Femtosecond lasers are widely used in industrial processing, national defense, medical and scientific research due to their ultra-short pulse width, extremely high peak power, and wide spectral width [[1], [2], [3]]. In the last two decades, femtosecond pulses generated by Yb3+-doped solid-state lasers operating in the 1 μm band have been one of the research hotspots of ultrafast lasers [[4], [5], [6], [7], [8], [9]]. Yb3+-doped gain media possess several advantages. Firstly, its quasi-three-level structure allows for a simple energy-level structure, effectively preventing additional losses such as upconversion and excited state absorption. Secondly, its quantum efficiency is as high as 90%. Moreover, the Stark energy level splitting of the Yb3+ ion energy level allows the spectral width to reach tens of nanometers, supporting the generation of ultrashort pulses [10]. The quasi-three-level structure of Yb3+-doped materials and the small splitting caused by the Stark effect will also lead to a decrease in the number of laser inversion particles and an increase in the absorption at the laser emission wavelength when the temperature increases [11]. Therefore, Yb3+-doped materials have higher requirements for thermo-optical properties. Thermal conductivity is a key parameter related to it [12]. The gain medium with low thermal conductivity will accumulate a lot of heat during the pumping process, which will reduce the conversion efficiency and laser performance. Table 1 shows the thermal conductivities of Yb:Lu2O3, Yb:LuAG and other common Yb-doped gain media.

Among them, Yb3+-doped lutetium garnets Yb:Lu2O3 (K = 11.0 W/m/K) [[19], [20], [21]] and Yb:LuAG (K = 7.4 W/m/K) [19,[22], [23], [24]] with high thermal conductivity have attracted the attention of researchers. In addition, the different atomic masses of the doped ions and the substituted ions will lead to a decrease in the thermal conductivity of the crystal with an increase in the concentration of the doped ions. In insulators such as Yb:Lu2O3 and Yb:LuAG, the heat transfer is mainly through the propagation of phonons. Due to the low mass difference between the doped ion Yb3+ (mass of 173 g/mol) and the substituted ion Lu3+ (mass of 175 g/mol), the phonon scattering rate at the mass defect is low, and the degree of thermal conductivity reduction is lower than that of Yb:YAG, Yb:SC2O3 and other crystals [13,[25], [26], [27]]. However, these materials also have some disadvantages. For example, their raw materials Lu2O3 with high purity are expensive, and the melting points of Lu2O3 and LuAG are relatively high [28]. In order to solve these problems, Kuwano et al. proposed the use of the mixed garnet laser gain medium LuYAG (whose parent garnet is LuAG and YAG) [[29], [30], [31]]. Due to the mixed composition, the Lu3+ ion concentration is reduced and the cost is reduced. LuYAG has a lower melting point than Lu2O3 and LuAG, which reduces the requirements for growth equipment [28]. Moreover, the disordered crystal structure generated by the mixing of the two compositions will cause an uneven broadening of the fluorescence line [12,32]. According to previous studies [32], the emission bandwidths of Yb:LuYAG, Yb:YAG and Yb:LuAG are 12.5 nm, 10 nm and 10 nm, respectively, and their absorption bandwidths are 22 nm, 20 nm and 22 nm, respectively. The emission bandwidth of Yb:LuYAG is wider than that of Yb:YAG and Yb:LuAG, and the absorption cross section and absorption bandwidth are comparable to those of Yb:YAG and Yb:LuAG [32], which is more suitable for generating femtosecond pulses. In 2015, Wang et al. reported a diode-pumped passively mode-locked Yb:LuYAG laser for the first time with a pulse duration of 2.4 ps [33]. In 2016, Pirri et al. realized a wide tuning range for the Yb:LuYAG laser from 998 to 1063 nm and obtained the highest continuous-wave (CW) laser output power of 8.7 W with a center wavelength of 1030 nm and a slope efficiency of 65.6% [28]. In 2021, using MoS2 as a saturable absorber, the passively Q-switched Yb:LuYAG laser with a pulse duration of 293 ns was realized by Lv et al. [34]. Very recently, Slimi et al. produced a Yb:LuYAG laser Q-switched by Cr4+:YAG generating 201 ns pulses with a pulse energy of 0.15 mJ at 39.7 kHz [12]. However, the shortest pulse generated by the Yb:LuYAG laser still stays at the picosecond level.

In this paper, the femtosecond Yb:LuYAG laser operation is studied for the first time. Employing SESAM as a saturable absorber, a 140 fs passively mode-locked Yb:LuYAG laser with a repetition rate of 95.4 MHz and an average output power of 618 mW is realized at a pump power of 3.78 W. In the CW operation, a maximum output power of 1.16 W with a center wavelength of 1051 nm is obtained. Notably, this represents the shortest pulse generated from a Yb:LuYAG laser to date.