the description of luminescent processes and their thermally induced changes, that may be also influenced by the optically active ions concentration, and thus by the various inter-ionic processes, is the key to the improved development of luminescence thermometry. A phosphor doped with only trivalent terbium ions was described, which, by using two excitation lines fitted to the 7 f 6 → 5 D 3 and 7 f 5 → 5 D 3 transitions, shows a luminescent signals with the opposite characteristics of intensity changes as a function of temperature. By modifying the concentration of tb 3+ ions, the probability of { 5 D 3 , 7 f 6 } ↔ { 5 D 4 , 7 f 0 } cross-relaxation was being altered, which turned out to have a beneficial effect on the properties of the described nanothermometers. the ratio of intensities for both excitations was found to be temperature dependent, which resulted in high relative sensitivities of temperature readout reaching 3.2%/°C for 190 °C and not reaching values below 2%/°C in the broad range of the temperature. extensive decay time measurements for 5 D 3 and 5 D 4 emissive levels were presented and the variability of both rise-and decay times as a function of terbium concentration and temperature was investigated. thanks to this, conclusions were drawn regarding thermally dependent optical processes occurring in a given and similar systems. The technique of high-resolution noncontact temperature measurement in the single-band-ratiometric (SBR) approach has been recently developed 1-6. It is an application of luminescence thermometry technique 7-10 which requires only one type of optically active center in any nanocrystalline matrix, and enables temperature readout with high resolution and significant relative sensitivity (S r). This approach has all the advantages of a ratiometric measurement 11-22 , and at the same time does not show susceptibility to partial signal interference by the possible changes in the transmission characteristics of the medium 23. The ratio of two spectrally separated bands is receptive to artificial alterations through various, not only thermal, factors modifying their shape, especially in biological media 24. The phosphors used in this approach may be based on different luminescent dyes or ions, whose emission intensity depends on the temperature in opposite ways, depending on the excitation line used. There can be many reasons for different temperature dependencies. The various conformational changes occurring in organic phosphors that result in a change in the shape of the absorption bands should be mentioned. In addition, it is possible to use an energy mismatch between energetic states, so that a given excitation line does not have a regular dependence of the resultant emission intensity on the temperature. Additionally, different types of energy transfers between optically active centers are possible, which may cause different dependence of emission intensity depending on the center which is excited using a given excitation wavelength. Nevertheless, one of the most promising...
Crystals of LiNbO3 single-doped with Sm3+, Tb3+, or Dy3+ and crystal of LiTaO3 single-doped with Tb3+ were grown by the Czochralski method. Luminescence spectra and decay curves for LiNbO3 samples containing Sm3+ or Dy3+ ions were recorded at different temperatures between 295 and 775 K, whereas those for samples containing Tb3+ ions were recorded at different temperatures between 10 and 300 K. Optical absorption spectra at different temperatures were recorded within the UV-blue region relevant to optical pumping of the samples. It was found that the effect of temperature on experimental luminescence lifetimes consists of the initial temperature-independent stage followed by a steep decrease with the onset at about 700, 600, and 150 K for Sm3+, Dy3+, and Tb3+ ions, respectively. Additionally, comparison of temperature impact on luminescence properties of LiNbO3:Tb3+ and LiTaO3:Tb3+ crystals has been adequately described. Experimental results were interpreted in terms of temperature-dependent charge transfer (CT) transitions within the modified Temperature—Dependent Charge Transfer phenomenological model (TDCT). Disparity of the onset temperatures and their sequence were explained based on the location of familiar zigzag curves connecting the ground state levels of rare earth ions with respect to the band-gap of the host. It was concluded also that LiNbO3:Sm3+ is suitable as an optical sensor within the 500–750 K temperature region whereas LiNbO3:Dy3+ offers the highest sensitivity at lower temperatures between 300 and 400 K.
Single crystals of Gd3Al2.5Ga2.5O12, single-doped with Pr3+ ions and double-doped with Pr3+ and Yb3+ ions, were fabricated by the Czochralski technique. Transition intensities and relaxation dynamics of Pr3+ ions were determined employing the Judd–Ofelt treatment. Crystal field splitting of excited multiplets of incorporated luminescent ions were determined on the basis of optical spectra recorded at liquid helium temperature. The Pr3+ → Yb3+ energy-transfer phenomena were determined analyzing the effect of Yb3+ concentration on luminescence spectra and decay curves of Pr3+ ions. It was concluded that the observed downconversion phenomenon involves a quantum-cutting mechanism consisting of a two-step energy transfer from Pr3+ to Yb3+. We observed a phenomenon of nonresonant conversion of femtosecond pulses of infrared light into visible Pr3+ emission that was weakly affected by the wavelength of incident light at least in the 1100–1600 nm region. It was concluded that excitation mechanisms consist of multiphoton absorption of incident infrared radiation involving interconfigurational 4f2–4f15d transitions of Pr3+ and/or indirect excitation of Pr3+ ions by energy transfer from electrons created in the conduction band of the host.
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