Study of Nonlinear Optical Properties of Gold Nanoparticles Dispersed in Degenerate and Oriented Nematic Liquid Crystals

Gold nanoparticles (AuNPs) come in various sizes and shapes. Their optical properties arise from surface plasmon resonance, collective excitation/oscillation of conduction electrons. The study of the nonlinear optical properties of these materials is essential for photonic and biomedical applications. On the other hand, liquid crystals have found useful applications in many fields because they can serve as a reconfigurable template to arrange and orient particles into ordered structures. They have been employed to guide nanoobjects into functional three-dimensional materials. In this work we studied the structural and nonlinear optical properties of 20 nm AuNPs that are dispersed in degenerate (non-oriented) and oriented nematic liquid crystals (NLCs) using polarizing microscopic technique and optical Z-scan technology, respectively. Polarizing microscopic images show AuNPs dispersing continuously in oriented NLCs while aggregating in degenerate NLCs. Nonlinearities are studied by curve fitting from Z-scan data to get nonlinear absorption coefficients and we considered three aspects: concentration of AuNPs, pulse laser frequency, and laser intensity. Our experiments demonstrated that AuNPs in oriented NLCs have stronger absorption, linear and nonlinear absorption coefficient of it is about twice larger than AuNPs in degenerate NLCs. Also, sample with low gold concentration, by weight percentage, shows the better nonlinearity.