Persistent spectral holeburning is known to occur in rare-earth doped silica-based glasses,1-3 where the mechanism is attributed to light induced changes in the coupling of tunnelling systems (TLS) which are characteristic of the amorphous state. A number of hole parameters such as lifetime, saturation depth and temperature dependence of the width, can provide important information on the density of states and dynamics of the TLS. We present the first data on spectral holeburning in a rare-earth doped heavy metal fluoride glass (HMF, Rennes) and compare with data obtained on a silicate glass (ED-2, Owens-Illinois). For this study we have chosen the well known 4I9/2 ← → 4F3/2 transition of the Nd3+ ion whose absorption peaks around 870nm (HMF) and 880nm (ED-2). Previous holeburning and photon echo measurements have been reported on the 4I9/2 ← → 4G5/2 transition of the Nd3+ of bulk ED-2 glass4 and on the 4I9/2 ← → 4F3/2 transition of the Nd3' transition of a pure silica fiber.3,5
The observation of statistical absorption fluctuations of an inhomogeneously broadened optical transition in a glass is reported. Frequency modulation spectroscopy in a 10-ppm
Nd3+-doped-silica optical fiber has also revealed spectral holes that persist for days at 4.2 K. Homogeneous linewidths ${\ensuremath{\Gamma}}_{h}$ measured using hole burning and fluctuations are in good mutual agreement but are significantly larger than those inferred from photon echo decays in the same system. This difference confirms recent predictions of a time-dependent ${\ensuremath{\Gamma}}_{h}$(t) arising from the tunneling dynamics of the glass.
We report what is, to the best of our knowledge, the first observation of transient spectral hole burning in erbium-doped fluorozirconate glass around 1.53 μm. Holes deeper than 12% were burnt. A study of the hole width as a function of power density, wavelength, temperature, and erbium concentration has been performed and underlines the role of spectral diffusion. Dynamics of refilling of the holes, involving optical pumping of the long-lived 4I13/2 excited state, was also investigated. The nearly linear temperature dependence of the hole width behaves as reported for other rare-earth ions and is interpreted within the framework of the two-level systems theory. Hole-refilling dynamics has been studied for three different erbium concentrations and is used to interpret the origin of the saturation.
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.
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.