Rotation of terahertz radiation due to phonon-mediated magnetoelectric coupling in chiral selenium

Abstract

The trigonal form of selenium (t-Se) has an unusual, quasi-one-dimensional, chiral crystal structure. First-principles calculations have helped us uncover a polar, optic phonon in t-Se that exhibits a diagonal magnetoelectric coupling with the electric field of the incident THz radiation, and induces a parallel magnetic field due to its inherent chirality. We show that this phonon-mediated, magnetoelectric mechanism is predicted to cause optical rotation in t-Se in the 1--3 THz range and is quite distinct from the high frequency (g80 THz) activity due to electronic excitations that has been reported previously. In the second part of the paper, we report our experimental results based on THz time-domain spectroscopy as well as direct measurements of optical rotation that confirm this prediction in an aligned, monocrystalline array of t-Se microrods. The Se microrod array not only exhibits a large birefringence ($\mathrm{\ensuremath{\Delta}}n=1.3$) but also rotates the polarization of THz radiation by \ensuremath{\sim}3\ifmmode^\circ\else\textdegree\fi{}/mm. To our knowledge, this is the only elemental solid known to rotate THz polarization, because the currently available THz rotators are based mainly on liquid crystals or metamaterials. The identification of new THz-active materials and a better understanding of the underlying physics are both clearly essential to the development of better sources, detectors and components.