  Excitation of coherent high-frequency magnons (quanta of spin waves) is
critical to the development of high-speed magnonic devices. Here we
computationally demonstrate the excitation of coherent sub-terahertz (THz)
magnons in ferromagnetic (FM) and antiferromagnetic (AFM) thin films by a
photoinduced picosecond acoustic pulse. Analytical calculations are also
performed to reveal the magnon excitation mechanism. Through spin pumping and
spin-charge conversion, these magnons can inject sub-THz charge current into an
adjacent heavy-metal film which in turn emits electromagnetic (EM) waves. Using
a dynamical phase-field model that considers the coupled dynamics of acoustic
waves, spin waves, and EM waves, we show that the emitted EM wave retains the
spectral information of all the sub-THz magnon modes and has a sufficiently
large amplitude for near-field detection. These predictions indicate that the
excitation and detection of sub-THz magnons can be realized in rationally
designed FM or AFM thin-film heterostructures via ultrafast optical-pump
THz-emission-probe spectroscopy.
