Broadband Optical Limiting via Reversible Polystyrene Nanoparticle Clouding: Phase Diagram for Laser Intensity and Concentration Thresholds

Optical limiting protects sensitive optical devices by attenuating high-intensity light to prevent damage through decreased light transmission. This study investigates ultra-broadband optical limiting via reversible clouding of polystyrene nanoparticles in an aqueous medium, induced near the phase separation boundary by optical trapping. Optical trapping via focused electromagnetic radiation locally increases the particle density, driving the system from homogeneous one-phase into heterogenous two-phase system that scatters incoming light. Experiments with both 1064 nm and 532 nm lasers show that the transmitted ratio, defined as incident / transmitted laser intensity, increases with increasing laser power, indicating aggregation-induced scattering. A threshold value that combines the laser power, laser wavelength and the numerical aperture of the lens at the trap is determined. Once above threshold, both trapping efficiency and aggregation rate depend weakly on laser power. These results demonstrate that optical gradient forces can reversibly induce colloidal clouding to achieve robust, wavelength-independent optical limiting.