Nonlinear absorption and refraction of neutral red dye in solid and solution forms

We report the results of our investigations on the nonlinear optical absorption and refraction of an organic chromophore, neutral red, both in Solution and in solid film forms, using z-scan technique. Z scan experiments were performed with the samples in solution form, in methanol and in solid film form in polyvinyl alcohol (PVA) matrix, using 532 nm, 20 ns, Nd-YAG laser pulses. The pre-focal peak and post focal valley in the closed aperture z-scan for solution sample are suggestive of a negative optical nonlinearity; where as, the open scan for the same indicates that excited state absorption (ESA) takes over saturable absorption (SA) when the input intensity is increased. For the solid film sample, on the other hand, the closed scan exhibits a sharp pre-focal valley followed by a post focal suppressed peak showing the predominance of two photon absorption (TPA) process. The open scan of the solid film has a dip, which is characteristic of ESA. As both solution and solid samples of this chromophore exhibit nonlinear absorption as well as refraction, neutral red can be a promising material for optical limiting applications.