The photostability and photodegradation of the PR dye.docx

We evaluated the stability of all-optical switching by characterizing of nonlinear optical properties of P-PR solution under cw laser irradiation for 2 hours. The evolution of diffraction rings was observed in real-time and recorded by a camera. Fig. 1(a) shows diffraction rings captured at different irradiation times. It is seen that the number of diffraction ring remains changed, while the overall size of diffraction pattern gradually decreases over time. Fig. 1(b) is the absorption spectrum of P-PR at different irradiation times. The absorbance at 532 nm shows no significant change, and no new absorption peaks are detected, meaning that minimal photodegradation and no formation of new substances during the irradiation process.As the irradiation time increases, the heat convection in the medium gradually stabilizes, leading to a reduction in the temperature gradient. And the refractive index distribution becomes more uniform. Therefore, the deflection angle of propagating laser beam decreases, which determining the size of the far-field diffraction pattern, as shown in Fig. 1(a). However, the number of diffraction rings remains largely unchanged, as it is determined by the peak phase shift at the beam center. Therefore, the number of diffraction ring remains constant, while the overall size of diffraction pattern becomes smaller over time. Due to the number of rings is unchanged, the nonlinear refractive index coefficients also remain unchanged. Therefore, P-PR has a stable nonlinear optical property under continuous wave laser irradiation.