D.S. Hamilton

Radiative and nonradiative relaxation measurements in Ce3+ doped crystals☆

Abstract Radiative and nonradiative transition rates from the lowest excited Ce3+ 5d level in a series of cerium doped oxide and fluoride crystals (Ce3+:LaF3, Ce3+:CaF2, Ce3+:YLiF4, Ce3+:Y3Al5O12, and Ce3+:YAIO3) have been determined by measuring the Ce3+ 5d → 4f fluorescence lifetime as a function of sample temperature. The onset of the thermally induced nonradiative transitions occurs for temperatures T∗ between 300 and 900 K depending on the host crystal. At temperatures lower than T∗, there is an additional but much weaker temperature dependance of the radiative lifetime. Analysis of the radiative lifetime shows that the radial integral 〈4f|r|5d〉 varies by about 20% from the mean value of 0.30 A among the set of five host crystals, and is a factor of about 1.5 smaller than the free-ion value.

Spatial-diffusion measurements in impurity-doped solids by degenerate four-wave mixing.

A method is introduced that measures the spatial-migration rate of electronic excitation in condensed media over distances of the order of 0.1 microm. Two volume holographic gratings of widely differing modulation periods are simultaneously produced, using a phase-conjugate wave geometry of degenerate four-wave mixing. Spatial migration results in a reduced scattering efficiency of one grating and is observed as a polarization rotation of the backward-going output wave. Upper limits are placed on the diffusion constants in pink ruby and Nd(3+)-doped silicate glass.

Electron traps and transfer efficiency of cerium-doped aluminate scintillators

Abstract Comparative measurements of thermoluminescence and scintillation light outputs of gamma-ray irradiated Ce;LuAlO 3 (LuAP) and Ce:YAlO 3 (YAP) reveal that electron trapping significantly depresses transfer efficiency in these scintillator materials, but fails to explain fully either their performance differential or their departures from ideal efficiency. In the limit of short radiation times, the ratio of integrated thermoluminescence light output to integrated scintillation light output is 0.14 in LuAP and 0.02 in YAP.

Optical gain and loss studies in Ce 3+ :YLiF 4

Transient and stable color centers created by optical pumping at 308 nm have been studied in Ce3+:YLiF4. Since the color centers are absorptive at the cerium emission wavelengths, they degrade the Ce3+:YLiF4 laser performance. The color centers are produced following a multiphoton ionization of the Ce3+ ions. Rate equations that incorporate both creation and photobleaching processes are developed to model the observed kinetics and intensity dependence of the color-center density. Measurements of the single-pass gain at 325 nm as a function of the repetition rate of the pump laser are interpreted in terms of the relaxation time of the transient centers.

Pressure dependence and thermal quenching of chromium photoluminescence in Cs2NaYCl6:Cr3+

Abstract Photoluminescence spectra of the chromium-doped elpasolite Cs 2 NaYCl 6 :Cr 3+ were measured at ambient and near liquid nitrogen temperatures as a function of pressure in a diamond anvil cell. A pressure-induced blue shift of the broad 4 T 2g → 4 A 2g fluorescence band was observed, culminating in a crossing of the 4 T 2g and 2 E g excited states as evidenced by the appearance of a structured 2 E g → 4 A 2g phosphorescence spectrum. Resolved vibronic structure of the latter permitted determination of vibration frequencies at high pressure. Evidence of a phase transition was observed near the highest pressures attained, ≈ 11 GPa. The pressure dependence of the peak energy of the broad band was interpreted in terms of a pressure-dependent local compressibility. Measurements of photoluminescence lifetimes as functions of pressure and temperature established that the activation energy for thermal fluorescence quenching increases linearly at the rate of 1668 ± 52 cm −1 GPa . A semi-empirical, two-accepting mode, linear- and quadratic-coupling model, calibrated at ambient pressure, predicts a rate of 1758 cm −1 GPa , in substantial agreement with experiment.

Infrared to visible up-conversion in thulium and holmium doped lutetium aluminum garnet

We report on the up-converted emissions at 367, 458 and 483 nm in thulium doped lutetium aluminum garnet (LuAG) under infrared excitation in the range of 730-800 nm. We also observe blue and strong green emission in thulium and holmium codoped LuAG with near infrared excitation. Excitation spectra, power dependence, temporal behavior and temperature dependence of up-converted emissions were measured to explain the up-conversion mechanisms. Several cross-relaxation mechanisms and excited state absorption are found to be responsible for the observed up-conversion processes.

Afterglow Suppression and Non-Radiative Charge-Transfer in CsI:Tl,Sm

Suppression of afterglow in co-doped CsI:Tl is found to be an order of magnitude more effective in CsI:Tl, Sm than in CsI:Tl, Eu. Rate equations predict that deep electron traps introduced by co-doping with samarium effectively scavenge electrons from shallow traps associated with thallium, thus suppressing afterglow in the time domain of tens of milliseconds. In addition, combined radioluminescence and thermoluminescence experiments suggest that electrons released by samarium recombine non-radiatively with trapped holes, thus providing a mechanism for suppression of hysteresis. Ab initio quantum chemistry calculations support the conclusion that non-radiative charge-transfer transitions in CsI:Tl, Sm are enabled by the presence of low-energy excited states of within the ground configuration.

UV-induced loss mechanisms in a Ce3+:YLiF4 laser

Abstract The optical pumping of Ce 3+ :YLiF 4 at 308nm leads to the formation of transient and stable color centers due to an excited-state absorption which photoionizes the cerium ions. Measurements of the temperature dependence of the transient center lifetime and the dependence of the single-pass gain on pump repetition rate are presented.

Utilization of synchrotron radiation for the measurement of fast fluorescence lifetimes of ions in solids

Abstract Synchrotron radiation from an electron storage ring has been used to measure the lifetime of the very short lived uv fluorescence of LaF 3 :Ce 3+ . The value obtained was 20 nsec. Several considerations which could improve and extend the experimental method are discussed.

Thermal quenching and electron traps in LSO

It is demonstrated by comparison of thermoluminescence and scintillation light outputs of LSO as functions of radiation time that a previously suggested thermal quenching correction is inappropriate. Approximate solutions of rate equations are employed to infer absolute trap concentrations and to explore the effects of thermal quenching on the shapes of thermoluminescence glow curves.