A terahertz active bandpass filter based on microplasma defects

Plasma defect photonic structures have demonstrated active bandpass filtering from microwave to W-band frequencies, but experimental extension to the terahertz (THz) regime has been hindered by insufficient plasma density and large structural dimensions. Here, we report a local-waveguide THz bandpass filter with a high-density atmospheric-pressure microplasma point defect embedded in a two-dimensional alumina ceramic rod array. Local waveguide channels formed adjacent to the defect facilitate electromagnetic coupling into the resonant cavity. A narrow passband is observed at 201.4 GHz with an insertion loss of 12.4 dB. Increasing the electron density from the plasma-off state to 4.86 × 1014 cm−3 experimentally blue-shifts the passband center frequency by approximately 3 GHz, demonstrating electron-density-controlled active tuning. The corresponding peak transmittance decreases from 0.058 to 0.037. Over the simulated maximum-electron-density range of 1014–1015 cm−3, the calculated quality factor ranges from approximately 481 to 569. Coupled microplasma and THz transmission simulations incorporating the spatially inhomogeneous electron-density distribution reproduce the measured passband response and its density-dependent tuning trend. Simulations predict that widening the plasma diameter from 0.5 to 0.65 mm shifts the center frequency from 199.8 to 203.4 GHz and broadens the full width at half maximum from 0.632 to 0.99 GHz, offering an additional structural degree of freedom for filter optimization. These results experimentally validate electron-density-controlled active tuning of THz narrowband filtering and provide a basis for dynamically reconfigurable THz filtering devices.