Orbital Angular Momentum Enables New Nonreciprocal Light-Matter Interactions

Research demonstrates that light’s orbital angular momentum (OAM) alters magneto-optical (MO) behaviour, reducing Faraday rotation through state-specific nonreciprocal operation. This arises from transverse momentum transfer inducing spin-orbit coupling, modifying the refractive index and linking structured light to MO response, enabling novel photonic and spintronic devices.

The manipulation of light’s inherent angular momentum presents a compelling route towards advanced photonic and spintronic devices, particularly within magneto-optical (MO) systems where light interacts with magnetic materials. Researchers are now demonstrating precise control over refractive index, a material’s ability to bend light, by leveraging the orbital angular momentum (OAM) of light, a property distinct from its more commonly studied spin angular momentum. Seth Nelson, Cong Yu, and colleagues from the Physics Department at Michigan Technological University detail their findings in a new study, titled ‘Optical Vortex Spin-Orbit Control of Refractive Index in Iron Garnets’, which reveals how structured light, specifically light carrying OAM, induces spin-orbit coupling within iron garnet materials, leading to a state-specific reduction in Faraday rotation and a modification of the material’s refractive index. This work establishes a fundamental link between structured light and magneto-optical responses, potentially enabling the development of novel optical components and ultrafast data storage technologies.
Structured light presents new methods for controlling interactions between light and matter, particularly within magneto-optical (MO) systems. Current research investigates the potential of orbital angular momentum (OAM) to induce non-reciprocal effects, phenomena where light propagation differs depending on direction, extending beyond those traditionally observed with spin angular momentum (SAM). A recent study demonstrates state-specific non-reciprocal operation using OAM states within an MO medium, resulting in a reduction of Faraday rotation, the rotation of light’s polarisation plane when traversing a magnetic material.

This effect originates from transverse momentum transfer into the MO medium, perturbing electronic transition rates and inducing spin-orbit coupling (SOC). SOC, a relativistic quantum mechanical effect, links an electron’s spin to its motion, influencing its interaction with electromagnetic fields. Researchers establish a direct link between structured light, specifically OAM beams, and the MO response of materials, revealing that OAM-dependent SOC modifies the refractive index, a measure of how light bends when entering a material. Interferometric techniques, such as Michelson and Sagnac interferometry, achieve high-precision measurements of these refractive index changes and other material parameters, providing crucial data for validating theoretical models.

Theoretical foundations, including quantum mechanics and Kramers-Kronig relations, underpin the experimental work, allowing for a deeper understanding of the fundamental principles governing light-matter interactions. These findings extend previous observations made with paraxial beams, which propagate with minimal divergence, and reveal a more fundamental mechanism governing OAM-driven non-reciprocal interactions, thereby challenging existing paradigms in the field. This research advances the integration of MO materials with structured light, offering the potential for the development of OAM-selective non-reciprocal photonic devices, chiral optical logic, memory elements, and ultrafast spintronic architectures.

The work extends the established understanding of SAM effects in MO systems by demonstrating the unique capabilities of OAM, carefully comparing and contrasting the advantages and limitations of using SAM versus OAM for manipulating magnetic materials. The ability to selectively control light-matter interactions based on OAM opens avenues for creating OAM-selective nonreciprocal components, chiral optical logic circuits, and advanced memory elements, promising to revolutionise various technological fields. Furthermore, the demonstrated control over spin-orbit interactions through structured light suggests potential applications in ultrafast spintronic architectures, opening up exciting possibilities for the development of next-generation electronic devices.

Future work will focus on optimising material properties and device geometries to maximise OAM-dependent nonreciprocal effects. Investigating different MO materials and exploring the potential for integrating these findings into more complex photonic circuits are key areas for further research, involving the development of advanced fabrication techniques and characterisation methods. Additionally, exploring the temporal dynamics of the induced SOC and its potential for developing ultrafast optical switches and modulators represents a promising avenue for future investigation.