Narrowband terahertz (THz) radiation is crucial for high-resolution spectral identification, but a narrowband THz source driven by a 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 multicycle THz pulse with a narrow linewidth down to ∼1.5 GHz, which is in contrast to 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.
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