Fine level splitting of aggregate neodymium centers in CaF2 crystals Original Research Article

A fine spectral structure of pair and quartet centers of Nd ions in CaF2 crystals was investigated by the selective laser absorption saturation and time resolved pump/probe technique at 8–30 K. The spectral hole–antihole pairs were observed under laser excitation of the pair Nd ions. It was shown that the main absorption lines reflect a splitting of the lowest Stark level of the 4G5/2 state caused by a strong coherent interaction between Nd ions in cluster with total splitting values 2.4 and 3.3 cm–1 for pair and quartet centers, respectively. Energy diagram, including singly and doubly excited states, was built for a strongly coherent interacting pair. The study of dynamics of the absorption saturation for different time delay between probe and pump pulses allowed us to estimate relaxation time (∼8 ns) among ground and excited state sub-levels.