Before the 1960s, all anti-Stokes emissions, which were known to exist, involved emission energies in excess of excitation energies by only a few kT. They were linked to thermal population of energy states above excitation states by such an energy amount. It was the well-known case of anti-Stokes emission for the so-called thermal bands or in the Raman effect for the well-known anti-Stokes sidebands. Thermoluminescence, where traps are emptied by excitation energies of the order of kT, also constituted a field of anti-Stokes emission of its own. Superexcitation, i.e., raising an already excited electron to an even higher level by excited-state absorption (ESA), was also known but with very weak emissions. These types of well-known anti-Stokes processes have been reviewed in classical textbooks on luminescence.1 All fluorescence light emitters usually follow the well-known principle of the Stokes law which simply states that excitation photons are at a higher energy than emitted ones or, in other words, that output photon energy is weaker than input photon energy. This, in a sense, is an indirect statement that efficiency cannot be larger than 1. This principle is

Upconversion and Anti-Stokes Processes with f and d Ions in Solids

Interpretation of europium(III) spectra

Although luminescent complexes of lanthanide ions and organic ligands have been studied intensively, relatively little attention has been paid to the natural (or ‘radiative’) lifetime of the lanthanide centered luminescent state in these systems. Here, the applicability of the well-known Judd–Ofelt theory to the emissive properties of Eu3+ complexes is investigated. Moreover, it is demonstrated experimentally that the radiative lifetime of the 5D0 excited state of Eu3+ can be calculated directly from its corrected emission spectrum, without using Judd–Ofelt theory. We also discuss briefly the possibility of finding the natural lifetimes of lanthanide ions other than Eu3+.

The emission spectrum and the radiative lifetime of Eu3+ in luminescent lanthanide complexes

Publisher Summary The intensities of intraconfigurational f–f transitions are reviewed in the chapter The chapter presents the transition mechanisms for lanthanide ions—namely, the magnetic dipole transition, the induced electric dipole transition, and the electric quadrupole transition It discusses the way experimental intensities can be determined from the spectra The chapter also focuses on the expression of the dipole strength and the oscillator strength in oriented and in randomly-oriented systems Correction factors for lanthanide ions in a dielectric medium are described in the chapter Although only a few magnetic dipole transitions exist for the trivalent lanthanide ions, magnetic dipole transitions are of interest, because their intensities are in a first approximation independent of the ligand environment and can, thus, be used as intensity standards The intensity of a magnetic dipole transition can be calculated exactly, provided that suitable wavefunctions are available These wavefunctions can be obtained from diagonalization of the energy matrix

Chapter 167 Spectral intensities of f-f transitions

Nanocrystalline Ln 3 + -doped YF 3 phosphors have been synthesized via a facile sonochemistry-assisted hydrothermal route. YF 3 nanoparticles are demonstrated to be a good host material for different lanthanides. Varying the dopants leads to different optical properties. In particular, the feasibility of inducing red, green, and especially blue emission in the Yb 3 + /Er 3 + co-doped YF 3 sample by up-conversion excitation in the near-infrared region is demonstrated. Such unusually strong 411 nm blue up-conversion emission has seldom been reported in other Yb 3 + /Er 3 + -doped systems. The up-conversion mechanisms have been analyzed.

Down/Up Conversion in Ln3+-Doped YF3 Nanocrystals

The up-conversion of infrared radiation into green, red, and 805-nm fluorescence has been studied for ${\mathrm{Er}}^{3+}$ and ${\mathrm{Tm}}^{3+}$ ions in both ${\mathrm{Yb}}^{3+}$-doped and non-${\mathrm{Yb}}^{3+}$-doped ${\mathrm{BaF}}_{2}$-${\mathrm{ThF}}_{4}$ fluoride glass over a wide temperature range and several dopant concentrations. It has been found that the addition of ${\mathrm{Tm}}^{3+}$ preferentially quenches the up-conversion efficiency of the green emission corresponding to a transition from the $^{4}\mathrm{S}_{3/2}$ level of ${\mathrm{Er}}^{3+}$. The decrease in emission amounts to a factor of about 50 for a concentration of 0.5 mol % ${\mathrm{TmF}}_{3}$ at 300 K. The quenching effect is discussed in terms of energy transfer between ${\mathrm{Er}}^{3+}$ and ${\mathrm{Tm}}^{3+}$ ions. A rate equation model and a Judd-Ofelt analysis used in conjunction with the appropriate optical data lead to an understanding of the up-conversion process. The absolute values of the energy transfer rates and the electron populations in the lower excited states of the excited ${\mathrm{Er}}^{3+}$ and ${\mathrm{Tm}}^{3+}$ ions can be determined. This information should enable researchers to tailor materials to improve the efficiency of the up-conversion and laser devices.

Energy transfer between Er 3 + and Tm 3 + ions in a barium fluoride – thorium fluoride glass

Spectroscopic properties and laser emission of Tm doped ZBLAN glass at 1.8 μm

The optical properties of ${\mathrm{Eu}}^{3+}$-doped fluorophosphate glasses of composition (in mol %) 60 ${\mathrm{NaPO}}_{3}$-${15\mathrm{B}\mathrm{a}\mathrm{F}}_{2}$-(25-x)${\mathrm{YF}}_{3}$-x${\mathrm{EuF}}_{3}$ (x=0.5, 2, 5, 10, 15, 20, and 25) have been investigated in the 4.2--300 K temperature range by using optical absorption spectroscopy and time-resolved resonant laser-induced fluorescence line narrowing. From the room-temperature absorption spectra Judd-Ofelt parameters have been obtained and used to calculate the spontaneous emission probabilities from the $^{5}$${\mathit{D}}_{0}$ state. The spectral features of the time-resolved fluorescence line-narrowed $^{5}$${\mathit{D}}_{0}$${\ensuremath{\rightarrow}}^{7}$${\mathit{F}}_{0,1}$ emission spectra obtained under resonant excitation at different wavelengths along the $^{7}$${\mathit{F}}_{0}$${\ensuremath{\rightarrow}}^{5}$${\mathit{D}}_{0}$ transition as a function of concentration and temperature reveal the existence of energy migration between discrete regions of the inhomogeneous broadened spectral profile. From the concentration and time dependence of the average rate of excitation transfer, the electronic mechanism ruling the ion-ion interaction can be identified as a dipole-dipole energy transfer process. At low temperatures the average transfer rate parameter slightly depends on wavelength showing a temperature independent behavior. Above 77 K the weak dependence of the transfer rate on excitation wavelength (weak dependence on energy mismatch) together with its T${\mathrm{}}^{3}$ temperature dependence point to a transfer mechanism consistent with a two-site nonresonant two-phonon assisted process. The estimated average crystal field strength grows monotonically with the $^{7}$${\mathit{F}}_{0}$${\ensuremath{\rightarrow}}^{5}$${\mathit{D}}_{0}$ energy suggesting a large variation in the local environment of ${\mathrm{Eu}}^{3+}$ ions in these glasses. The slight increase with concentration of the $^{5}$${\mathit{D}}_{0}$ fluorescence decays together with their single exponential character suggest that the transfer process may be fast enough to drive the system of excited centers to thermal equilibrium. \textcopyright{} 1996 The American Physical Society.

Time-resolved fluorescence-line narrowing and energy-transfer studies in a Eu 3 + -doped fluorophosphate glass

Anti-Stokes cooling between room temperature and 77 K in a fluorochloride glass (CNBZn) and a fluoride glass (BIG) doped with 1 mol % of ${\mathrm{YbF}}_{3}$ has been demonstrated by using collinear photothermal deflection and conventional laser excitation spectroscopies under high photon irradiances. The cooling efficiency for CNBZn glass which is \ensuremath{\sim}2.0% relative to the absorbed laser power at 1010 nm and 300 K falls about 20% at 77 K. The cooling efficiency for BIG glass was only \ensuremath{\sim}0.6% at room temperature. A model accounting for the photon-ion-phonon interaction is in good agreement with the observed temperature dependence of the cooling process and shows its relation with the vibrational properties of the material.

Anti-Stokes laser-induced internal cooling of Yb 3+ -doped glasses

The ${\mathrm{Tm}}^{3+}$\ensuremath{\rightarrow}${\mathrm{Tm}}^{3+}$ and ${\mathrm{Tm}}^{3+}$\ensuremath{\rightleftarrows}${\mathrm{Ho}}^{3+}$ energy transfers have been studied in both Tm-doped and (Tm,Ho)-doped indium-based fluoride glasses over a wide temperature range for several dopant concentrations. The regime of diffusion among the ${\mathrm{Tm}}^{3+}$ ions is clearly established and the cross-relaxation $^{3}$${\mathit{H}}_{4}$, $^{3}$${\mathit{H}}_{6}$${\ensuremath{\rightarrow}}^{3}$${\mathrm{F}}_{4}$${,}^{3}$${\mathit{F}}_{4}$ has been proved and its efficiency calculated. The temperature dependence of the thermal equilibrium observed between the lowest excited states $^{3}$${\mathit{F}}_{4}$ of ${\mathrm{Tm}}^{3+}$ and $^{5}$${\mathit{I}}_{7}$ of ${\mathrm{Ho}}^{3+}$ ions in codoped glasses is explained in terms of the Boltzmann distribution of the state populations governed by efficient forward and backward energy transfers. When ${\mathrm{Tm}}^{3+}$ ions are excited in the $^{3}$${\mathit{H}}_{4}$ level near 0.8 \ensuremath{\mu}m, it is shown that two cross-relaxation processes compete: one between ${\mathrm{Tm}}^{3+}$ ions and the other between ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Ho}}^{3+}$ ions. Their efficiencies are shown to strongly depend on the relative Tm and Ho concentrations.

J.L. Adam

Papers

Energy transfer between Er 3 + and Tm 3 + ions in a barium fluoride – thorium fluoride glass

Spectroscopic properties and laser emission of Tm doped ZBLAN glass at 1.8 μm

Time-resolved fluorescence-line narrowing and energy-transfer studies in a Eu 3 + -doped fluorophosphate glass

Anti-Stokes laser-induced internal cooling of Yb 3+ -doped glasses

Fluorescence mechanisms in Tm 3+ singly doped and Tm 3+ , Ho 3+ doubly doped indium-based fluoride glasses