Determination of radiative and multiphonon non-radiative relaxation rates of upconversion materials

The radiative and multiphonon non-radiative relaxation rates of lanthanide ions are intrinsic parameters to characterize the optical properties, which are the basic data for the theoretical model and numerical simulation of lanthanide upconversion systems. However, there are complex energy transfer processes, such as energy migration, energy transfer upconversion, and cross-relaxation in the lanthanide-doped upconversion materials, so it is difficult to accurately measure the intrinsic radiative and multiphonon relaxation rates. Therefore, a method to determine the relaxation rates of multi-level upconversion systems is proposed based on multi-wavelength excitation and level-by-level parameter calculations in this paper. For a dilute doped multi-level luminescence system excited at low powers, a model based on the measurements of steady-state emission spectra and luminescence decay curves is established through the macroscopic rate equations at multi-wavelength excitation, which can be used for the level-by-level calculation of the multi-level radiative and multiphonon relaxation rates. With the dilute doped β-NaYF₄:Er³⁺ six-level luminescence system as an example, the measurement method and the model are introduced in detail. Under the experimental conditions of neglecting the energy transfer effect between ions, the materials are excited by five lasers with central wavelengths of 1523 nm, 980 nm, 808 nm, 660 nm, and 520 nm to form five subsystems. The steady-state emission spectra and luminescence decay curves of the luminescence system excited by each wavelength were recorded. The intrinsic relaxation rates including 11 radiative relaxation rates and 4 multiphonon relaxation rates in the β-NaYF₄:Er³⁺ six-level system were determined based on the established model and method, which experimentally verified the applicability of the method proposed in this paper. This work will provide basic data for the analysis and regulation of the luminescence properties of lanthanide upconversion systems.

Synthesis, characterization and optical spectroscopy of GdPO4 :Er3

A series of Er³⁺ -doped GdPO₄ phosphors was synthesized using a conventional solid-state reaction. The monazite structure (space group P12₁ /n₁ ) of the obtained materials was confirmed using X-ray diffraction and Fourier transform infrared spectroscopy. Their optical spectra (excitation, emission, absorption, decay curves) were measured at room temperature in the visible and near-infrared (NIR) regions. The UV-visible-NIR optical absorption spectrum of GdPO₄ :7% Er³⁺ was analyzed based on Judd-Ofelt (J-O) theory and the J-O intensity parameter (Ω₂ , Ω₄ , Ω₆ ) was calculated. J-O intensity parameters were used to evaluate spontaneous emission properties such as branching ratios, transition probabilities, and radiative lifetime. The calculated quantum efficiency of the 1.5 μm emission (⁴ I_13/2 -⁴ I_15/2 ) was calculated to be 89%. This result proved that GdPO₄ :Er³⁺ is suitable for use in optical amplifiers and is a potential host for laser applications. The most interesting transitions, located at about 540 nm, and 1.0 and 1.5 μm were investigated as a function of doping level and of temperature, to assess the conditions needed for the highest emission performance and to explore the range of application, in particular in the fields of lighting, thermal sensing, and of phosphors for bio-imaging.

Mid-infrared luminescence properties of Dy-doped silver halide crystals

The absorption and the kinetics of the emission in the mid-infrared (mid-IR) were investigated in AgCl(x)Br(1-x) crystals doped with Dy(3+) ions. Strong emission bands were detected at 3, 4.4, and 5.5 μm and attributed to the (6)H(13/2)→(6)H(15/2), (6)H(11/2)→(6)H(13/2), and (6)F(11/2)+(6)H(9/2)→(6)H(11/2) transitions. Various optical parameters were calculated for the Dy(3+) doped crystals, using the Judd-Ofelt approximation and the rate equations. The measured results and the calculated parameters indicate that these doped crystals could be used for the development of mid-IR solid-state lasers or mid-IR fiber lasers.

Optical characterization of Nd (3+):AgBr

The luminescence of silver bromide crystals, doped with neodymium, was investigated over the visible and near-infrared spectral ranges. The emission, excitation, and absorption spectra were measured over a broad temperature range. The absolute luminescence quantum yield was estimated by comparing the luminescence with that of a neodymium-doped phosphate glass, for which the manufacturer gives a value of 0.4. The Judd-Ofelt analysis was applied to both materials, and transition rates, branching ratios, and quantum efficiencies were calculated for all the observed bands. Good agreement was obtained between theory and experiment.

Spin-lattice relaxation by the direct process in optically excited Er3+ in LaF3

Spin-lattice relaxation within the lower Stark component of the metastable 4S3 /2 state of Er3+ ions in LaF3 has been studied by an optical method at high magnetic fields B and at low crystal temperature. The relaxation rate increases as B5, as expected for a direct-process relaxation within Kramers doublets at large Zeeman splittings.

Properties of neodymium laser materials

The optical properties of neodymium-doped crystalline and glass laser materials have been studied in an attempt to further the development of laser materials (emitting at 1.06 micro) for specific applications. The 1.06-micro fluorescence lifetimes and relative fluorescence conversion efficiencies for the more promising materials such as YAG:Nd and glass:Nd have been measured over the temperature range 300-500 K. Also determined were the effects of sensitization of YAG:Nd by Cr(3+) (the enhancement of 1.06-micro, fluorescence and the Cr(3+) ? Nd(3+) energy transfer times). Further absorption spectra and fluorescence lifetimes have been determined for a large group of crystalline and glass laser media under room temperature conditions. The study of these optical properties and their thermal dependences in this temperature region provide information that is useful in solving laser device problems such as optimization of operating temperature and of pump characteristics. Presented in this paper are the results obtained from the above experimental study of the optical properties together with a collection of data from the literature of optical, thermal, and physical properties of these laser materials.