Nonlinear change of refractive index of Co3O4 thin films induced by semiconductor laser (λ=405 nm) irradiation

Nonlinear change of the refractive index (n) and extinction coefficient (k) of ${\mathrm{Co}}_3{\mathrm{O}}_4$ thin films induced by a laser with $\text{λ=405\hspace{0.17em}}\mathrm{nm}$ $\text{(hν=3.06\hspace{0.17em}}\mathrm{eV})$ irradiation was evaluated using equipment having an ellipsometric optical configuration. Nonlinear refractive index ${\mathrm{(n}}_2)$ and extinction coefficient ${\mathrm{(k}}_2)$ were ${\text{+1.0×10}}^{\text{−10}}\hspace{0.17em}{\mathrm{m}}^2/\mathrm{W}$ (positive) and ${\text{−2.6×10}}^{\text{−11}}\hspace{0.17em}{\mathrm{m}}^2/\mathrm{W}$ (negative), respectively. $n_2$ and $k_2$ at $\text{λ=650\hspace{0.17em}}\mathrm{nm}$ $\text{(hν=1.91\hspace{0.17em}}\mathrm{eV})$ were ${\text{−5.5×10}}^{\text{−11}}\hspace{0.17em}{\mathrm{m}}^2/\mathrm{W}$ and ${\text{−8.7×10}}^{\text{−11}}\hspace{0.17em}{\mathrm{m}}^2/\mathrm{W}$ (both have negative signs). From these results and the fact that the ${\mathrm{Co}}_3{\mathrm{O}}_4$ thin film has the band-gap energy of 2.06 eV, the band filling effect can be seen as one of the most probable models describing the large nonlinear change of n and k of ${\mathrm{Co}}_3{\mathrm{O}}_4$ thin film.