In the endeavor to miniaturize and improve technologies that manipulate light, MIT scientists have introduced a novel platform that extends the boundaries of contemporary optics via nanophotonics, the control of light at the nanoscale, or billionths of a meter.
The outcome is a new category of ultracompact optical devices that are not just smaller and more efficient than current technologies, but also dynamically adjustable, or switchable, from one optical mode to another. Until now, this has been a challenging synthesis in nanophotonics.
This research is presented in the July 8 edition of Nature Photonics.
“This research signifies a notable advancement toward a future where nanophotonic devices are not only compact and effective but also reprogrammable and responsive, able to dynamically react to external stimuli. The combination of emerging quantum materials and established nanophotonics architectures will undoubtedly propel both fields forward,” states Riccardo Comin, MIT’s Class of 1947 Career Development Associate Professor of Physics and project leader. Comin also works with MIT’s Materials Research Laboratory and Research Laboratory of Electronics (RLE).
Alongside Comin are colleagues Ahmet Kemal Demir, an MIT graduate student in physics; Luca Nessi, a former MIT postdoc now at Politecnico di Milano; Sachin Vaidya, a postdoc in RLE; Connor A. Occhialini PhD ’24, currently a postdoc at Columbia University; and Marin Soljačić, the Cecil and Ida Green Professor of Physics at MIT.
Demir and Nessi are co-first authors of the Nature Photonics publication.
Toward innovative nanophotonic materials
Traditionally, nanophotonics has depended on materials like silicon, silicon nitride, or titanium dioxide. These serve as the fundamental components for devices that direct and trap light while utilizing structures such as waveguides, resonators, and photonic crystals. The latter are periodic configurations of materials that influence light propagation, similar to how a semiconductor crystal affects electron movement.
Though highly effective, these materials are restricted by two key limitations. The first concerns their refractive indices, which measure how intensely a material interacts with light; higher refractive indices result in greater interaction with the light, bending it more sharply and reducing its speed. The refractive indices of silicon and other conventional nanophotonic materials are often limited, which restricts how tightly light can be confined and how small optical devices can be constructed.
The second significant limitation of traditional nanophotonic materials is that once a structure is created, its optical properties are nearly immutable. There is generally no way to significantly adjust its light response without physically altering it. “Tunability is crucial for various next-generation photonics applications, facilitating adaptive imaging, precision sensing, reconfigurable light sources, and trainable optical neural networks,” notes Vaidya.
Introducing chromium sulfide bromide
These ongoing challenges can potentially be addressed by chromium sulfide bromide (CrSBr). CrSBr is a layered quantum material featuring a rare combination of magnetic order and strong optical response. At the heart of its remarkable optical characteristics are excitons: quasiparticles formed when a material absorbs light and an electron is excited, leaving behind a positively charged “hole.” The electron and hole remain linked by electrostatic attraction, forming a neutral particle capable of strongly interacting with light.
In CrSBr, excitons dictate the optical response and exhibit high sensitivity to magnetic fields, allowing them to be manipulated with external controls.
Due to these excitons, CrSBr showcases an unusually high refractive index that enables researchers to shape the material to construct optical structures like photonic crystals that are an order of magnitude thinner than those created from traditional materials. “We can fabricate optical structures as thin as 6 nanometers, or just seven layers of atoms stacked together,” states Demir.
Importantly, by applying a modest magnetic field, the MIT team was able to continuously and reversibly adjust the optical mode. In other words, they showcased the capability to dynamically alter light flow through the nanostructure, all without any moving components or changes in temperature. “This level of control stems from a substantial, magnetically induced shift in the refractive index, far exceeding what is typically attainable in established photonic materials,” explains Demir.
In fact, the interaction between light and excitons in CrSBr is so potent that it leads to the emergence of polaritons, hybrid light-matter particles that inherit characteristics from both components. These polaritons enable novel forms of photonic behavior, including enhanced nonlinearities and new regimes of quantum light transport. Unlike traditional systems that necessitate external optical cavities to reach this regime, CrSBr supports polaritons intrinsically.
While this demonstration employs standalone CrSBr flakes, the material can also be incorporated into current photonic platforms, such as integrated photonic circuits. This makes CrSBr immediately pertinent to practical applications, where it can act as a tunable layer or component in otherwise passive devices.
The MIT findings were accomplished at very low temperatures of up to 132 kelvins (-222 degrees Fahrenheit). Although this is below room temperature, there are compelling applications, including quantum simulation, nonlinear optics, and reconfigurable polaritonic platforms, where CrSBr’s unparalleled tunability might justify operation in cryogenic settings.
In other words, states Demir, “CrSBr is so distinct compared to other common materials that even enduring cryogenic temperatures will be justified, hopefully.”
That said, the team is also investigating related materials with higher magnetic ordering temperatures to facilitate similar functionalities under more accessible conditions.
This research was funded by the U.S. Department of Energy, the U.S. Army Research Office, and a MathWorks Science Fellowship. The work was partially conducted at MIT.nano.