Near- and mid-infrared photoluminescence in Ho³⁺ doped and Ho³⁺-Yb³⁺ codoped low-phonon-energy germanotellurite glasses

Intense infrared emissions at 1.20 μm (Ho³⁺: ⁵I₆→⁵I₈ transition) and 2.0 μm (Ho³⁺: ⁵I₇→⁵I₈ transition) wavelengths from holmium-ytterbium (Ho³⁺-Yb ³⁺) codoped low-phonon-energy germanotellurite glasses and fibers were observed. In comparison to Ho³⁺-singly doped glass, the incorporation of Yb³⁺ as sensitizer increases the quantum efficiency of the 1.20 μm wavelength emission from 2.4% to 7.9% through efficient energy transfer from Yb³⁺(²F_5/2) to Ho ³⁺(⁵I₆). Emission of 1.38 μm originating from the Ho³⁺: (⁵F₄,⁵S ₂)→⁵I₅ transition was also recorded under 488 nm excitation. The observation of both 1.20 and 1.38 μm wavelength emissions is primarily due to the low phonon energy of the germanotellurite glasses and is 770 cm^-1 in accordance to the Raman spectrum. Excellent gain performance is predicted by the long lifetime and the large stimulated emission cross-section. The results suggest that low-phonon-energy germanotellurite glass is a promising candidate for optical amplification at relatively unexplored 1.20 and 1.38 μm wavelength regions, and lasing operation at the eye-safe 2.0 μm wavelength region.