Dynamical renormalization of the magnetic excitation spectrum via high-momentum nonlinear magnonics

Sustaining the growth of the data volume generated by artificial intelligence and the internet of things demands to develop schemes for data storage and processing operating at terahertz frequencies, unrestrained by thermal throttling. The optical drive of coherent magnetic collective excitations, namely magnons, represents a promising route. The ability to arbitrarily and nonthermally increase the magnon frequencies with laser pulses could enable this progress. However, this effect has not been reported to date. To achieve it, here, we explore the optical resonant excitation of high-momentum magnons, which experimentally are observed to couple to low-momentum magnons, modifying the frequencies and amplitudes thereof. This evidence, not caused by laser heating, is explained with a resonant light-scattering mechanism coupling high- and low-momentum eigenmodes across momentum space. Our results disclose routes to inducing instabilities and phase transitions via mode softening and potentially even light-driven Bose-Einstein condensation of magnons and superconductivity mediated by high-momentum spin-fluctuations.