An Ultra-wideband Elliptical Bowtie Antenna With Depressed Bias for THz Vacuum Photomixers

In this paper, a novel ultra-wideband elliptical bowtie antenna with depressed bias is presented for the THz vacuum photomixer. The proposed antenna obtains a broad frequency bandwidth of 0.2-2 THz with a port reflection
$\vert S_{11}\vert$
lower than -10 dB and a smoothly varied radiation resistance with an average value of ~60Ω. Under the collimation by an extended hemispherical lens, its radiation gain covers a range of 7.0-16.7 dBi within the corresponding bandwidth. Besides, to decrease the electron collision with the isolator and meanwhile lower the energy of the collected electron, a depressed bias structure in the vacuum channel is employed: a ring-shaped gate structure with 20 V positive bias is placed in the middle to accelerate the electron and the other antenna arm connected to the anode is biased with a depressed voltage of 5 V to decelerate the accelerated photoelectrons from the gate. Due to the introduction of such a structure, the electron transmission rate has been increased from 69.8% to 96.3%.