Crystal field (CF) excitations in the ground multiplet ${}^3{H}_6$ of $\mathrm{T}{\mathrm{m}}^{3+}$ ions were investigated using terahertz transmission spectra of magnetoelectric (ME) $\mathrm{TmA}{\mathrm{l}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ and $\mathrm{T}{\mathrm{m}}_{0.05}\mathrm{Y}{\mathrm{b}}_{0.1}{\mathrm{Y}}_{0.85}\mathrm{A}{\mathrm{l}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$. These excitations were identified as mainly magnetic dipole transitions from the ground singlet $A_1$ to the next excited doublet $E$, split by the CF 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 CF with $D_3$ symmetry. Strong natural optical activity was observed near the CF transitions resulting in a polarization plane rotation up to 25°. The optical activity is quantitatively described by contributions from magnetic and electric dipole transitions to dynamic ME susceptibility and considering the classification of local distortions.

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• Magnetoelectric effect
• Multiferroics
• Electromagnetic wave theory
• Terahertz spectroscopy
• Terahertz techniques