First Tunable, Chip-Based Vortex Microlaser

Vortex lasers are named for the way light spirals around their axis of travel, thanks to a orbital angular momentum, or OAM. Different OAM modes correspond to the direction and spacing of those spirals, and given a sensitive enough laser and detector, could be another property in which information could be transmitted. (Source: U. Pennsylvania)

As computers get more powerful and connected, the amount of data that we send and receive is in a constant race with the techno­logies that we use to transmit it. Electrons are now proving insufficiently fast and are being replaced by photons as the demand for fiber optic internet cabling and data centers grow. Though light is much faster than elec­tricity, in modern optical systems, more infor­mation is transmitted by layering data into multiple aspects of a light wave, such as its amplitude, wavelength and polari­zation. Increasingly sophis­ticated multi­plexing techniques like these are the only way to stay ahead of the increasing demand for data, but those too are approaching a bottleneck. We are simply running out of room to store more data in the conven­tional properties of light.

To break through this barrier, engineers are exploring some of light’s harder-to-control properties. Now, two studies from the University of Penn­sylvania’s School of Engineering and Applied Science have shown a system that can manipulate and detect one such property known as the orbital angular momentum, or OAM, of light. Critically, they are the first to do so on small semi­conductor chips and with enough precision that it can be used as a medium for trans­mitting information. The matched pair of studies was done in colla­boration with researchers at Duke University, North­eastern University, the Polytechnic University of Milan, Hunan University and the U.S. National Institute of Standards and Tech­nology.

One study, led by Liang Feng, demons­trates a microlaser which can be dynamically tuned to multiple distinct OAM modes. The other, led by Ritesh Agarwal, shows how a laser’s OAM mode can be measured by a chip-based detector. Both studies involve colla­borations between the Agarwal and Feng groups at Penn. Such vortex lasers, named for the way their light spirals around their axis of travel, were first demons­trated by Feng with quantum symmetry-driven designs in 2016. However, Feng and other researchers in the field have thus far been limited to trans­mitting a single, pre-set OAM mode, making them impractical for encoding more information. On the receiving end, existing detectors have relied on complex filtering techniques using bulky components that have prevented them from being integrated directly onto a chip, and are thus incom­patible with most practical optical communi­cations approaches.

Together, this new tunable vortex micro-trans­ceiver and receiver represents the two most critical components of a system that can enable a way of multiplying the information density of optical communi­cation, potentially shattering that looming bandwidth bottle­neck. The ability to dynami­cally tune OAM values would also enable a photonic update to a classic encryption technique: frequency hopping. By rapidly switching between OAM modes in a pre-defined sequence known only to the sender and receiver, optical communi­cations could be made impossible to intercept. “Our findings mark a large step towards launching large-capacity optical communi­cation networks and confronting the upcoming information crunch,” says Feng.

In the most basic form of optical communi­cation, transmitting a binary message is as simple as repre­senting 1s and 0s by whether the light is on or off. This is effectively a measure of the light’s amplitude which we experience as brightness. As lasers and detectors become more precise, they can consistently emit and distin­guish between different levels of amplitude, allowing for more bits of information to be contained in the same signal. Even more sophis­ticated lasers and detectors can alter other properties of light, such as its wavelength, which corresponds to color, and its polarization, which is the orien­tation of the wave’s oscillations relative to its direction of travel. Many of these properties can be set inde­pendently of each other, allowing for increasingly dense multi­plexing.

Orbital angular momentum is yet another property of light, though it is considerably harder to manipulate, given the complexity of the nanoscale features necessary to generate it from computer-chip-sized lasers. Circularly polarized light carries an electric field that rotates around its axis of travel, meaning its photons have a quality known as spin angular momentum, or SAM. Under highly controlled spin-orbit inter­actions, SAM can be locked or converted into another property, orbital angular momentum, or OAM.

In this new study, Feng, Zhang and their colleagues began with a microring laser, which consists of a ring of semiconductor, only a few microns wide, through which light can circulate inde­finitely as long as power is supplied. When additional light is pumped into the ring from control arms on either side of the ring, the delicately designed ring emits circu­larly polarized laser light. Critically, asymmetry between the two control arms allows for the SAM of the resulting laser to be coupled with OAM in a parti­cular direction. This means that rather than merely rotating around the axis of the beam, as circularly polarized light does, the wavefront of such a laser orbits that axis and thus travels in a helical pattern. A laser’s OAM mode corresponds to its chirality, the direction those helices twist, and how close together its twists are.

“We demonstrated a microring laser that is capable of emitting five distinct OAM modes,” Feng says. “That may increase the data channel of such lasers by up to five times.” Being able to multiplex the OAM, SAM and wavelength of laser light is itself unpre­cedented, but not parti­cularly useful without a detector that can differen­tiate between those states and read them out. In concert with Feng’s work on the tunable vortex microlaser, the research on the OAM detector was led by Agarwal and Zhurun Ji, a graduate student in his lab.

“OAM modes are currently detected through bulk approaches such as mode sorters, or by filtering techniques such as modal decom­position,” Agarwal says, “but none of these methods are likely to work on a chip, or interface seamlessly with electronic signals.” Agarwal and Ji built upon their previous work with Weyl semi­metals, a class of quantum materials that have bulk quantum states whose electrical properties can be controlled using light. Their experi­ments showed that they could control the direction of electrons in those materials by shining light with different SAM onto it.

Along with their colla­borators, Agarwal and Ji drew on this phenomenon by designing a photo­detector that is similarly responsive to different OAM modes. In their new detector, the photo­current generated by light with different OAM modes produced unique current patterns, which allowed the researchers determine the OAM of light impinging on their device. “These results not only demonstrate a novel quantum pheno­menon in the light-matter interaction,” Agarwal says, “but for the first time enable the direct read-out of the phase information of light using an on-chip photo­detector. These studies hold great promise for designing highly compact systems for future optical communi­cation systems.”

Next, Agarwal and Feng plan to colla­borate on such systems. By combining their unique expertise to fabricate on-chip vortex micro­lasers and detectors that can uniquely detect light’s OAM, they will design integrated systems to demons­trate new concepts in optical communi­cations with enhanced data trans­mission capabilities for classical light and upon increasing the sensitivity to single photons, for quantum applications. This demons­tration of a new dimension for storing information based on OAM modes can help create richer super­position quantum states to increase information capacity by a few orders of magnitude. (Source: U. Pennsylvania)

Reference: Z. Zhang et al.: Tunable topological charge vortex microlaser, Science 368, 760 (2020); DOI: 10.1126/science.aba8996 Z. Ji et al.: Photocurrent detection of the orbital angular momentum of light, Science 368, 763 (2020); DOI: 10.1126/science.aba9192

Link: Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, USA

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