We report on a high-resolution optical and magneto-optical spectroscopy, luminescence, and electron paramagnetic resonance (EPR) studies of yttrium orthophosphate single crystals doped with erbium, which are promising telecom-wavelength materials for applications in quantum electronics and quantum information processing. An observation of the hyperfine structure in optical spectra of
167Er isotope in
Er:YPO4 is presented. Energies and symmetries of 40 crystal-field levels of
Er3+ in
Er:YPO4 and
g factors of some of them were determined and successfully modeled on the basis of crystal-field calculations. The obtained set of crystal-field parameters was used in modeling the hyperfine structure observed in the optical and EPR spectra of
Er:YPO4 single crystals.
The frequency-field and orientation dependences of the electron paramagnetic resonance (EPR) spectra are measured for impurity Tm 3+ ions in yttrium orthosilicate (Y 2 SiO 5 ) single crystals by stationary EPR spectroscopy in the frequency range of 50–100 GHz at 4.2 K. The position of the impurity ion in the crystal lattice and its magnetic characteristics are determined. The temperature dependences of the spin–lattice and phase relaxation times are measured by pulse EPR methods in the temperature range of 5–15 K and the high efficiency of the direct single-phonon mechanism of spin–lattice relaxation is established. This greatly shortens the spin–lattice relaxation time at low temperatures and makes impurity Tm 3+ ions in Y 2 SiO 5 a promising basis for the implementation of high-speed quantum memory based on rare-earth ions in dielectric crystals.