Highly efficient narrowband terahertz pulse generated from a radial plasma oscillator via mid-infrared lasers.

Plasma-based terahertz (THz) sources are currently limited to low laser-to-THz conversion efficiencies. Generating narrowband THz radiation remains particularly challenging for high-power THz applications. Here we demonstrate an unprecedented 0.88% laser-to-THz energy conversion efficiency from the recently proposed radial plasma oscillator (RPO) scheme [M. Kumar et al., Phys. Rev. Lett. 134, 015001 (2025)] driven by mid-infrared laser pulses at [Formula: see text] and [Formula: see text]. Theoretical modelling and quasi-3D particle-in-cell simulations have been used to achieve THz pulse energies up to 0.341 mJ, with a peak field strength of 6.94 GV/m and a narrow spectral width ~ 15.5%. It is also shown that the high efficiency is well-preserved under field-ionized and collisional plasma conditions, making it a practical pathway toward compact narrowband high-power THz sources.