## 3 Results and Discussion
 \\ \hline \({}^{6}\)H\({}_{15/2}\)\(\rightarrow\)\({}^{6}\)P\({}_{7/2}\) & 352 & 29.321 & 0.451 \\ \({}^{6}\)H\({}_{15/2}\)\(\rightarrow\)\({}^{4}\)F\({}_{7/2}\) & 393 & 25.945 & 0.520 \\  
to 3.706 eV, and the values of indirect (\(E_{g}\)) show a decrease from 3.189 to 2.556 eV with increasing dopant contents, as listed in Table 5. The declines in \(E_{g}\) values may be due to structural changes due to the addition of Dy\({}^{3+}\) ions. The addition of Dy\({}_{2}\)O\({}_{3}\) can contribute to an increase in electron localisation that increases donor centres in the glass matrix. This increment causes a decrease in \(E_{g}\) values [35]. This is also because a new extrinsic band is formed by Dy\({}^{3+}\) on the grid between the boron and oxygen ions. As a consequence, there is an amount of possible reduction in (B-O-B) [36]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **E\({}_{\text{dir}}\) (eV)** & **E\({}_{\text{indir}}\) (eV)** & **\(E_{u}\) (eV)** & **\(A_{c}\) [nm]** \\ \hline
**LABD-0.0** & 3.580 & 3.189 & 3.520 & 362.36 \\
**LABD-0.2** & 3.706 & 3.166 & 3.337 & 368.83 \\
**LABD-0.4** & 3.566 & 2.919 & 3.272 & 376.44 \\
**LABD-0.6** & 3.585 & 2.886 & 3.075 & 385.81 \\
**LABD-0.8** & 3.473 & 2.7005 & 3.352 & 394.15 \\
**LABD-1** & 3.650 & 2.556 & 3.058 & 401.81 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Direct and indirect band gaps, Urbach energy (\(E_{u}\)) and cut-off wavelength (\(\lambda_{c}\)) of the samples studied. Figure 6: Bang gap energy: (**a**) [PERSON]’s plot for allowed direct transitions (\(n=2\)); (**b**) [PERSON]’s plot for allowed indirect transitions (\(n=1/2\)). Figure 7 indicates that the energy of Urbach decreases with changes in the Dy\({}_{2}\)O\({}_{3}\) concentration. The reduction in the energy of Urbach is attributed to the creation of fewer defects, as reported [37]. Figure 8 and Table 6 show a slight gradual increase in refractive index values from 2.29 to 2.35 with increasing Dy\({}_{2}\)O\({}_{3}\) concentration that can be attributed to the increase in electronic polarizability from 2.67 to 2.75 [17]. The sample has a higher refractive index, as it has a smaller bandgap value that reflects the compactness of the glass network structure. Meanwhile, the increment in  
Table 6 presented the optical properties of the glass samples. It is observed that the molar refractivity (R\({}_{\rm M}\)), reflection loss (R\({}_{\rm L}\)) and refractive index (\(n\)) has a significant influence on the polarizability (\(\alpha_{m}\)) which demonstrates that the refractive index of the glasses does not solely depend on the density. It is known that the samples containing NBOs have great polarizability compared with samples containing bridging oxygen (BOs). The results presented here are in agreement with another work [38]. The theoretical optical basicity (\(\Lambda\)) is a calculation of oxygen's capacity to contribute a negative charge load in glasses. To classify the covalent/ionic ratios of the glass, the theoretical optical basicity may be used, because the increment in (\(\Lambda\)) values indicate the declining covalence. Table 6 indicates that the values of \(\Lambda\) are within the range 1.240-1.285 and found to increase with increasing Dy\({}_{2}\)O\({}_{3}\) concentrations. Here, the increment in optical basicity values means the ability of oxide ions to transfer electrons in the cations surrounding them [7]. Figure 9 represents the decline in the metallization criterion with an increase in the Dy\({}_{2}\)O\({}_{3}\) concentration. The obtained values confirm the non-metallic nature of the current glass samples [8]. From Table 3, the results show that the refractivity R\({}_{\rm M}\), \(\alpha_{\rm M}\), \(\varepsilon\) and optical dielectric constant increase with increasing Dy\({}_{2}\)O\({}_{3}\), meaning an increase in NBOs inside the glass matrix.