We investigated the hole-burning phenomena and homogeneous linewidth in
Pr3+-doped
Y2O3-based crystals under the systematic control of disorder in the crystal. When divalent ions are doped into an
Y2O3:Pr3+ crystal, holes due to rearrangement of local structure around
Pr3+ ions are burned. The burning curves are successfully analyzed in terms of a distributed tunneling rate model for hole formation. The burning efficiency from this analysis and the homogeneous linewidth become larger as the crystal includes a larger amount of divalent ions. The interaction of
Pr3+ ions with oxygen vacancies is thought to be one of the origins of the hole production and the line broadening. The interaction length between
Pr3+ ions and oxygen vacancies is discussed.
The persistent spectral hole-burning phenomenon was investigated for Sm2+ ions-doped Al2O3-SiO2 glasses prepared by the sol-gel method. The efficiency of hole-burning, burned at low temperature ~77K, was proportionally increased with the content of OH groups surrounding the rare-earth ions. The proposed mechanism for hole-burning was the optically activated rearrangement of the OH bonds surrounding the rare-earth ions. The burned-hole was thermally refilled and erased above ~200 K. On the other hand, the glasses obtained by heating in H2 gas or irradiating with x-rays showed PSHB up to room temperature. When heated in H2 gas, the H2 molecules react with oxygen ions to form H2O. Removal of the generated H2O causes the number of oxygen ions surrounding rare-earth ions to decrease, resulting into the reduction of the ions. The hole burning in the H2-treated glasses was performed by the electron transfer between the rare-earth ions and the trapping centers. In contrast, in the x-ray irradiated glass, it was concluded that the rear-earth ions are deuced into the divalent state by electron transfer from the oxygen defect center. The hole defect centers are trapped in oxygen ions bound with Al3+ ions. The spectral hole burning of the x-ray irradiated glasses could be burned by the reverse reaction of the reduction of the rare-earth ions. A short distance between the Sm2+ and oxygen defect centers brought a high-speed hole burning, that is, 30 times faster than in a similar H2 gas treated glass.
We have observed persistent spectral hole burning in a family of centers in CaF2:Pr3+:D− and SrF2:Pr3+:D− crystals in which the Pr3+ ion has multiple D−-ion near neighbors. The hole burning is caused by light-induced D- motion that leads to persistent changes in the local crystal field of the Pr3+ ion. Depending on the center and the temperature, side-hole or antihole structure is observed that has its origin in the second-order hyper-fine interaction. The variation in the hyperfine splittings between centers is consistent with the separation between the ground state and the first excited state.
Laser induced hole burning in optical absorption bands of low temperature solids has been investigated with emphasis on mechanisms for population hole burning, the interactions with the environment which limit the hole widths and the use of hole burning for ultra high resolution spectroscopy. Hole burning was observed in a wide range of solids including organic and organometallic molecules in crystals and rare earth ions in crystals and glasses. The mechanisms for hole burning also varied widely and a new mechanism involving optically induced nuclear spin flips of near neighbor nuclei was discovered. Examples of hole burning in new classes of materials are presented here for the first time, namely inorganic glasses (silicate glasses containing Nd(3+), Pr(3+) or Eu(3+)) and undoped stoichiometric compounds (e.g., EuP5O14). Hole recovery was used to determine nuclear spin lattice relaxation rates in praseodymium doped crystals. For allowed optical transitions (e.g., singlet states of zinc porphyrin, dibromoindigo and color centers in alkali halides) the hole widths at 2K are limited by population decay to approximately 50 MHz. For longer lived optical levels in crystalline materials, e.g., those containing rare earth ions, the hole widths are narrower (less than 10 MHz) and are limited by dynamical interactions with surrounding nuclei. In amorphous materials such as europium doped silicate glass, hole widths are much broader, being limited by low frequency tunneling motions. Hole burning measurements were complemented by time domain coherent transient (photon echo, and optical free decay) measurements of homogeneous linewidths which enabled us to measure optical resonances as narrow as 760 Hz in Eu(3+):Y2O3.
Persistent and time-resolved transient holes were burnt into the
1D2${\ensuremath{\leftarrow}}^{3}$
H4 transition of
Pr3+ and the
5D0${\ensuremath{\leftarrow}}^{7}$
F0 transition of
Eu3+ in silicate glasses. The holewidths were studied as a function of burning-fluence density, wavelength, and temperature between 0.4 and 12 K. Transient holes arising from optical pumping of ground-state nuclear hyperfine levels were found for both ions at T3 K. Their lifetime decreases with increasing temperature. They are accompanied by symmetrically placed antiholes. For
Eu3+, two pairs of antiholes were found which are attributed to the two europium isotopes,
151Eu and
153Eu. For
Pr3+, an additional mechanism leading to persistent hole burning dominates above 3 K. The holewidth follows a ${\mathit{T}}^{(1.3\ifmmode\pm\else\textpm\fi{}0.1)}$ dependence for
Pr3+ and a ${\mathit{T}}^{(1.0\ifmmode\pm\else\textpm\fi{}0.1)}$ dependence for
Eu3+. The temperature dependence is the same as that of the contribution of the two-level systems to the specific heat measured on the same samples and in the same temperature range. The quadrupole splittings of the
7F0 ground state and the crystal-field splittings of the
7F1 and
7F2 levels of
Eu3+ as a function of excitation wavelength indicate that the strength of the noncubic component of the crystal field varies monotonically for excitation across the
5D0${\ensuremath{\leftarrow}}^{7}$
F0 absorption band, falling to zero at the long-wavelength edge. The hole width shows a similar variation with excitation frequency. No influence of the experimental time scale on the hole width was detected for either sample between ${10}^{\mathrm{\ensuremath{-}}2}$ and
102 s.
Due to their narrow homogeneous linewidths, rare-earth ions in inorganic crystals at low temperatures have recently been given considerable attention as test materials for experiments in coherent quantum optics. Because these narrow linewidth transitions have been buried in a wide inhomogeneous line, the scope of experiments that could be carried out in these materials has been limited. However, here we present spectroscopic techniques, based on spectral hole burning and optical pumping, which allow hyperfine transitions that are initially buried within an inhomogeneously broadened absorption line to be studied with no background absorption from other transitions. A sequence of hole-burning pulses is used to isolate selected transitions between hyperfine levels, which makes it possible to directly study properties of the transitions, e.g., transition strengths, and gives access to information that is difficult to obtain in standard hole-burning spectroscopy, such as the ordering of hyperfine levels. The techniques introduced are applicable to absorbers in a solid with long-lived sublevels in the ground state and where the homogeneous linewidth and sublevel separations are smaller than the inhomogeneous broadening of the optical transition. In particular, this includes rare-earth ions doped into inorganic crystals and in the present work the techniques are demonstrated in spectroscopy of
Pr3+ in
Y2SiO5. Information on the hyperfine structure and relative transition strengths of the $^{3}H_{4}\text{\ensuremath{-}}^{1}D_{2}$ hyperfine transitions in
Pr3+:Y2SiO5 has been obtained from frequency-resolved absorption measurements, in combination with coherent and incoherent driving of the transitions.