Ultrafast optical excitation reshapes magnon spectra by modifying exchange interactions. This enables dynamic control of spin waves, advancing high-speed, energy-efficient spintronic and magnonic device technologies.
Study: Photoengineering the magnon spectrum in an insulating antiferromagnet. Image Credit: LIGHT_ONLY/Shutterstock
In a recent research article published in the journal Nature Physics, researchers demonstrated that femtosecond optical pumping of charge-transfer excitations in the antiferromagnet DyFeO3 can coherently reshape the spin-wave spectrum without destroying long-range magnetic order.
Optical Control of Magnons in Insulating Antiferromagnets
Magnons represent collective oscillations of the antiparallel spin structure native to antiferromagnets, occurring at terahertz frequencies, which are crucial for future high-speed spintronic technologies. The research focuses on the ability of femtosecond laser pulses to manipulate the magnon spectrum by tuning the fundamental exchange interaction, the quantum mechanical force that governs the magnetic order. Such control is pivotal to the development of ultrafast, energy-efficient spin-based devices.
Resonant Optical Excitation of Charge-Transfer Transitions
At the heart of this optical control is the resonant pumping of charge-transfer (CT) electronic transitions within DyFeO3. These CT transitions involve electron movement between ions and play a central role in determining the strength of magnetic exchange interactions. When the material is excited with femtosecond pulses above the bandgap, transient photoexcited carriers are generated. These carriers drastically weaken the exchange interaction in a nanoscale near-surface region, inducing a profound reshaping of the magnon spectrum on ultrafast timescales.
Experimental Approach: Ultrafast Pump-Probe Spectroscopy
The authors utilize an advanced optical pump-probe technique to observe ultrafast spin dynamics. A 50-femtosecond Ti:Sapphire laser system delivers pump pulses that resonantly excite CT electronic states in the sample. Subsequently, time-delayed probe pulses monitor the evolution of magnons by measuring changes in the polarization state of the reflected or transmitted light (Faraday and Kerr effects).
This setup enables femtosecond temporal resolution and spectral specificity to track real-time modifications in magnon frequencies and amplitudes. Experiments performed at cryogenic temperatures ensure magnetic order is stable and that observed effects are nonthermal in origin.
Observation of Magnon Spectrum Renormalization
Time-resolved measurements reveal the emergence of a broadband continuum of in-gap magnon states with frequencies substantially lower than the equilibrium magnon gap. Fourier-transform analysis shows an asymmetric broadening and a redshift of magnon spectral lines under strong optical pumping. Crucially, these new low-frequency states cannot be attributed to heating or thermal population effects because their frequencies lie outside thermally accessible ranges.
Moreover, the amplitude of magnons at zero momentum grows super linearly with pump fluence, indicating a nonlinear but nonthermal origin related to exchange interaction quenching. The optical excitation thus transiently engineers a new magnetic state with modified magnon dispersion.
Spatial Localization and Near-Surface Exchange Quench
The study finds that the optical excitation effects are strongly confined within the optical penetration depth, on the order of nanometers, near the sample surface. Numerical simulations incorporating a spatially varying quench of the exchange interaction confirm that this localized reduction creates a potential well trapping magnons with finite in-plane momentum. These trapped magnons exhibit reduced frequencies and velocities, directly linking the optically modified exchange to spectral changes. The spatial profile of the quenched exchange explains both the broad spectral linewidths and the experimental fluence dependence of magnon amplitudes.
Theoretical Modelling of Optically Induced Exchange Quench
To quantitatively describe the observed phenomena, the research leverages numerical solutions of coupled spin evolution equations in a one-dimensional lattice model. The simulations introduce a depth-dependent exchange quench matching the experimental pump penetration profile.
Results show excellent agreement with data, reproducing the amplitude growth of zone-center magnons and diminished propagation of finite-momentum magnons at high fluences. Alternative scenarios, such as changes in anisotropy or local-moment size, fail to reproduce the experimental spectral line shapes and dynamics. This validates that the primary optical effect is a transient, near-surface quench of the exchange interaction induced by resonant CT photoexcitation.
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Exclusion of Competing Mechanisms
The optical control of magnons observed is not due to simple heating, change in Dzyaloshinskii-Moriya interaction, or nonlinear magnon-magnon scattering. The authors provide evidence that these alternative effects are either too weak or contradictory to account for their results. For example, the magnon gap frequency shifts opposite to what thermal effects predict, and measurements in phases without spin canting show no comparable dynamics.
Furthermore, modifications to magnetic anisotropy alone do not trigger the robust magnon renormalization found here. These arguments reinforce that the exchange quench driven by electronic photoexcitation is the dominant mechanism.
Implications for Ultrafast Magnonics and Spintronics
The demonstrated optical control of exchange interactions opens exciting opportunities for designing reconfigurable magnonic devices operating at terahertz frequencies. By dynamically tuning magnon bands on nanosecond to picosecond timescales, it becomes possible to create ultrafast magnonic crystals and spintronic elements with functionalities controlled solely by light. This approach surpasses prior strategies relying on nonlinear magnon interactions or resonant terahertz excitation, offering a new paradigm based on electronic resonance manipulation.
Journal Reference
Radovskaia V., Andrei R., et al. (2026). Photoengineering the magnon spectrum in an insulating antiferromagnet. Nature Physics. DOI: 10.1038/s41567-026-03230-6, https://www.nature.com/articles/s41567-026-03230-6