Light is the fastest and cleanest carrier of information we know. The science and technology of guiding, manipulating, and detecting light—photonics—underpins every facet of modern life, from the global internet and medical imaging to precision manufacturing and autonomous sensing.
For decades, the photonics industry has flourished in an environment of predictability. Much of linear optics is mapped out; Maxwell’s equations provide an exact playbook that allows engineers to predict the performance of a device with near-certainty. This certainty is enabled by mature, reliable materials like silicon, silica, and silicon nitride, which are incredibly well-characterized. As a result, photonics has become an engineer’s field as much as a physicist’s: You can design, simulate, fabricate, and almost always get exactly the expected result.
In stark contrast lies the world of quantum materials. These are solids where the collective, cooperative behavior of electrons gives rise to entirely new states of matter. They promise breakthroughs in computing, energy, and sensing because their properties can go far beyond the static limitations of ordinary materials. This field is characterized by fundamental complexity and surprise. Even qualitatively, the many-body interactions are so intertwined that there is no simple equivalent to Maxwell’s equations to neatly guide us. Spin, charge, valley, and lattice degrees of freedom are all coupled, meaning every knob a researcher turns pulls on several others at once. It remains very much a physicist’s playground, where experiments frequently reveal unexpected phenomena that theory struggles to catch up with.
This intellectual and technological orthogonality—the certainty of photonics meeting the surprise of quantum matter—is profoundly interesting. It presents an opportunity to inject the rich, dynamic tunability of emergent phenomena into the stable platform of integrated optics. This is the fertile middle ground where our work resides.
A new spin on light: Magnetic control in CrSBr
Our recent efforts have focused on an unusual two-dimensional (2D) magnet: CrSBr, a van der Waals antiferromagnetic semiconductor. This material has a layered structure with strong in-plane bonds, but very weak out-of-plane interactions. In its most stable, low-temperature state (below 132 K), the spins on the chromium atoms within each layer align together, but adjacent layers feature spins that point in opposite directions (see Fig. 1).1
The truly extraordinary feature here is the material’s active response to external control. By applying a magnetic field, we can continuously tune the relative orientation of the spins in adjacent layers from a fully compensated (antiferromagnetic) to a fully spin-polarized (ferromagnetic) state. This magnetic state transition drastically modifies the material’s optical properties. Specifically, strong light-matter excitations, known as “excitons,” have a resonance energy that varies significantly with the magnetic order. Since the refractive index determines nearly all linear optical behavior, this variation means that the index itself is strongly and dynamically modified by the magnetic field (see Fig. 2).