Nonlinear optical properties of methyl red under CW irradiation

• Methyl red (MR) doped PMMA is investigated by using Z-scan at different wavelengths.
• n2 and ΔnΔn are obtained under different scanning speed at 473 nm and 532 nm.
• We compare the results of CW Z-scan and pulse Z-scan at 532 nm.
• The cumulative thermal effect has great influence to the NLO properties of MR.

Organic materials have wide potential application in nonlinear optical devices. The nonlinear optical (NLO) properties of methyl red (MR) doped polymethyl methacrylate (MR-PMMA) are investigated under CW laser irradiation at 473 nm, 532 nm and 632.8 nm, respectively. By combining Kramers–Kronig (K–K) relation and CW Z-scan technique, the effective refractive index n2 and the change of refractive index ΔnΔn are obtained under different scanning speed at 473 nm and 532 nm. ΔnΔn is positive at 473 nm, while ΔnΔn is negative at 532 nm. The experimental result is consistent with that of K–K relation. With the scanning speed decreasing, the NLO properties of MR-PMMA are enhanced. With different laser powers at 632.8 nm, MR-PMMA has only nonlinear absorption rather than nonlinear refraction. Meanwhile, the sample is investigated under pulse laser irradiation at 532 nm. Through the comparison of results of CW Z-scan and pulse Z-scan, the influence of the cumulative thermal effect on NLO properties of material is investigated. The results indicate that, under CW irradiation near the absorption peak wavelength, the cumulative thermal effect has great influence to the NLO properties of MR-PMMA.