Laser Site-Selective Spectroscopy and Magnetic Hyperfine Splittings of Ho${}^{3+}$ doped Y${}_2$SiO${}_5$

Laser site-selective spectroscopy and high-resolution absorption measurements have been used to determine 51 crystal-field energy levels for one of the Ho${}^{3+}$ centres in Y${}_2$SiO${}_5$. This centre is denoted as Site 2 and has been tentatively assigned as the seven-fold coordinated centre. High resolution absorption measurements reveal complex hyperfine patterns that obey and approximate selection rule. The application of a magnetic field along the three optical axes reveals the presence of avoided crossings below 0.5 Tesla, in both the ground and excited states.