Interionic Processes Research Articles

Enhancement of the sensitivity of single band ratiometric luminescent nanothermometers based on Tb3+ ions through activation of the cross relaxation process

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 7F6 → 5D3 and 7F5 → 5D3 transitions, shows a luminescent signals with the opposite characteristics of intensity changes as a function of temperature. By modifying the concentration of Tb3+ ions, the probability of {5D3, 7F6} ↔ {5D4, 7F0} 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 5D3 and 5D4 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.

Optical spectroscopic investigation of Ba3Tb(PO4)3 single crystals for visible laser applications

Optical spectroscopic characterization of Ba3Tb(PO4)3 single crystals grown by the Czochralski technique was performed in order to evaluate the potential for visible laser application. Powder X-ray diffraction patterns at different temperatures were recorded to calculate the thermal expansion for temperatures between 298 and 1073 K. Refractive indices in the visible spectral region were determined and fitted into a Sellmeier equation. Absorption spectra were measured in the UV and visible range to obtain the ground-state absorption cross-sections, which were used for a Judd-Ofelt analysis. Electrostatic and spin-orbit interaction parameters were fitted, allowing for more precise Judd-Ofelt calculations. The Judd-Ofelt intensity parameters were calculated and employed to study the radiative transition probabilities, branching ratios and radiative lifetime associated to the potential upper laser level 5D4. The fluorescence spectrum shows several lines in the visible range, which can be assigned to 5D4 → 7FJ (J=6,5,4,3,2,1,0) transitions of Tb3+. These data enabled the determination of the branching ratio of the transitions from the 5D4 excited state. The stimulated emission cross-section of the 5D4 → 7F5 transition was found to be 5.9 × 10−22 cm2 at a wavelength of 549.6 nm by using the Füchtbauer-Ladenburg equation. Fluorescence decay curves were recorded to obtain the fluorescence lifetime. The resulting value of 3.17 ms is in reasonable agreement with the theoretical value of 3.54 ms pointing towards a high radiative quantum efficiency of 90% despite a high doping ion density of 3.5 × 1021 cm−3 and revealing the low effect of detrimental inter-ionic processes in Tb3+.

Role of Interionic Processes in the Efficiency and Operation of Erbium-Doped Fluoride Fiber Lasers

Great increase in the output power of erbium-doped fluoride fiber lasers has recently been achieved, but the predicted high slope efficiency was not observed. We conduct an investigation into the possible underlying reasons, based on the newly available spectroscopic data, and our measurements on the running fiber laser are compared against computer simulation. The results indicate that interionic processes in the present fibers are much weaker than previously expected and only allow for an operation with 15-30% efficiency where the excited-state absorption is shown to have an important contribution.

Optical spectroscopy of heavily Ho3+-doped BaY2F8 crystals

The ultraviolet, visible, and near IR (0.8–2.4μm) luminescence spectra of BaY2F8 single crystals heavily doped with Ho3+ ions (10 and 30mol%) have been investigated at room temperature and 12K, together with the luminescence decay curves (up to 300μs) of the visible emission. Excitation in the visible region gives rise to very strong emission bands originating from the first 5I7 level and located around 2070nm. However the 5I7 emission is not observed upon excitation at wavelengths shorter than 300nm. The inter-ionic processes are found to shorten the decay times of the levels emitting in the visible region with respect to the corresponding radiative lifetimes.

Optical losses in YVO4: RE (RE = Nd3+, Er3+, Tm3+) laser crystals

The relevance of processes contributing to depletion of pump and upper laser levels has been assessed based on experimental data obtained during measurement of excited state absorption, steady state emission and dynamics of excited states as a function of excitation power and activator concentration. It has been concluded that the excited state absorption in YVO4: Nd and YVO4: Er is not significant except for that from the 4 I 11/2 level of Er3+. In these systems, the interionic processes are dominant. In particular, the reported decrease of the YVO4: Er laser slope efficiency when the Er3+ concentration increased from 0.5 to 1 at % is due mainly to the up-conversion by energy transfer from the pump level and upper laser level. Excited state absorption cannot contribute to depletion of excited states involved in the 3 F 4-3 H 6 laser operation near 1800 nm in the YVO4: Tm crystal. However, the heavy doping required to enhance the cross-relaxation process which feeds the upper laser level brings about the migration-accelerated energy transfer to energy sinks.

Modelling of high-power diode-pumped erbium 3 μm fibre lasers

We present theoretical calculations that relate to the cw operation of a high-power Er3+, Pr3+:ZBLAN double-clad fibre laser. Using the measured energy-transfer, energy-transfer-upconversion and cross-relaxation parameters relevant to Er3+-doped and Er3+, Pr3+-codoped ZBLAN, we compare the theoretical calculations from the model with recent experimental measurements. The model is then used to analyse the important pump and interionic processes occurring in Er3+, Pr3+:ZBLAN fibre lasers. The theoretical results indicate that energy transfer from Er3+ to Pr3+ leads to a fast depletion of the lower laser level and, due to the cw threshold condition, to low population densities in both laser levels. Thus, ground-state bleaching, pump excited-state absorption and energy-transfer upconversion amongst the Er3+ ions are avoided.

Upconversion, lifetime quenching, and ground-state bleaching in Nd 3+:LiYF 4

Since the Nd 3+:LiYF 4 system has some advantage over Nd 3+:YAG and Nd 3+:YVO 4 for high-power scaling of diode-end-pumping, this system has been investigated under strong excitation, in this case using a Ti:sapphire pump. The interionic processes responsible for fluorescence saturation have been determined, due allowance being taken for the significant ground-state bleaching under these conditions. Their temperature dependence, which is relevant to scaling consideration, has been investigated theoretically, and found to be rather small over a wide temperature range. By comparing the experimental data with finite-element rate-equation calculations, the influence of interionic upconversion is determined quantitatively, and a published value of the upconversion parameter is confirmed. The spatial dependence of ground-state bleaching and quenching of the fluorescence lifetime is calculated. Analytical expressions are derived, including the influence of interionic upconversion, for the dependence of ground-state bleaching, excitation density, and storage time on pump parameters and dopant concentration.

The route toward a diode-pumped 1-W erbium 3-μm fiber laser

A rate-equation analysis of the erbium 3-/spl mu/m ZBLAN fiber laser is performed. The computer calculation includes the longitudinal spatial resolution of the host material. It considers ground-state bleaching, excited-state absorption (ESA), interionic processes, lifetime quenching by co-doping, and stimulated emission at 2.7 /spl mu/m and 850 mn. State-of-the-art technology including double-clad diode pumping is assumed in the calculation. Pump ESA is identified as the major problem of this laser. With high Er/sup 3+/ concentration, suitable Pr/sup 3+/ co-doping, and low pump density, ESA is avoided and a diode-pumped erbium 3-/spl mu/m ZBLAN laser is predicted which is capable of emitting a transversely single-mode output power of 1.0 W when pumped with 7-W incident power at 800 nm. The corresponding output intensity which is relevant for surgical applications will be in the range of 1.8 MW/cm/sup 2/. Compared to Ti:sapphire-pumped cascade-lasing regimes, the proposed approach represents a strong decrease of the requirements on mirror coatings, cavity alignment, and especially pump intensity. Of the possible drawbacks investigated in the simulation, only insufficient lifetime quenching is found to have a significant influence on laser performance.

Population mechanisms of the green Er3+:LiYF4 laser

In computer simulations the mechanisms that lead to room-temperature continuous-wave green upconversion lasing in Er3+:LiYF4 are investigated. The rate-equation system considers the full erbium level scheme up to 2H9/2, ground-state depletion, excited-state absorption on the pump and laser wavelengths, three interionic processes, stimulated emission, and the crystal and resonator data of the experiments. Experimental results performed at the University of Hamburg, Germany, are reproduced in the simulation. The influence of different parameters as pump wavelength, absorption cross sections, interionic parameters, dopant concentration, and temperature is investigated. An avalanche effect which exploits the strong cross relaxation from the upper laser level and the upconversion from 4I13/2 leads to an efficient population of the upper laser level. At higher dopant concentrations the cross relaxation becomes detrimental to stimulated emission due to the depletion of the upper laser level. This concentration dependence can be considered as a general behavior of rare-earth-doped avalanche lasers.

Fluorescence and lasing properties of NdNa 5(WO 4) 4, K 3Nd(PO 4) 2 and Na 3Nd(PO 4) 2

Optical spectroscopic characterization of Ba3Tb(PO4)3 single crystals grown by the Czochralski technique was performed in order to evaluate the potential for visible laser application. Powder X-ray diffraction patterns at different temperatures were recorded to calculate the thermal expansion for temperatures between 298 and 1073 K. Refractive indices in the visible spectral region were determined and fitted into a Sellmeier equation. Absorption spectra were measured in the UV and visible range to obtain the ground-state absorption cross-sections, which were used for a Judd-Ofelt analysis. Electrostatic and spin-orbit interaction parameters were fitted, allowing for more precise Judd-Ofelt calculations. The Judd-Ofelt intensity parameters were calculated and employed to study the radiative transition probabilities, branching ratios and radiative lifetime associated to the potential upper laser level 5D4. The fluorescence spectrum shows several lines in the visible range, which can be assigned to 5D4 → 7FJ (J=6,5,4,3,2,1,0) transitions of Tb3+. These data enabled the determination of the branching ratio of the transitions from the 5D4 excited state. The stimulated emission cross-section of the 5D4 → 7F5 transition was found to be 5.9 × 10−22 cm2 at a wavelength of 549.6 nm by using the Füchtbauer-Ladenburg equation. Fluorescence decay curves were recorded to obtain the fluorescence lifetime. The resulting value of 3.17 ms is in reasonable agreement with the theoretical value of 3.54 ms pointing towards a high radiative quantum efficiency of 90% despite a high doping ion density of 3.5 × 1021 cm−3 and revealing the low effect of detrimental inter-ionic processes in Tb3+.