Terahertz optical activity near crystal field transitions of Tm3+ ions in magnetoelectric alumoborates

TL;DR

This study investigates terahertz optical activity near crystal field transitions of Tm$^{3+}$ ions in magnetoelectric alumoborates, revealing strong natural optical activity linked to magnetic and electric dipole transitions.

Contribution

It provides a detailed analysis of the optical activity near crystal field transitions in Tm-based magnetoelectric materials, highlighting the role of local crystal field distortions.

Findings

Observed polarization plane rotation up to 25 degrees near CF transitions.

Resolved fine structure of modes at low temperatures.

Quantitatively described optical activity via magnetoelectric susceptibility contributions.

Abstract

Crystal field (CF) excitations in the ground multiplet $^3H_6$ of Tm$^{3+}$ ions were investigated using terahertz transmission spectra of magnetoelectric TmAl$_3$(BO$_3$)$_4$ and Tm$_{0.05}$Yb$_{0.1}$Y$_{0.85}$Al$_3$(BO$_3$)$_4$. These excitations were identified as mainly magnetic dipole transitions from the ground singlet A$_1$ to the next excited doublet E, split by the crystal field of the D$_3$ symmetry. The fine structure of the modes was resolved at low temperatures. It manifested differently in lightly doped and in pure Tm borates, consistent with different distortions of the local crystal field with the D$_3$ symmetry. Strong natural optical activity was observed near the CF transitions resulting in a polarization plane rotation up to 25 degrees. The optical activity is quantitatively described by contributions of magnetic and electric dipole transitions to dynamic magnetoelectric susceptibility and taking into account the classification of local distortions.