The Non-linear Optical Properties Of Silver/polydiacetylene Hybrid System

During the past decade, much significant research has been directed toward understanding organic nonlinear optical (NLO) materials and their application in information processing due to their large NLO response and short response time. Among these organic NLO materials, polydiacetylene (PDA), classified as a one-dimensional-conjugated polymer, possesses excellent third-order NLO response properties in the range of femtoseconds to sub-picoseconds, which is attributed to delocalized-conjugated electrons along the main chain backbone. However, the third-order nonlinear susceptibilityχ(3)(ω) was only between 10-9 esu and 10-11 esu which limit the application of these nonlinear optical materials. So how to improve the effective third-order NLO susceptibilityχ(3)(ω) of PDA system has became a porimary issue. It is worth nothing that nano metal and semi-conductor fine particles exhibited the enhancement of NLO properties induced by quantum confinement effect. characteristic interactions between exciton, plasmon and their resonance effect in the hybridized systems between organic microcrystals and inorganic nano fine particles would make it possible to exhibit novel optical properties. Thus we introduced the metal nanoparticles into PDA system to prepare PDA/Ag nanoparticles composite vesicles, composite films and composite crystal. Then we change the size, array and concentration of Ag nanoparticles in this PDA/Ag nanocomposite system to enhance the third-order nonlinear optical properties, which would be useful in the development of new PDA-based optical and NLO devices.Two kinds of nanometer-size silver-coated polydiacetylene (PDA) composite vesicles were prepared by simple and effective approaches, the average diameter was about 100 nm which was measured by transmission electron microscopy (TEM). Silver nanoparticles were coated onto the outer and inner surface (I) or only on the outer surface of PDA vesicles (II), respectively. As a reference sample, the mixing solution of Ag colloidal nanoparticles and PDA vesicles was prepared. NLO properties of these three samples have been evaluated by means of a Z-scan technique. Open and closed aperture Z-scan measurements were used to evaluate the nonlinear refraction and nonlinear absorption properties. Compared with pure PDA vesicles, the Ag colloidal nanoparticles coating led to the obvious enhancement of NLO properties for PDA vesicles. The obtained nonlinear refractive index for pure PDA vesicles and PDA/Ag composite vesicles with yellow Ag nanoparticles coated on the outer surface of PDA vesicles was 1.2×10-14 cm2 W-1 and 7.3×10-14 cm2 W-1, respectively. The nearly seven times improvement is observed as a result of local field enhancement under the surface plasmon resonance of silver nanoparticles at the interface.As to further study the influence of surface plasmon field of Ag nanoparticles to PDA/Ag composite system, we changed the size of Ag nanoparticles on the surface of PDA vesicles. We found that PDA/Ag (50 nm) composite vesicles showed maximum NLO properties enhancement effect (nearly 13 times) and this size-dependent enhancement effect of Ag colloidal nanoparticles should be attributed to mediation of a size-dependent light-confinement effect and a size-dependent dielectric constant of Ag spheres. These experimental results were well in agreement with the theory calculation using modified Mie theory. Furthermore, these PDA/Ag composite vesicles were successfully immobilized onto the solid substrate by Langmuir-Blodgette (LB) method and their linear and nonlinear optical properties were characterized, respectively. Obviously, PDA/Ag composite vesicles Langmuir-Blodgett (LB) films promoted the enhancement of the third-order NLO properties. Control of suitable immobilization spacing and relative position of the neighboring PDA/Ag composite vesicles in LB films should lead to stupendous enhancements in the third-order NLO response.In addition, the distance between the adjacent Ag nanoparticles can also affect the surface plasmon resonance field, thereby affecting the effective third-order NLO susceptibility of PDA/Ag composite system. Silver nanoparticles with different concentration were adulterated in above polymerized NADA (PNADA) films and the third-order nonlinear optical (NLO) properties were investigated in detail. The third-order nonlinear optical susceptibility Reχ(3) increased unceasingly when the volume fraction of silver nanoparticles changed from 0 to 58 %, and the maximum appeared when the volume fraction of silver nanoparticles reached 58 %. When the volume fraction of silver nanoparticles increased over 58 %, Reχ(3) was observed to decrease contrarily. Obviously, Reχ(3) reached maximum when the the volume fraction of silver nanoparticles in PNADA/Ag composite films was 58 %. This concentration-dependent enhancement effect of Ag colloidal nanoparticles should be attributed to the effect of localized field coupling among Ag nanoparticles. When the volume fraction of silver nanoparticles in the composite films is lower (the distance between Ag nanoparticles was far), the interaction between localized fields was relatively weak, and the electric field mainly localized in the surface of Ag nanoparticles. With the concentration of Ag nanoparticles increasing, the distance between nanoparticles gradually decreased. When the particle distance was reduced to a certain degree, the coupling effects will markedly enhance the local field, and thus the local field was mainly distributed between the two particles. With the distance between particles further decreasing, particle clusters phenomenon would occur, which would reduce the local field.According to the relationship between the third-order nonlinear susceptibility of PDA/Ag composite system and the spacing between Ag nanoparticles, we designed a thermosensitive silver/polydiacetylene hybrid nano-crystal with tunable non-linear optical properties. At first, we fabricate the core-shell colloidal material comprising silver/polydiacetylene (Ag/PDA) hybrid nano-crystals stabilized with a thermally responsive poly (N-isopropylacrylamide) (PNIPAM). While the Ag/PDA hybrid nano-crystals cores provide the nonlinear optical properties, the PNIPAM shells can swell or collapse as a function of temperature, which allows for adjusting the relative spatial distances between neighboring Ag/PDA hybrid nano-crystals, further control the localized surface plasmon resonance enhancement and the resulting third-order nonlinear susceptibility of this colloidal material. Because of the anchoring and stabilization of PNIPAM brush to the Ag/PDA hybrid nano-crystals, the tuning of spatial distribution of nano-crystals is completely reversible with heating/cooling cycles. The facile preparation procedures and their efficiency of response to the surrounding media render these novel hybrid colloidal material potential candidates for applications in sensors, catalysis, and optic/electronic devices.