Optical Limiting in Organic Molecular Nano/Microcrystals: Nonlinear Optical Effects Dependent on Size Distribution

Several recent investigations have revealed interesting size-dependent linear optical responses in molecular nanocrystals. However, little is known about similar effects in the nonlinear optical behavior of such nanomaterials. The current study addresses optical limiting characteristics and nonlinear absorption/scattering effects in nano/microcrystals of a strongly zwitterionic diaminodicyanoquinodi methane molecule fabricated in the colloidal state through a reprecipitation-digestion protocol. Optical limiting response observed for 800 nm ferntosecond laser pulses is found to be appreciable and dependent on the size distribution of the molecular crystals in the colloid. Z-scan studies reveal that the ultrashort pulses lead to multiphoton absorption with saturation effects, both becoming more pronounced with smaller particle sizes. The nano/microcrystals in the colloidal state exhibit strong nonlinear scattering; when thermal effects are excluded, these responses are found to be largely independent of the distribution of particle sizes.