Fabry–Pérot Cavities for the Terahertz Spectral Range Based on High-Reflectivity Multilayer Mirrors

We constructed Fabry–Perot cavities using well-characterized broadband dielectric multilayer mirrors based on z-cut quartz with a clear aperture of 20 mm. Each quartz wafer thickness was measured with an error significantly less than 1%, which enabled the accurate simulation of the cavity mode frequency positions. Characterization methods in transmission geometry involved white-light interferometry, terahertz Fourier transform, and time-domain spectroscopy. The latter enabled the determination of the cavities phase response. The first transmission stopband with strong attenuation of the multilayer mirrors appears between 0.5 and 0.9 THz. With four dielectric layers, a reflectivity of at least 98.6% and a full-width at half-maximum of more than 53% with respect to the center design frequency at 0.75 THz are obtained. A loaded 2-mm cavity made from two mirrors, each with two layers, reached a Q-factor of at least 54. In all cases, we find a good agreement between the experimental transmission and phase response of our structures and the simulations based on the transfer matrix formalism.