Lanthanide-based luminescent hybrid materials.

Interpretation of europium(III) spectra

Temperature (T) is probably the most fundamental parameter in all kinds of science. Respective sensors are widely used in daily life. Besides conventional thermometers, optical sensors are considered to be attractive alternatives for sensing and on-line monitoring of T. This Review article focuses on all kinds of luminescent probes and sensors for measurement of T, and summarizes the recent progress in their design and application formats. The introduction covers the importance of optical probes for T, the origin of their T-dependent spectra, and the various detection modes. This is followed by a survey on (a) molecular probes, (b) nanomaterials, and (c) bulk materials for sensing T. This section will be completed by a discussion of (d) polymeric matrices for immobilizing T-sensitive probes and (e) an overview of the various application formats of T-sensors. The review ends with a discussion on the prospects, challenges, and new directions in the design of optical T-sensitive probes and sensors.

Luminescent probes and sensors for temperature.

Lanthanide-doped upconversion nanoparticles (UCNPs) have attracted considerable interest due to their superior physicochemical features, such as large anti-Stokes shifts, low autofluorescence background, low toxicity and high penetration depth, which make them extremely suitable for use as alternatives to conventional downshifting luminescence bioprobes like organic dyes and quantum dots for various biological applications. A fundamental understanding of the photophysics of lanthanide-doped UCNPs is of vital importance for discovering novel optical properties and exploring their new applications. In this review, we focus on the most recent advances in the development of lanthanide-doped UCNPs as potential luminescent nano-bioprobes by means of our customized lanthanide photophysics measurement platforms specially designed for upconversion luminescence, which covers from their fundamental photophysics to bioapplications, including electronic structures (energy levels and local site symmetry of emitters), excited-state dynamics, optical property designing, and their promising applications for in vitro biodetection of tumor markers. Some future prospects and efforts towards this rapidly growing field are also envisioned.

Lanthanide-doped upconversion nano-bioprobes: electronic structures, optical properties, and biodetection

Lanthanide-based thermometers: At the cutting-edge of luminescence thermometry

The measurement of branching ratios, cross sections and radiative lifetimes for rare earth ions in solids is considered The methods are applied to Tm and Ho in YLF as a test case De-activation rates for electric dipole and magnetic dipole emission are calculated for many of the lower lying manifolds in Tm:YLF and Ho:YLF in the context of the Judd-Ofelt theory to determine radiative lifetimes Measured values for the branching ratios as well as the absorption and emission cross sections are also presented for many of the excited state manifolds From these measurements, a methodology is developed to extract measured values for the radiative lifetimes These results are compared with the Judd-Ofelt theory as a guide for consistency and for determining the accuracy of the Judd-Ofelt theory in predicting branching ratios and radiative lifetimes The parameters generated by the methods covered here have potential applications for more accurate modeling of Tm:Ho laser systems

Branching ratios, cross sections, and radiative lifetimes of rare earth ions in solids: Application to Tm3+ and Ho3+ ions in LiYF4

previous ideas and forming a complete picture. So it was in August 1962 when there appeared in the literature, simultaneously and independently, two identical formulations of a theory. One by Brian R. Judd at the University of California at Berkeley and the other by a Ph.D. student, George S. Ofelt, at the Johns Hopkins University in Baltimore. Judd and Ofelt had never met personally, and were not aware of each other’s interest in the intensities of rare earth ions in solids. While there are some differences in the two formulations, the approach and the assumptions used to arrive at the final result are remarkably similar. The titles of the two articles reflect a thought along similar lines. Judd referred to Optical Absorption, while Ofelt referred to Crystal Spectra, and each to Intensities of Rare-Earth Ions. The formulations as originally published by Judd and Ofelt came to be known as the Judd-Ofelt theory of the intensities of rare earth ions. Regarding these publications, the late Professor Brian G. Wybourne has said, “I suggest that the coincidence of discovery was indicative that the time was right for the solution of the problem” [1]. In the light of the advancement in the understanding of complex atomic spectra in the quarter century preceding the 1962 publications of Judd and Ofelt, this suggestion of Wybourne is based on very sound reasoning. The rare earths comprise Y, Sc, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. The last 15 make up the lanthanide series. Most of these elements were discovered over a period of time stretching from the late 18th century to the early 20th century. Promethium (Pm) was the last to be discovered in 1947 at Oak Ridge National Laboratory. So, with the exception of Promethium, all the rare earths were discovered in the span of a little more than a century. Part of the reason why they are called rare earths is two-fold. First, they are very difficult to chemically extract from the earth. Second, they do not exist in nature in high abundance. In the universe, the rare earths are approximately 10 6 times less abundant than the

Judd-Ofelt theory: principles and practices

A review of Tm and Ho materials is presented, covering some fundamental aspects on the spectroscopy and laser dynamics in both single and co-doped systems. Following an introduction to 2-μm lasers, applications and historical development, the physics of quasi-four level lasers, energy transfer and modeling are discussed in some detail. Recent developments in using Tm lasers to pump Ho lasers are discussed, and seen to offer some advantages over conventional Tm:Ho lasers. This article is not intended as a complete review, but as a primer for introducing concepts and a resource for further study.

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Review of Tm and Ho materials; spectroscopy and lasers

Lanthanide series ions are considered in the context of acquiring spectroscopic parameters and their application to modelling of quasifour-level lasers. Tm:Ho codoped crystals of YLiF4 (YLF) and the isomorphs LuLiF4 (LuLF) and GdLiF4 (GdLF) as 2.0 μm lasers are used for illustration of the experimental and theoretical techniques presented here. While these materials have similar physical properties, they differ in the strength of the crystal field at the site of optically active lanthanide dopant ions such as Tm3+ and Ho3+. This is due in part to the size of the Lu3+, Y3+, and Gd3+ ions, which comprise part of the host lattice, but ionicity plays a role as well. This selection of lanthanide: host materials provides a useful basis on which to assess laser materials with regards to changes in the strength of the crystal field at the dopant ion site. It is demonstrated that Tm:Ho:LuLF has a larger crystal field splitting than Tm:Ho:YLF and Tm:Ho:GdLF, leading to smaller thermal populations in the Ho lower laser level. To assess this effect quantitatively, the energy levels of the first ten manifolds in Ho:LuLF have been determined. Measurement of Ho:XLiF4 (X=Y,Lu,Gd) emission cross sections at 2.0 μm, Tm:XLiF4 pump absorption cross sections around 0.78 μm, manifold to manifold decay times and energy transfer parameters in Tm:Ho:XLiF4 systems are also determined to provide a consistent set of parameters to use in laser modeling. The techniques presented here are applicable to any lanthanide series ion in a crystalline host. A theoretical laser model has been developed that is easily adapted to any lanthanide ion in a crystal host. The model is used to predict diode side-pumped laser performance of Tm:Ho:LuLF and Tm:Ho:YLF using input parameters determined from the spectroscopy presented here. An explanation is presented for the improved performance of Tm:Ho:LuLF over Tm:Ho:YLF by modeling the laser. A demonstration that small changes in lower laser thermal population can substantially alter laser performance is noted, an effect that has not been fully appreciated previously.

Spectroscopy and modeling of solid state lanthanide lasers: Application to trivalent Tm3+ and Ho3+ in YLiF4 and LuLiF4

Tm-doped ZBLAN and Tm-doped silica glass are compared spectroscopically and the fiber lasing of the Tm 3
F
4 →3
H
6 transition around 1.9 μm in ZBLAN and silica fibers is compared. The spectroscopy of these materials indicates that Tm:ZBLAN possesses advantages over Tm:silica glass due to the lower phonon energies. The phonon energy in these glass hosts influences both the pump manifold lifetime, the Tm 3
H
4, and the upper laser manifold lifetime, the Tm 3
F
4. The maximum phonon energy in Tm:ZBLAN, ∼500 cm-1 , compared to Tm:silica, ∼1100 cm-1, leads to better Tm–Tm self quenching towards populating the Tm 3
F
4, as well as better Tm 3
F
4→3
H
6 quantum efficiency. A spectroscopic analysis using the Judd–Ofelt theory and measured lifetimes are used to assess the merits of Tm:ZBLAN over Tm:silica as a fiber laser material. Diode-pumped fiber lasing experiments show that Tm:ZBLAN possesses advantages over Tm:silica that are believed to be due to a lower phonon energy. Data is presented for launched pump energy versus laser energy, fiber length versus slope efficiency, and output mirror reflectivity versus slope efficiency. Tm:ZBLAN is demonstrated to possess higher slope efficiencies and lower thresholds, than Tm:silicate. A grating tuned Tm:ZBLAN laser is also demonstrated for tunable operation between 1.893 μm and 1.955 μm.

Brian M. Walsh

Papers

Branching ratios, cross sections, and radiative lifetimes of rare earth ions in solids: Application to Tm3+ and Ho3+ ions in LiYF4

Judd-Ofelt theory: principles and practices

Review of Tm and Ho materials; spectroscopy and lasers

Spectroscopy and modeling of solid state lanthanide lasers: Application to trivalent Tm3+ and Ho3+ in YLiF4 and LuLiF4

Comparison of Tm:ZBLAN and Tm:Silica fiber lasers; Spectroscopy and tunable pulsed laser operation around 1.9 μm