Optical limiting based on liquid-liquid immiscibility

Nonionic surfactants are used to stabilize a dispersed droplet phase in a continuous liquid phase when two immiscible liquids are mixed. As both liquid phases approach the index-matched condition, interfacial scattering is suppressed, and the mixture takes on the characteristics of a Christiansen-Shelyubskii filter. If, in addition, one of the liquids exhibits a substantial nonlinear optical response, then interfacial light scattering can be reversibly turned on when a laser beam incident upon the filter exceeds a critical fluence. To demonstrate this effect, an organic phase (dichloroethane) was dispersed in an aqueous phase containing sodium thiocyanate (NaSCN) using an alkyl end-capped polyethylene glycol ether. Optical limiting was observed through this transparent medium under conditions where the focused second-harmonic output of a Q-Switched Nd:YAG laser was on the order of about 50 mJ/cm(2). An open-aperture z-scan technique was used to quantify the limiting behavior. Since the thiocyanate anion is both isostructural and isoelectronic with carbon disulfide which exhibits a large optical nonlinearity, the mechanism of optical limiting is proposed to be associated with a nonlinear shift in the aqueous fluid index of refraction, resulting in an index mismatch-between the disparate phases at high laser fluence. Index mismatch between the two phases leads to multiple reflections, loss of coherence, and a significant transmission decrease due to Tyndall scattering. Fundamental studies of such systems are used to verify theoretical predictions of the limiting effect, and aid in the design and development of improved sub nanosecond limiters based upon this optical deflection approach.