# Diode-Pumped Solid-State Q-Switched Laser with Rhenium Diselenide as Saturable Absorber
## 2 Preparation and Properties of Rhenium Diselenide  
In the experiment, based on the feature of weak-layered dependence, there was no need to strictly control the uniformity of the \(\text{ReSe}_{2}\) SA. So, the ultrasonic exfoliation method was chosen to prepare \(\text{ReSe}_{2}\) SA. This method was simple and efficient. The \(\text{ReSe}_{2}\) dispersion solution was prepared by adding 0.5 mg \(\text{ReSe}_{2}\) powder to 200 mL of 15% alcohol. The mixture was placed in the ultrasonic cleaner for 48 h, and then stored at room temperature for 24 h. For the SA fabrication, the supernatant was made to spin on sapphire base and put in a drying cabinet for 24 h. Additionally, the properties of the \(\text{ReSe}_{2}\) were characterized by using a Raman spectrometer (HORIBA LabRAM HR Evolution, Paris, France), a scanning electron microscope (SEM, Zeiss Sigma 500, Oberkochen, Germany), a transmission electron microscope (TEM, JEM-2100, Tokyo, Japan), and a home-built Yb-doped mode-locked laser.  
Raman scattering experiments reported that \(\text{ReSe}_{2}\) had the abundant spectra [16]. As shown in Figure 1a, the powdered \(\text{ReSe}_{2}\) Raman spectrum is displayed. There were marked three main Raman shifts, corresponding to the \(\text{E}_{\text{g}}\) (124 cm\({}^{-1}\)) and \(\text{A}_{\text{g}}\) (159 cm\({}^{-1}\) and 173 cm\({}^{-1}\)) modes. The \(\text{E}_{\text{g}}\) and \(\text{A}_{\text{g}}\) were equivalent to in-plane and out-of-plane vibrational modes, respectively. At the same time, because of the low-symmetry of \(\text{ReSe}_{2}\), many small peaks were noticed in the experimental scope [32]. The morphology of \(\text{ReSe}_{2}\) powder was elucidated by the SEM image, which is reproduced in Figure 1b. SEM image shows that the pre-dispersed \(\text{ReSe}_{2}\) powder is a multi-layer structure with micro level in size.  
To test the shape of ReSe\({}_{2}\) through ultrasonic exfoliation, the TEM image of the dispersion solution is shown in Figure 2. Compared with the SEM image, ReSe\({}_{2}\) was effectively stripped into nano-scale sheets. The optical resolution of the TEM microscope was 50 nm.  
A power-dependent transmission technique was employed for investigating the saturable absorption properties of ReSe\({}_{2}\) SA. For this experiment, the pump source was a home-built nonlinear polarization rotation mode-locked Yb-doped fiber laser with 68 ps pulse width at 1064 nm and repetition rate of 38.6 MHz. Based on the formula:  
\[T(I)=1-T_{ns}-\Delta T\times\exp(-I/I_{sat}) \tag{1}\]  
where \(T(I)\) is the transmittance, \(T_{ns}\) represents the non-saturable absorbance, \(\Delta T\) is the modulation depth, \(I\) signifies the input intensity of laser, and \(I_{sat}\) is the saturation intensity [30]. The experimental results are shown in Figure 3. Fitting the experimental results, the saturation intensity, non-saturable absorbance, and modulation depth were 6.37 MW/cm\({}^{2}\), 16.84%, and 1.89%, respectively. In subsequent experiments, attempts were made to change raw material parameters and optimize the preparation process. ReSe\({}_{2}\) SA materials with different parameters are expected to be obtained, thus making them more suitable for a variety of applications.  
Figure 1: (**a**) Raman spectrum and (**b**) SEM image of ReSe\({}_{2}\) powder.  
Figure 2: Transmission electron microscope (TEM) image of ReSe\({}_{2}\) dispersion solution.