[en] A possibility to realize optical transitions within the ⁴I_15/2 ground state of Er³⁺ ion in Si, between levels split by crystal-field, has been investigated by pump-probe technique. The study has been conducted in the THz range on a sublimation MBE-grown Si/Si:Er multinanolayer structure, which allows to take advantage of the preferential formation of a single type of Er-related centers. We present preliminary results, which show absorption band around the wavelength of 43 μm. A resonant transition at this wavelength is predicted for this material from high-resolution photoluminescence measurements. The experimentally observed absorption decay time is most likely due to non-radiative recombination by phonon emission