Mid-infrared (MIR) lasers with wavelengths of 3?5 μm have significant application value in fields such as national defense, military use, biomedical applications, and gas sensing. However, current mainstream light source technologies, such as super-continuum laser methods and optical parametric oscillators, generally suffer from inherent drawbacks like complex system structures and low electro-optical conversion efficiency. In contrast, rare-earth ion-doped solid-state laser media—which offer high beam quality, good output stability, and a compact structure—are considered an ideal approach for achieving efficient 3?5 μm laser output. This study uses LiYF₄ single crystals grown by the Bridgman method as the matrix, taking full account of their excellent optical and physicochemical properties. Additionally, holmium ions (Ho³⁺) are regarded as ideal active ions for this wavelength band due to their ⁵I₅→⁵I₆ energy-level transition, which corresponds to 3.9 μm emission. By successfully preparing Ho³⁺/Nd³⁺ co-doped LiYF₄ single crystals, Nd³⁺ efficiently absorbs 808 nm LD pump light and, through inter-ion energy transfer, effectively excites Ho³⁺—alleviating the self-terminating effect of the lower laser level of Ho³⁺ ions and thus achieving 3.9 μm mid-infrared emission. The successful development of this single crystal provides a new material for the development of efficient and environmentally friendly 3?5 μm mid-infrared lasers.

High-quality LiYF₄ single crystals doped with Ho³⁺ (molar fraction: 0.6%), doped with Nd³⁺ (1.0 %), and co-doped with Ho³?/Nd³? (0.6%/1.0%) were prepared using the Bridgman method. The raw materials used were LiF, YF₃, HoF₃, and NdF?, each with a purity of 99.999%, in molar ratios of n(LiF)∶n(YF₃)∶n(HoF₃)∶n(NdF₃)=50∶(50－α－γ)∶α∶γ (α=0,0.6; γ=0,1.0). The XRD patterns of the samples were measured using a Bruker D8 Advance X-ray diffractometer (Germany) with a scanning range from 10° to 90°. The absorption spectra of the crystals were determined using a Cary 5000 UV/VIS/NIR spectrophotometer within a wavelength range of 400 to 2200 nm. The mid-infrared fluorescence spectra and fluorescence decay curves of the samples were measured using FSP920/FSP980 mid-infrared fluorescence spectrometers and FLS920 UV-Vis-NIR fluorescence spectrometers. The doping concentrations of Ho³? and Nd³? ions in the grown crystals were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES).

Ho³⁺ and Nd³⁺ ions successfully replaced Y³⁺ lattice sites in LiYF₄ single crystals (Fig. 1, Table 1), indicating that the doping of Ho³⁺ and Nd³⁺ does not disrupt the LiYF₄ crystal structure. Compared to Ho³⁺∶LiYF? single crystals, the higher spectral intensity parameter Ω? and fluorescence branching ratio of Ho³⁺ ions in Ho³⁺/Nd³⁺∶LiYF₄ single crystals (Table 2) confirm that the co-doping of Nd³⁺ increases the asymmetry of the local crystal field environment around Y³⁺ ions and enhances the fluorescence emission efficiency of Ho³⁺ ions. The full width at half maximum (FWHM) values of Ho³⁺/Nd³⁺∶LiYF₄ single crystals are 124 nm and 91 nm, which are higher than those of Ho³⁺∶LiYF₄ single crystals (116 nm and 87 nm)—suggesting that the co-doping of Nd³⁺ ions facilitates the broadening of the mid-infrared laser emission range (Fig.4). Additionally, the emission cross-sections at 2.9 μm and 3.9 μm are 1.48×10?²⁰ cm² and 0.19×10?²⁰ cm², respectively (Fig. 5), demonstrating the significant laser output advantages of Ho³⁺/Nd³⁺∶LiYF₄ single crystals. Finally, the energy transfer efficiency (ET1) from Nd³⁺∶⁴F_3/2 to Ho³⁺∶⁵I₅ is 91.42% (Fig. 8), the energy transfer efficiency (ET2) from Ho³⁺∶⁵I₆ to Nd³⁺∶⁴I_15/2 is 52.91%, and the energy transfer efficiency (ET3) from Ho³⁺:⁵I₇ to Nd³⁺:⁴I_13/2 is 61.43% (Fig. 9)—validating the efficient sensitizing and quenching effects of Nd³⁺ ions on Ho³⁺ ions.

The fully sealed crucible descent method is an appropriate process for growing high-quality Ho³⁺/Nd³⁺∶LiYF₄ single crystals. Under 808 nm LD pumping, both 2.9 μm and 3.9 μm mid-infrared fluorescence emissions can be observed. The absorption cross-sections and emission cross-sections (for 2.9 μm and 3.9 μm, respectively) are 1.29×10^-20 cm2 and 1.48×10^-20 cm², as well as 0.12×10^-20 cm² and 0.19×10^-20 cm²—with effective emission bandwidths of 124 nm and 91 nm. The incorporation of Nd³⁺ ions enhances the lifetime of the Ho³⁺:⁵I₅ energy level through energy transfer from Nd³⁺:⁴F_3/2 to Ho³⁺:⁵I₅ (transfer efficiency η=91.42%) and also promotes the energy transfer processes from Ho³⁺:⁵I₆ to Nd³⁺:⁴I_15/2 and from Ho³⁺:⁵I₇ to Nd³⁺:⁴I_13/2. These energy transfer mechanisms strengthen the 3.9 μm and 2.9 μm mid-infrared fluorescence emissions. The doping of Nd³⁺ plays roles in sensitization and deactivation. Therefore, Nd³⁺/Ho³⁺ co-doped LiYF₄ single crystals are a promising medium for mid-infrared lasers.