Component design and stable SESAM mode-locking of the disorder middle entropy Yb:CaSrBaF₆ single crystal

To the best of our knowledge, this is the first report on continuous and passively mode-locked operation of the multi-component fluoride CaSrBaF₆ crystal. A novel disorder laser material, Yb:CaSrBaF₆ (Ca_0.33Sr_0.33Ba_0.33F₂) of multi-component middle entropy crystal was designed and grown by temperature gradient technique (TGT) for the first time. X-ray diffraction (XRD) and X-ray fluorescence (XRF) analysis of Yb:CaSrBaF₆ crystal reveals that Ca²⁺, Sr²⁺, and Ba²⁺ of near equal atomic ratio (1:1:1) have formed a homogeneous single-phased fluorite solid solution. The first principle calculation further shows that Ca²⁺, Sr²⁺, Ba²⁺ ions tend to be evenly distributed in the matrix crystal. The total formation energy is the lowest -547.17 ev and the structure is also the most stable at this time. The spectral properties of the crystal are systematically characterized. The emission cross section of ²F_5/2→²F_7/2 transition at 1040 nm is 0.62 × 10^-20 cm² with the larger full width at half maximum (FWHM) of 60.5 nm. The evenly disordered distribution of various cations and lattice distortion effect leads to the more diverse local structure and the diversity of luminescence, which can cause non-uniform broadening of the spectrum. Meanwhile, the Yb:CaSrBaF₆ crystal generated a continuous wave (CW) output power of 1.128 W, corresponding to a slope efficiency of 32% and an optical-to-optical conversion efficiency of 28.7% at 1055.4 nm. By implementing a semiconductor saturable absorber mirror (SESAM) for stable mode-locked laser operation, when the absorbed pump power reached 3.79 W, the laser ran into continuous wave mode-locking (CWML) regime, the maximum average output power of 123 mW was generated and the pulse duration of 89 ps was achieved at a pulse repetition rate of 54.6 MHz, with a pulse energy of 2.25 nJ and a pulse peak power of 25 W. Better laser performance could be expected after optimizing pump core diameter and elimination of dispersion. All results show that Yb:CaSrBaF₆ crystal is regarded as a what we believe to be novel laser materials, which also provide a reference for the development of disordered material and other rare earth ions doping.