Efficient extraction of terahertz wave totally reflected at semiconductor surface by double metal gratings

We propose double metal grating as a way of extracting terahertz (THz) wave which is totally reflected at the surface of THz emitter such as quantum cascade lasers, THz parametric oscillators, etc. By tuning the grating parameters, efficiencies as high as 60% for vertical output and 70% for tilted output is predicted assuming practical material loss. The high efficiency is realized by the control of Fano resonance between the direct and resonant scatterings, the latter introduced as the Fabry-Perot resonance between the two gratings. Then, the results are analyzed using temporal coupled mode theory, to show how the high efficiency is realized.