We present a method for generating a narrowband terahertz (THz) emission using a radially oscillating plasma oscillator formed in a plasma slab with a longitudinal paraboliclike density profile. Through quasi-three-dimensional particle-in-cell simulations, we observe more stable, longer-lived plasma oscillations near the vertex of the density profile, resulting in temporally coherent THz emission. In contrast, a linear density gradient leads to faster decoherence of the plasma oscillator. The radiated THz pulse has a narrow spectral width of approximately 0.66 THz ($\sim 3.36\%$ relative bandwidth), with a spectral energy density efficiency of $0.01\%$ per THz, in good agreement with analytical results. These findings demonstrate the potential of this scheme to efficiently concentrate THz energy into a compact, narrower spectral band, offering a promising route toward the development of next-generation narrowband THz sources for applications in THz-driven particle acceleration and ultrafast wireless communication.

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• Laser-plasma interactions
• Terahertz generation in plasmas
• Laboratory plasma
• Particle-in-cell methods