In recent years, ~ 3 μm mid-infrared lasers have attracted much attention due to wide applications in biomedical treatments, molecular fingerprint identification and optical parametric oscillation (OPO) pumping source.

Generally, two methods are effective for the realization of mid-infrared laser. One method is a non-linear approach combined with OPO technique, which is structure-complicated and instable. The other one depends on pumping the mid-infrared laser crystal directly, which is more conducive to the development of the miniaturization and practicality.

Recently, a research team from Shanghai Institute of Optics and Fine Mechanics, Chinese Academic of Sciences, has succeeded in developing a new kind of low-phonon-energy laser crystal: Nd,Er:LaF₃.The crystal growth, structure, spectra, and self-termination effect were investigated. Their achievement was published in Journal of Alloys and Compounds.

In their experiment, for the first time, the Nd, Er:LaF₃ crystal was grown successfully by the Bridgman method. The Er:LaF₃ crystal was also grown for comparison. It remained hexagonal crystal structure free from a second phase. The segregation coefficients of Nd³⁺ and Er³⁺ were 0.96 and 0.76 respectively, indicating that it was easy to dope Nd³⁺ and Er³⁺ ions into LaF₃ host.

Furthermore, the intense 2.7 μm emission in Nd,Er:LaF₃ crystal was obtained under the 808 nm LD excitation. With the help of Judd-Ofelt theory and absorption spectra, the fluorescence branching ratio, emission cross-section and measured fluorescence lifetime at 2.7 μm were calculated to be 28.5%, 1.3×10^-20 cm² and 5.48 ms, respectively.

Dual-function of Nd³⁺ ions were analyzed by comparing the mid-infrared fluorescence spectra with lifetimes of the upper and the lower energy levels in singly-doped and doubly-doped crystals.

They found that the Nd³⁺ ions not only sensitized the Er³⁺ ions but also deactivated the Er³⁺:4I_13/2 energy level effectively by lowering the lifetime of the lower energy level from 22.60 ms to 4.37 ms with the deactivation efficiency of 80.7% in Nd,Er:LaF₃ crystal.

It's worth noting that no self-termination effect exists in the Nd,Er:LaF₃ crystal. All the results show that Nd,Er:LaF₃ crystal is a promising candidate for 2.7 μm lasers pumped by a commercial 808 nm LD.

This work was supported by the National Natural Science Foundation of China, the National Key R&D Program of China; Strategic Priority Research Program and the Major Project of Shanghai Science and Technology Research Foundation.

Characterization of Nd,Er:LaF3 crystal. (Image by SIOM)