Magnonics is a research field complementary to spintronics, in which the quanta of spin waves (magnons) replace electrons as information carriers, promising less energy dissipation 1-3 . The development of ultrafast nanoscale magnonic logic circuits calls for new tools and materials to generate coherent spin waves with frequencies as high, and wavelengths as short, as possible 4,5 . Antiferromagnets can host spin waves at THz frequencies and are therefore seen as a future platform for the fastest and the least dissipative transfer of information 6-11 . However, the generation of short-wavelength coherent propagating magnons in antiferromagnets has so far remained elusive. Here we report the efficient emission and detection of a nanometer-scale wavepacket of coherent propagating magnons in antiferromagnetic DyFeO3 using ultrashort pulses of light. The subwavelength confinement of the laser field due to large absorption creates a strongly nonuniform spin excitation profile, enabling the propagation of a broadband continuum of coherent THz spin waves. The wavepacket features magnons with detected wavelengths down to 125 nm that propagate with supersonic velocities of more than 13 km/s into the material. The long-sought source of coherent shortwavelength spin carriers demonstrated here opens up new prospects for THz antiferromagnetic magnonics and coherence-mediated logic devices at THz frequencies.Antiferromagnetic insulators (AFMs) are prime candidates to replace ferromagnets (FMs) as active media in the quest towards high-speed spin transport and large spectral bandwidth operation [6][7][8] . Integration of AFMs in future wave-based technologies 3 crucially requires the realization of coherent (ballistic) transport of antiferromagnetic spin waves over large distances 5 . In this regard, non-uniform spin-wave modes with short wavelengths (λ ≲ 100 nm) are of particular importance: they can operate at THz clock rates, exhibit high propagation velocities and enable the miniaturization of devices down to the nanoscale. Phase-coherent ballistic spin transport in AFMs
We report a theory of optical generation and detection of the propagating spin waves in antiferromagnetic materials relevant for the ultrafast pump-probe experiments. We derive and solve the equations of motion for antiferromagnetic spins in response to the light-induced effective magnetic field in the linear regime.Different forms of the excitation and the properties of the generated spin waves are analysed. We theoretically show the selective detection of the spin waves by the magneto-optical Kerr effect. The developed formalism is readily applicable to inform future experiments on antiferromagnetic opto-magnonics.
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