Accurate THz Measurements of Permittivity and Permeability of BiFeO$_3$ Thin Films

TL;DR

This paper presents a precise method for measuring the complex permittivity and permeability of BiFeO$_3$ thin films in the THz range using time domain spectroscopy, revealing magnetoelectric effects and phononic resonances.

Contribution

The study introduces a combined transmission and reflection measurement approach with a retrieval method to accurately determine permittivity and permeability of BiFeO$_3$ films in the THz region.

Findings

High-accuracy permittivity and permeability measurements

Detection of magnetoelectric effects

Observation of phononic resonances

Abstract

The electrodynamic properties of BiFeO$_3$ 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 $\tilde{\varepsilon}$ 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.