research team from the U.S. and Italy has now demonstrated chip-scale microring lasers, a mere 9 µm in diameter, that can churn out single-mode OAM vortex beams whose topological charge and polarization can be designed on demand (Science, doi: 10.1126/science.aaf8533). The group believes that the microlasers could have applications in next-gen optoelectronic devices for both classical and quantum optical communications.

A “grand challenge” of microintegration

Circularly polarized light can carry two types of angular momentum: spin angular momentum (SAM), associated with the circular polarization direction itself, and orbital angular momentum (OAM), a property of “vortex” beams with helical wavefronts that depends on the spatial distribution of the light field. While SAM can take only one of two values, OAM is in principle unbounded. That has stoked a lot of interest in OAM as an additional degree of freedom that can be exploited in optical communications to help meet the rapidly increasing demand for network capacity.

A key problem in moving OAM into the communications system, however, lies in the light sources. While a great deal of fruitful work on OAM has been done in the nearly 25 years since the property was first described, much of it has relied on bulk optics such as spiral wave plates and spatial light modulators (or, more recently, metamaterials and planar optics) to generate complex OAM beams. Integrating these techniques to create OAM microlasers on chips, and thereby leverage OAM as an information carrier in modern optical communications, remains (in the words of the new study’s authors) a “grand challenge.”

Breaking symmetry

To meet that challenge, the research team—led by OSA Member Liang Feng and OSA Fellow Natalia Litchinitser of the University of Buffalo, USA—turned to a natural potential source of OAM: the optical whispering-gallery modes (WGMs) that circulate inside of microring resonators. Such resonators are known to be a good fit with CMOS technology, and have a long pedigree in creating novel kinds of lasers. Moreover, the WGMs circulating within the ring resonators carry large amounts of OAM.

But OAM lasing from a ring resonator also has a problem. Because of the resonator’s mirror symmetry, it tends to generates both clockwise- and counterclockwise-circulating WGMs. That means that the OAMs carried within those counter-circulating WGMs will tend to cancel each other out. Getting past that stumbling block requires a way to somehow break the mirror symmetry of the ring cavity.

The Buffalo team accomplished that symmetry-breaking feat by exploiting the peculiar physics of so-called exceptional points. These are mathematically permitted singularities in certain systems in which the eigenvalues of two different states “coalesce” into one.

The team began with a 500-nm-thick, 9-µm-diameter InGaAsP ring built on an InP substrate. The researchers discovered that, by carefully tweaking the refractive-index structure of that microresonator (specifically by capping the ring with alternating Ge and Ge/Cr structures, tuned to the refractive index and gain/loss of the cavity), they could find the “sweet spot”—an exceptional point within the system, at which the eigenstates of the two opposite WGM modes circulating within the cavity did indeed coalesce into a single value. The result? A resonator capable of driving single-mode, OAM-carrying laser emissions.

Opening up a communications channel?

The researchers proved out the concept by fabricating such an OAM microlaser with overlay electron beam lithography. Using a 1064-nm beam as a pump, they then coaxed the cavity into lasing, and were able to establish the OAM vortex character of the resulting beam through a variety of measurements that analyzed its spatial intensity profile.

Moreover, the team stressed that the method allows specific properties of the OAM beam, such as polarization and topological charge, to be “designed on demand.” That flexibility, coupled with the ability to integrate these vortex lasers at the chip scale, could, the paper concludes, “offer novel degrees of freedom” for next-gen optical communications, both classical and quantum.

In addition to Feng, Litchinitser and others at the University of Buffalo, the team also included Stefano Longhi, from  Politecnico di Milano, Italy.