THEIA · Time-varying Horizons with Electron-spin Interaction in Altermagnets

Nanophotonics are being revolutionized by the study of time-varying media, where abrupt changes of a system’s optical properties unlock new forms of control of light. Using nonlinear optics, one can switch-on optical properties of various materials on femtosecond time scales, but the switch-off time scales remain too slow and significantly constrain technological applications. To overcome this barrier, this project proposes to explore a new promising family of materials, altermagnets. Their unique crystal structure gives them a strong potential for nonlinear optics and time-varying photonics, as their switching using nonlinear optics will exhibit both fast spin dynamics and slower electron dynamics. This is due to their electronic bands being separated in energy by spin in the absence of magnetic fields, and being reliant on this altermagnetic phase. This will go beyond the state of the art in the field, as materials currently in use have slow switch-off times and fewer degrees of freedom to leverage. For example, transparent conductive oxides are known to have high switching contrast but slow electron-phonon relaxation. Semiconductors show higher speeds but low switching contrast, and the use of long-lived resonant metasurfaces to enhance this contrast only results in a slow switch-off speed. This project aims at performing a first linear and nonlinear optical characterisation of altermagnetic materials, and establishing them as a prime platform for time-varying nanophotonics. I will first determine the individual role of electron and spin dynamics by decoupling them through Z-scan measurements. Then, I will demonstrate another key properties, optical isolation, in a nonreciprocal pump-probe experiment. The final part of the project will accurately measure the dynamics of the materials with few-femtosecond pulses, and use these new dynamics to create a synthetic motion device which will control pulses in momentum and frequency by mimicking relativistic effects.