Out-coupling characteristics of partial resonator mirror for compact terahertz free-electron laser

Terahertz light sources with small size and high output enable a variety of new applications. Free-electron laser (FEL) is the most powerful light source in the terahertz (THz) range with perfect wavelength tunability. However, the size of the FEL facility is too large. We are developing a table-top THz FEL using a small microtron accelerator. Through the development of a high-performance and compact undulator and a new waveguide-mode resonator, we confirmed that an FEL size of 1.5 × 2 m² is possible. One of the reasons we could design the small FEL is because we do not use electromagnets to force the electron beam into and out of the FEL resonator. We have developed an out-coupling mirror of the FEL resonator for a wide spectral range from 0.5 to 1 THz to have a structure in which the electron and THz beam transmit simultaneously without any bending magnets. The out-coupling mirror has wire-grid-polarizer (WGP) structure in the center. This paper discusses optimizing the WGP's parameters like wire thickness and period to get appropriate reflectance and transmittance in the 0.5–1 THz region and has low electron beam loss in the waveguide-based resonator using the COMSOL Multiphysics simulation. Simulations found the optimized value of wire thickness and period as 20 and 100 µm, respectively. We further calculated the TE transmittance of the WGP, which is 1-030% for the optimized values, depending on the frequency, ranging from 0.5 to 1 THz. Experiments using the THz time-domain spectroscopy method validated that the measured results agreed with those of the simulations.