  Narrowband terahertz (THz) radiation is crucial for high-resolution spectral
identification, but a narrowband THz source driven by femtosecond (fs) laser
has remained scarce. Here, it is computationally predicted that a
metal/dielectric/magnetoelastic heterostructure enables converting a fs laser
pulse into a multi-cycle THz pulse with a narrow linewidth down to ~1.5 GHz,
which is in contrast with the single-cycle, broadband THz pulse from the
existing fs-laser-excited emitters. It is shown that such narrowband THz pulse
originates from the excitation and long-distance transport of THz spin waves in
the magnetoelastic film, which can be enabled by a short strain pulse obtained
from fs laser irradiation of the metal film when the thicknesses of the metal
and magnetoelastic films both fall into a specific range. These results
therefore reveal an approach to achieving optical generation of narrowband THz
pulse based on heterostructure design, which also has implications in the
design of THz magnonic devices.
