Fluorescence and magnetic resonance spectra of Nd3+ and Er3+ ions in single crystals of LaCl3

Abstract

This article presents the results of experiments in which the 19 429.67-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ emission of an ${\mathrm{Ar}}^{+}$ laser is used to pump the ${Z}_{1}$, $^{4}\mathrm{I}_{9/2}$, \ensuremath{\mu}=(5/2\ensuremath{\rightarrow}${F}_{1}$, $^{2}\mathrm{G}_{9/2}$, \ensuremath{\mu}=(1/2 transition of ${\mathrm{Nd}}^{3+}$ ions dilutely substituted at ${\mathrm{La}}^{3+}$ sites in ${\mathrm{LaCl}}_{3}$ crystals in an \ensuremath{\sim}1.6-K bath of liquid He. In some of the work ${\mathrm{Er}}^{3+}$ ions are also present. When an external magnetic field is applied electron paramagnetic resonance (EPR) absorption of microwave energy by either ${\mathrm{Nd}}^{3+}$ or ${\mathrm{Er}}^{3+}$ results in marked intensity changes of ${\mathrm{Nd}}^{3+}$ fluorescence emissions. The essential new feature of this work is the observation of the importance of the effects of the heating of the crystal above the bath temperature by both optical and microwave absorption. The heating by phonons generated in the radiationless transitions associated with the optical pumping cycle results in the emission of 17 E and D ${\mathrm{Nd}}^{3+}$ fluorescences not observed in previous work with the crystal at \ensuremath{\sim}1.6 K. Because of the encapsulation of the crystals in silica envelopes in these experiments, the heating effects are especially marked at higher laser powers.These new fluorescences result from the state population redistributions effected by the rise in temperature of the crystal. EPR microwave absorption by either ${\mathrm{Nd}}^{3+}$ or ${\mathrm{Er}}^{3+}$ accompanied by spin-lattice relaxation results in a further increase in crystal temperature and further ${\mathrm{Nd}}^{3+}$ state population redistributions with concommitant changes in the intensities of the fluorescence emissions from these states of ${\mathrm{Nd}}^{3+}$. This affords a new mechanism for optically detected magnetic resonance (ODMR) of these rare-earth ions. This new mechanism for ODMR produces optical signals strikingly different from those previously studied which result from the changes in the populations of the two components of the ground ${Z}_{1}$ doublet of ${\mathrm{Nd}}^{3+}$ produced by ${Z}_{1}$ microwave absorption or by microwave absorption by the ${\mathrm{Er}}^{3+}$ ground doublet with subsequent transfer of Zeeman quanta between ${\mathrm{Er}}^{3+}$ and ${\mathrm{Nd}}^{3+}$ and resultant changes in optical pumping rates and fluorescence emissions. The factors contributing to the new mechanism for ODMR also account for the observed changes with temperature in the characters of the decays of the ODMR signals when microwave irradiation is terminated. With constant intensity laser irradiation in the absence of external magnetic fields the 28 observed fluorescence emissions oscillate in synchrony at frequencies (1/2 to (1/3 Hz, when the laser irradiation intensity lies in a certain narrow range. Several mechanisms considered as explanations of this phenomenon have failed to account for its observed features. It is clear, however, that these oscillations of emission intensity are accompanied by oscillations in the temperature of the crystal and of the populations of the emitting states.