Accurate THz measurements of permittivity and permeability of BiFeO3 thin films

The electrodynamic properties of BiFeO₃ films in the THz region are investigated via time domain spectroscopy. Combining the use of transmission ($\tilde{T}$) and reflection ($\tilde{R}$) measurements under normal incidence, the refractive index and impedance of the samples under test are evaluated using a retrieval method. From $\tilde{T}$ ($\tilde{R}$) data two complex functions describing the refractive index ${\tilde{n}}_T$ (${\tilde{n}}_R$) and impedance ${\tilde{z}}_T$ (${\tilde{z}}_R$) are extracted from the independent minimization of the error functions given by the difference between the theoretical model and measurements. Knowledge of the pairs (${\tilde{n}}_T$, ${\tilde{n}}_R)$ and (${\tilde{z}}_T$, ${\tilde{z}}_R)$ enables to calculate with a high accuracy both complex permittivity $\tilde{\epsilon }$ and permeability $\tilde{\mu }$ of the sample. Signatures of magnetoelectric effects and phononic resonances are observed in the permittivity and permeability functions and discussed in detail.