Investigation of a Low-Loss Broadband Pill-box Window Operation over 1THz

A low-loss broadband pill-box window for Terahertz (THz) Vacuum Electron Devices (VEDs) is proposed in this letter. To enlarge the THz vacuum window geometric size benefiting the fabrication process, a novel strong coupling over-mode operation has been proposed and employed without introducing extra oscillation points over a broad operation band. At the same time, the monocrystalline diamond (MD) layer is used to withstand stresses during the brazing process as well as the atmospheric pressure. The window frames use an Oxygen Free Copper (OFC) - Kovar heterogeneous integrated structure, ensuring both vacuum tightness and low-loss transmission. The fabricated device demonstrated exceptional vacuum tightness, with a measured leak rate of 6.95×10⁻¹⁰ Pa·m³/s. The cold test result of the sealed THz MD window demonstrates a recorded low insertion loss (-3.67 dB) at 1 THz, and a 70 GHz bandwidth (0.960 THz – 1.030 THz). The cold test measurements closely match simulation predictions, validating the effectiveness of the strong coupling design in suppressing oscillatory points and the heterogeneous integrated structure in reducing the transmission loss.