Minimization of reflection at the boundaries of a finite-size coupled terahertz cavity in a metal air-gap waveguide

Introducing antireflection slits at the boundaries of a finite-size coupled terahertz cavity in a metal air-gap waveguide can minimize reflections at the boundaries. The optimal design parameters for the minimal reflection were obtained using the one-dimensional antireflection coating theory and finite-difference time-domain simulations. We experimentally demonstrated that the optimized antireflection slits significantly reduce the finite-size effects such as the strong Fabry-Perot oscillations in the transmission spectrum and the variations in the group velocity curve.