NIR laser irradiation of Er0.5Yb2.5Al5O12 single crystal: photoluminescence from ultraviolet to NIR-II†

Abstract

Under excitation at 980 nm, Er³⁺ and Yb³⁺ co-doped into a single crystal may emit light from ultraviolet to NIR-II, fulfilling the application requirements for solid-state lasers covering a wide wavelength range. However, due to extensive non-radiative transitions between Er³⁺ energy levels, achieving ultraviolet emission via multiphoton up-conversion processes in garnet single crystals is a challenge and has been rarely reported. Achieving ultraviolet emission from a garnet single crystal requires high contents of Er³⁺ and Yb³⁺, whereas excessive doping usually results in poor crystal quality, such as cracking and opacity. Here, a high-quality Er_0.5Yb_2.5Al₅O₁₂ single crystal with high contents of Er³⁺ and Yb³⁺ has been grown by an optical floating zone method. It exhibits a cubic garnet structure, a density of 6.590 g cm⁻³ and a transmittance exceeding 87%. For comparison, an Er_0.5Y_2.5Al₅O₁₂ single crystal was also grown. Under excitation at 980 nm, due to the energy transfer from Yb³⁺ to Er³⁺, ultraviolet emission at 382 nm from the Er_0.5Yb_2.5Al₅O₁₂ single crystal was observed for the first time, indicating that this crystal can serve as an ultraviolet laser crystal. Additionally, a stronger emission band within 1450–1700 nm, falling within the third telecommunication window (1530–1565 nm), was also obtained, indicating its suitability as a NIR-II laser crystal used in telecommunication. Furthermore, the four-photon up-conversion transition process of Er³⁺ and the energy transfer process from Er³⁺ to Yb³⁺ in the Er_0.5Yb_2.5Al₅O₁₂ single crystal excited by a 1550 nm laser were also investigated.