A fridge that can do your shopping for you and tell you when food has gone bad is a shiny, exciting not-to-far-away future. A less exciting element of the Internet of Things (IoT) is the amount of data it is going to generate, the need to store it, and pass it from one point to another. Every cloud has a physical server somewhere, and information has to get from there to other places, even within the server, but data transfer can become a real bottleneck for data processing efficiency.

Likewise, Artificial Intelligence is becoming commonplace but it, too, requires vast amounts of data transfer. Blockchain technology, increased media use and virtual reality will all contribute to error messages and prompts to increase your storage and data communication bandwidth.

Spintronics is a field that explores the spin properties of electrons and it has the potential to revolutionise data storage and transfer by offering new types of memory devices that can store data more efficiently. Similarly, photonics can offer greater capacity than traditional technologies to encode information on light photons using their polarization, akin spin for electrons, but only if you can control it.

In research published in Nature Nanotechnology, physicists from TMOS, the ARC Centre of Excellence for Transformative Meta-Optical Systems, including Associate Investigators from City University of New York, the Australian National University and the Airforce Research Laboratory, have developed a new method for designing metasurfaces. This method can engineer electromagnetic spin by generating a new type of photonic mode in an innovative Dirac-like waveguide. This advances previous research into low-loss information transfer that uses signal transmission along topological interfaces.

Traditionally, topological waveguides are built with abrupt edges between its various interfaces. These edges create boundary modes—electromagnetic waves that behave differently where edges exist than they do across the bulk of the material. These boundary modes can be used productively in many ways, but they have only one spin direction and lack radiation control.

Lead investigator Prof. Alexander B. Khanikaev and his team have taken a new approach to metasurface interfaces. Instead of a hard edge, they have smoothed the boundaries by patterning a gradual shift into the metasurface slab. Rather than discrete shapes butted up together, they’ve made small variations to the design, in this case a pattern of holes that form repeating hexagons, so the shapes gradually join. This has generated brand-new modes of electromagnetic wave never seen before in a metasurface, with radiative properties that are very exciting. At a single frequency, two modes of different spin could co-exist, one radiating more than the other. By hitting the metasurface with a circularly polarized laser, Kiriushechkina et al. were able to pick up a specific mode spin. This was proven in the laboratory by each mode propagating at different lengths when excited.

This method could soon lead to an ability to independently control the spin of both modes. This would create a binary degree of freedom, which opens up significant opportunities for the field of spin-photonics and the eventual development of data storage systems that use binary photon spin to encode and manipulate information.

Co-first author Dr Daria Smirnova says “The proof-of-concept experiment conclusively validated our theoretical findings and modeling. Curiously enough, the effect can be explained by merging the Dirac formalism with handy electrodynamics to describe radiative nature of the modes designed.”

Khanikaev says, “The possibility to engineer a binary spin-like structure of light on a chip and the possibility to manipulate it on demand opens truly exciting opportunities to encode information in it, especially quantum information. Our team, in collaboration with our colleagues from TMOS and AFRL, are currently working on creating quantum interconnects based on such photonic spin, but also on elementary quantum logic operations on a silicon photonic chip. As such, we believe that, in the long run, integrated Dirac photonic systems can become a viable platform for integrated quantum photonics.”

TMOS Centre Director Dragomir Neshev says, “This cross-institution teamwork has advanced the field of meta-optics significantly. It is an extraordinary achievement and a prime example of why Centres of Excellence exist. They facilitate the sharing of knowledge and expertise in a way that is often limited by a researcher’s own networks. I’m excited to see what comes next from these collaborators.”

For media enquiries or more information about this research, please contact connect@tmos.org.au

Spin-dependent properties of optical modes guided by adiabatic trapping potentials in photonic Dirac metasurfaces

Svetlana Kiriushechkina, Anton Vakulenko, Daria Smirnova, Sriram Guddala, Yuma Kawaguchi, Filipp Komissarenko, Monica Allen, Jeffery Allen & Alexander B. Khanikaev

Nature Nanotechnology, 27 April 2023

The Dirac-like dispersion in photonic systems makes it possible to mimic the dispersion of relativistic spin-1/2 particles, which led to the development of the concept of photonic topological insulators. Despite recent demonstrations of various topological photonic phases, the full potential offered by Dirac photonic systems, specifically their ability to emulate the spin degree of freedom—referred to as pseudo-spin—beyond topological boundary modes has remained underexplored. Here we demonstrate that photonic Dirac metasurfaces with smooth one-dimensional trapping gauge potentials serve as effective waveguides with modes carrying pseudo-spin. We show that spatially varying gauge potentials act unevenly on the two pseudo-spins due to their different field distributions, which enables control of guided modes by their spin, a property that is unattainable with conventional optical waveguides. Silicon nanophotonic metasurfaces are used to experimentally confirm the properties of these guided modes and reveal their distinct spin-dependent radiative character; modes of opposite pseudo-spin exhibit disparate radiative lifetimes and couple differently to incident light. The spin-dependent field distributions and radiative lifetimes of their guided modes indicate that photonic Dirac metasurfaces could be used for spin-multiplexing, controlling the characteristics of optical guided modes, and tuning light–matter interactions with photonic pseudo-spins.