Electromagnon excitation in cupric oxide measured by Fabry-Perot enhanced terahertz Mueller matrix ellipsometry

Here we present the use of Fabry-Perot enhanced terahertz (THz) Mueller matrix ellipsometry to measure an electromagnon excitation in monoclinic cupric oxide (CuO). As a magnetically induced ferroelectric multiferroic, CuO exhibits coupling between electric and magnetic order. This gives rise to special quasiparticle excitations at THz frequencies called electromagnons. In order to measure the electromagnons in CuO, we exploit single-crystal CuO as a THz Fabry-Perot cavity to resonantly enhance the excitations signature. This enhancement technique enables the complex index of refraction to be extracted. We observe a peak in the absorption coefficient near 0.705 THz and 215 K, which corresponds to the electromagnon excitation. This absorption peak is observed along only one major polarizability axis in the monoclinic a-c plane. We show the excitation can be represented using the Lorentz oscillator model, and discuss how these Lorentz parameters evolve with temperature. Our findings are in excellent agreement with previous characterizations by THz time-domain spectroscopy (THz-TDS), which demonstrates the validity of this enhancement technique.Funding Agencies|National Science Foundation [DMR 1808715]; Air Force Office of Scientific Research [FA9550-18-1-0360]; Nebraska Materials Research Science and Engineering Center [DMR 1420645]; EPSRC [EP/N034872/1]; University of Nebraska Foundation; J.A. Woollam Foundation