Tailoring the optical limiting response of methyl orange via protonation

Abstract

In order to realize effective optical limiting (OL), it is necessary to utilize materials that exhibit distinct traits, such as a large and rapid nonlinear optical response, high photothermal stability, and cost-effectiveness. In the present study, the nonlinear optical (NLO) response of methyl orange (MO) is modified by inducing conformational and structural changes through appropriate modification of pH conditions. The NLO properties of MO under two different pH conditions were investigated via the z-scan technique under 7 nanoseconds excitation in the visible region (532 nm). From z-scan results, the nonlinear absorption coefficient of pristine MO and protonated-MO (P-MO) was estimated. Compared to MO, the nonlinear absorption coefficient of P-MO increased by 3.6 times, and it can be attributed to the redistribution of energy levels of MO, which favored resonant absorption in the system. Molecular energy shift and the corresponding apparent red shift in the absorption spectrum of MO are confirmed from both experimental and theoretical analysis. The mechanism behind the nonlinear absorption (NLA) and OL activity of MO is found to be effective two-photon absorption. The closed aperture (CA) z-scan data reveal a negative nonlinear refraction response of MO and P-MO. These findings suggest an efficient way to tune the NLO response of MO through protonation, and this method readily applies to numerous nonlinear optical applications, including but not limited to OL.