Efficient conversion of the ultrashort THz pulse into a narrowband oscillation

Abstract Efficient conversion of ultrashort pulses into coherent, narrowband radiation with tunable frequency remains a major challenge in terahertz (THz) photonics. Here, we propose a compact mechanism that exploits Sommerfeld standing waves in thin conductive coupled rods to achieve dynamic spectral narrowing of ultrashort pulses. In a system of two parallel rods separated by a deeply subwavelength gap, coupled symmetric and antisymmetric modes are excited by an incident pulse. While the dipole-active symmetric mode rapidly decays, the dipole-passive antisymmetric mode accumulates energy and emits narrowband quadrupole and magnetic dipole radiation. This self-induced spectral compression arises from strong field localization and internally driven charge redistribution, enabling conversion efficiencies around 25%. Our approach offers a simple metasurface-compatible platform for tunable THz waveform control, with potential applications in spectroscopy, sensing, and quantum photonics.