Temperature-Dependent Spectroscopy of Praseodymium-Doped Oxides

Trivalent praseodymium <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\text{Pr}^{3+})$</tex> is a well-known active ion for lasers in the visible spectral range offering laser transitions in the blue, green, orange, and red. Currently, the most efficient lasers based on praseodymium utilize fluoride host crystals such as <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{LiYF}_{4}$</tex> (YLF) [1]. Oxide hosts could offer new laser wavelengths, exhibit higher mechanical and thermal robustness and better thermal conductivity, yet they suffer from low output powers and efficiencies [1]. More efficient lasing might be possible at temperatures accessible with thermoelectric cooling. <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{YAlO}_3$</tex> (YAP) is one of the most established oxides for praseodymium [1]. However, room-temperature lasing with Pr:YAP has only been achieved in the red and deep red. Green and blue lasers based on Pr:YAP only operate at cryogenic temperatures [2], for reasons not fully understood, yet. Therefore, we performed polarized high-resolution temperature-dependent spectroscopy to determine the absorption, and emission cross sections, as well as the fluorescence lifetime <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$({ }^{3} \mathrm{P}_{0},{ }^{1} \mathrm{D}_{2})$</tex> for Pr(0.5%):YAP and Pr(0.2%):LuAP. Parts of the results for Pr:YAP are shown in Figure 1. Emission cross sections were calculated via the Füchtbauer-Ladenburg equation. The <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${}^{3}\mathrm{P}_{0}$</tex> fluorescence lifetime was measured by exciting at 448 nm and detecting at 501 nm, where no fluorescence from the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${}^{1}\mathrm{D}_{2}$</tex> level is possible. The lifetime measurements of the 0.5 %-doped Pr:YAP presented in Figure la) show a non-exponential decay with an effective lifetime of the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${}^{3}\mathrm{P}_{0}$</tex> level of 8.6 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu\mathrm{s}$</tex> at room temperature. At temperatures below 40 K, a single-exponential decay is observed and the lifetime increased to 11.9 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu\mathrm{s}$</tex>. The data indicate the presence of cross relaxation, which can partly explain lower laser efficiencies at elevated temperatures. Figure 1b) shows the emission cross sections in the green spectral range (547 nm), where lasing has been achieved at temperatures below 150 K [2]. The emission cross sections for this transition increases by a factor of 6 between 300 K and 8 K. In comparison, the cross sections for the deep red laser at 747 nm depicted in Figure 1c) are one order of magnitude higher at room temperature and only change by a factor of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\sim 2.4$</tex>. Thus, it is less sensitive to changes in temperature then the green laser [2]. The significant enhancement of the emission cross sections of the green transition compared to the red transition explains the possibility of low temperature laser performance of this laser by sufficiently reducing the laser threshold coinciding with reduced losses for both transitions caused by non-resonant cross relaxation.