  Electromagnetic resonators, which are based on optical cavities or electronic
circuits, are key elements to enhance and control light-matter interaction. In
the THz range, current optical cavities exhibit very high-quality factors with
$(\lambda/2)^3$ mode volumes limited by diffraction, whereas resonant
electronic circuits show low quality factor but provide strong subwavelength
effective volume ($10^{-6} \lambda^3$). To overcome the limitations of each
type of resonator, great efforts are being devoted to improving the
performances of current methods or to the emergence of original approaches.
Here, we report on an optical resonator based on Tamm modes newly applied to
the THz range, comprising a metallic layer on a distributed Bragg reflector and
demonstrating a high-quality factor of 230 at $\sim$1 THz. We further
experimentally and theoretically show a fine-tuning of the Tamm mode frequency
(over a 250 GHz range) and polarization sensitivity by subwavelength
structuration of the metallic layer. Electromagnetic simulations also reveal
that THz Tamm modes are confined over a $\lambda$/2 length within the
distributed Bragg reflector and can be ideally coupled to both bulk materials
and 2D materials. These THz Tamm cavities are therefore attractive as basic
building blocks of lasers, for the development of advanced THz optoelectronic
devices such as sensitive detectors, high-contrast modulators, narrow filters,
and polarizers, as well as for THz cavity quantum electrodynamics in
nanostructures.
