In a Y2SiO5:Pr3+ crystal characterized by two non-equivalent Pr3+ optical centers, the interaction mechanisms between Pr3+ ions as well as between Pr3+ ions and a crystal lattice have been studied. It has been shown that along with the electron-phonon interaction, the interaction between Pr3+ ions and two-level systems made an additional contribution to the homogeneous linewidth of the 3H4↔3P0 optical transition. Two different mechanisms of the Pr3+ interionic coupling have been found. The first mechanism is associated with a dipole-dipole interaction between non-equivalent Pr3+ optical centers, while the other one is governed by a cooperative cross relaxation of the excitation energy among the same type Pr3+ optical centers.
Optical read-out and manipulation of the nuclear spin state of single rare-earth ions doped in a crystal enable the large-scale storage and the transport of quantum information. Here, we report the photo-luminescence excitation spectroscopy results of single Pr3+ ions in a bulk crystal of LaF3 at 1.5 K. In a bulk sample, the signal from a single ion at the focus is often hidden under the background signal originating from numerous out-of-focus ions in the entire sample. To combine with a homemade cryogenic confocal microscope, we developed a reflecting objective that works in superfluid helium with a numerical aperture of 0.99, which increases the signal by increasing the solid angle of collection to 1.16π and reduces the background by decreasing the focal volume. The photo-luminescence excitation spectrum of single Pr3+ was measured at a wing of the spectral line of the 3H4 right arrow 3P0 transition at 627.33 THz (477.89 nm). The spectrum of individual Pr3+ ions appears on top of the background of 60 cps as isolated peaks with intensities of 20–30 cps and full-width at half-maximum widths of approximately 3 MHz at an excitation intensity of 80 W cm−2.
Laser-selective excitation and optical-absorption techniques have been used to establish comprehensive sets of energy levels for both the ${\mathit{F}}^{\mathrm{\ensuremath{-}}}$ and hydrogenic varieties of the tetragonal symmetry (
C4v) centers in
SrF2 and
CaF2. Polarization of the various spectral lines was used to determine the symmetry labels of many of the energy levels. Reversals of the ordering of some levels between
SrF2 and
CaF2 are found and, in particular, the two lowest levels (
D1 and
D2) of the
1D2 multiplet, which are the originating levels for many of the observed emission transitions, have reversed-symmetry label assignments in
SrF2 and
CaF2. Differences were found between the ground-state Zeeman splittings for the
C4v ${\mathit{F}}^{\mathrm{\ensuremath{-}}}$ and ${\mathit{D}}^{\mathrm{\ensuremath{-}}}$ centers for both
SrF2 and
CaF2. Fluorescence lifetimes of the
C4v hydrogenic centers show a strong dependence on the particular charge-compensating ion present through nonradiative decay processes involving the respective local mode phonons. The upconversion spectra observed for ${\mathit{F}}^{\mathrm{\ensuremath{-}}}$, ${\mathit{T}}^{\mathrm{\ensuremath{-}}}$, and ${\mathit{D}}^{\mathrm{\ensuremath{-}}}$ varieties of the
C4v centers in
SrF2 and for the
C4v ${\mathit{F}}^{\mathrm{\ensuremath{-}}}$ center in
CaF2 are all consistent with excitation of specific subsets of nearby
C4v centers.
Excited-state absorption spectra obtained by a pulsed pump–probe technique specially designed for the near-UV spectral domain were recorded in three Pr3+-doped fluoride crystals (KY3F10,LiYF4, and BaY2F8). These excited-state absorption spectra correspond to optical transitions from the 3P0 or the 1D2 metastable levels of the 4f2 configuration up to the first energy levels of the 4f5d electronic configuration. The position, the anisotropic behavior, and the line shape of the observed excited-state absorption bands are discussed and compared with ground-state absorption (from level 3H4) or direct excitation spectra available in the literature. Subsequently, two-step excitation into the 4f5d high-lying energy levels of Pr3 by means of 3P0 and 1D2 is shown to be an efficient way to produce broadband UV fluorescence by use of pump beams in the blue and the near UV.
Rare earth ions in crystals exhibit narrow spectral features and hyperfine-split ground states with exceptionally long coherence times. These features make them ideal platforms for quantum information processing in the solid state. Recently, we reported on the first high-resolution spectroscopy of single ions in yttrium orthosilicate nanocrystals via the transition at a wavelength of 488 nm. Here we show that individual praseodymium ions can also be detected on the more commonly studied transition at 606 nm. In addition, we present the first measurements of the second-order autocorrelation function, fluorescence lifetime, and emission spectra of single ions in this system as well as their polarization dependencies on both transitions. Furthermore, we demonstrate that by a proper choice of the crystallite, one can obtain narrower spectral lines and, thus, resolve the hyperfine levels of the excited state. We expect our results to make single-ion spectroscopy accessible to a larger scientific community.