Electronic structure of non-Kramers Tb3+ centers in garnet crystals and evidence of their energy and spin transfer to Ce3+ emitters

The electronic structure of non-Kramers Tb3+ centers in single crystals of yttrium aluminum garnet (YAG) was studied using a new-generation high-frequency magnetic resonance spectrometer that allows measurements of electron paramagnetic resonance (EPR), electron spin echo (ESE), the photon echo on hyperfine components of Stark levels in zero magnetic field, and optically detected magnetic resonance (ODMR). It was created on the basis of a highly stable microwave bridge operating in the near-terahertz region (0.094 or 0.130 THz), both in continuous-wave (cw) and pulse modes, a nonresonant system for supplying microwave power to a sample, and a cryogen-free magneto-optical cryostat. EPR, ESE, and cw measurements at two frequencies allowed us to reliably and with high accuracy determine the parameters of Tb3+ centers that occupy yttrium dodecahedral positions in YAG: g(parallel to) = 15.8 +/- 0.2, g(perpendicular to) approximate to 0 (parallel to corresponds to one of the < 100 > crystal axes), the zero-field splitting Delta = 2.705 +/- 0.005 cm(-1), and the hyperfine interaction constant A = 0.197 +/- 0.005 cm(-1). In addition to the Tb(3+ )centers in a regular environment, EPR spectra with lower intensity and a resolved hyperfine structure were found for at least three types of additional Tb3+ centers. They have symmetry, g factors, and hyperfine interaction constants which are close to those of the main Tb3+ centers but differ in the zero-field splitting parameters Delta that strongly depend on the crystal field. They were ascribed to Tb3+ centers with nearby antisite Y-Al defects. Similarity with several types of Ce3+ centers in YAG crystals, the types, and relative concentrations of antisite defects are discussed. For one of the additional Tb3+ centers, splitting of the energy levels in the zero field turned out to be close to an energy of 94 GHz microwave quanta, and intense echo signals were observed in weak magnetic fields and in the zero field corresponding to the EPR transitions between the hyperfine components of the Tb3+ spin levels. ODMR spectra of Tb3+ centers in YAG crystals, containing Ce and Tb, were obtained by monitoring the intensity of the Ce3+ luminescence, which implies the transfer of energy from Tb3+ ions to Ce3+ ions with conservation of spin. This result seems to be important, since these systems are of interest for quantum communication and computations. Taking into account that cerium does not have isotopes with a nuclear spin and that terbium has 100% Tb-159 having nuclear spin I = 3/2 and a sufficiently large nuclear magnetic moment, the Tb3+ - Ce3+ system in garnet crystals can be promising for coherent information processing. Tb3+ ions can play a role of qubits and Ce3+ ion with a lifetime of about 50 ns, and almost unity quantum efficiency of optical transitions can be used as a single-ion readout arrangement. The prospects for using this system as hardware for quantum communication and computations are discussed.