Selective laser spectroscopy of impurity Ho$^{3+}$ centers in SrY$_2$O$_4$ single crystal

Detailed studies of emission and excitation spectra of impurity Ho$^{3+}$ centers in the SrY$_2$O$_4$ single crystal performed by means of the site-selective laser spectroscopy are reported. The energy levels patterns for the $^5I_8$, $^5I_7$, $^5I_6$, $^5S_2$ and $^5F_4$ multiplets of Ho$^{3+}$ ions which substitute for the Y$^{3+}$ ions at the two structurally nonequivalent sites Y1 and Y2 with the local Cs symmetry are constructed by making use of the measured spectra. These energy schemes are well described in the framework of the crystal field theory, the sets of 15 crystal field parameters are determined for Ho$^{3+}$ ions at both Y1 and Y2 sites. The calculated parameters of the EPR spectra agree with the earlier published experimental data.