Coherent nonlinear magnonics

Abstract

Coherent manipulation of interacting collective modes offers a route to ultrafast information processing in quantum materials. Here we summarize a sequence of terahertz experiments that establishes complementary nonlinear energy-conversion pathways between the quasi-ferromagnetic (qFM) and quasi-antiferromagnetic (qAFM) magnon modes of the canted antiferromagnetic ErFeO3. Polarization-resolved time-domain measurements first show that intense single-cycle terahertz magnetic fields drive the qFM mode and generate a qAFM response that is forbidden in linear excitation. Two-dimensional terahertz spectra identify the off-diagonal coherence transfer, its quadratic field scaling, and its strong enhancement when the qAFM frequency approaches twice the qFM frequency. A complementary two-pulse experiment then excites both modes and isolates field-assisted downconversion, magnonic difference-frequency generation, and stimulated degenerate parametric amplification of the lower-frequency qFM coherence. At the frequency-matching condition near 200 K, the downconversion and difference-frequency features merge into a strongly enhanced response reproduced by nonlinear spin-dynamics calculations. Together, these results establish canted antiferromagnets as tunable platforms for polarization-selective magnon conversion and amplification at terahertz rates.