## 3 Results and Discussion
The broadening of this band may be ascribed to the superposition of three electron transition in 'd' orbitals corresponding to the \({}^{2}\)B\({}_{1g}\)\(\rightarrow\)\({}^{2}\)E\({}_{g}\),\({}^{2}\)B\({}_{1g}\)\(\rightarrow\)\({}^{2}\)A\({}_{1g}\), and \({}^{2}\)B\({}_{1g}\)\(\rightarrow\)\({}^{2}\)B\({}_{2g}\) transitions of Cu\({}^{2+}\) ions. Figure 3: (**a**) The DTA traces; and (**b**) variation of T\({}_{g}\) and T\({}_{x}\) as a function of Na\({}_{2}\)CO\({}_{3}\) concentration in the PSAB glass system. It was also observed that the absorption edge shifted towards higher wavelength, and the optical bandgap decreased with increasing CuO concentration. The reason for this behavior was explained by other researchers [27; 28]. According to the report, the octahedral coordinated Cu\({}^{2+}\) ions act as modifiers to induce nonbridging oxygens (NBOs) in the glass network. Therefore, when CuO concentration increases, this resulted in an increase of NBOs. This leads to an increase in the degree of localization of electrons, thereby increasing the donor centers in the glass matrix. The presence of larger concentrations of these donor centers decreases the optical bandgap and shifts the absorption edge towards the higher wavelength side. In order to investigate the effect of CuO and Na\({}_{2}\)CO\({}_{3}\) codopant on transmittance, the glass samples were prepared by adding 3 wt% Na\({}_{2}\)CO\({}_{3}\) into 0.2 wt% and 0.5 wt% CuO-doped PSAB glass system. Figure 5b displays the transmittance of codoped PSAB glasses for different concentrations of CuO. It was noticed that the transmittance decreased from 20.7% to 13.4% and 3.19% to 0% at 810 nm with the addition of 3 wt% Na\({}_{2}\)CO\({}_{3}\) into 0.2 wt% and 0.5 wt% CuO-doped PSAB glass system, respectively. It was also clear that the same trend was noticed when CuO was added into the glass system; however, the codoping into the system reached 100% absorption at the laser wavelength. With the addition of Na\({}_{2}\)O alkali oxide into the glass system, a bond in the network was broken, resulting in an increase of NBOs and the relatively mobile Na ion became a part of the structure, which caused lower melting and working temperatures by decreasing viscosity [29]. The results indicate that codoping into the PSAB system not only increased the CTE but also decreased the transmittance, i.e., increased the absorption at 810 nm. Figure 4: (**a**) TMA traces; and (**b**) variation of CTE for different temperature ranges, dependent on Na\({}_{2}\)CO\({}_{3}\) dopant concentration; inset shows the variation of T\({}_{g}\), dependent on Na\({}_{2}\)CO\({}_{3}\) concentration. The flow button test was performed, in order to investigate flow ability of PSAB glass on substrate. The heat reaction of glass frits was divided into three stages [30,31]. In the first stage, the square shape changed to a trapezoid and then to a square. The pellet was eventually formed into a swollen sphere in the second stage. Thus, the shape of the pellet changed into an ellipsoid with an angle \(>\)90\({}^{\circ}\) at T\({}_{\rm s}\). This is known as the softening point. In the third stage, the wetting angle of the molten glass was \(<\)90\({}^{\circ}\). At higher temperatures (\(>\)T\({}_{\rm s}\)), the swollen sphere started to spread over and then the molten glass was eventually puddled on the substrate. As shown in Figure 6, the flow button test showed the shapes of pellets as a function of sintering temperature at (a) 400 \({}^{\circ}\)C, (b) 450 \({}^{\circ}\)C, (c) 500 \({}^{\circ}\)C, and (d) 550 \({}^{\circ}\)C. Height and diameter of pellets were measured as a function of sintering temperature. The wetting angle of the pellet was divided into three stages as shown in Figure 6. At the first stage, a cylinder type of pellet was sintered at 400 \({}^{\circ}\)C for 30 min, and the volume of the pellet decreased by 23% to reduce the surface energy.