An insight into phenomenal optical non-linearities arising from synergistic relationship between selected BODIPYs and noble metal nanoparticles

Materials with large and quick non-linear optical response, excellent photo thermal stability, cost effectiveness and off resonant non-linear absorption (NLA) are essential requirements for a good optical limiting (OL). Among them, organic systems and pi-conjugated polymers are getting special interest because of their flexibility in structural modification that leads to the tuning of optical and electronic properties that are suitable for working under large bandwidth. Here, we report a drastic enhancement of non-linear optical activity and exceptional OL action of two organic-inorganic hybrid system based on BODIPY and Au and Ag nanoparticles (NPs). The organic system taken was certain set of BODIPYs with different substitution at the para and meta positions. All the compounds were found to be exhibiting good reverse saturable absorption (RSA) behaviour and negative non-linear refraction property. An attempt was made to improve the non-linear optical (NLO) property of the BODIPY by forming nanohybrids with Au and Ag NPs, and the best NLO active candidate among the studied system was chosen for it. A significant enhancement in NLO property was observed on hybrid system formation. The optical limiting (OL) threshold of the hybrid (1 J/cm(2)) was reduced almost 1/5 times than that of the parent compound (5.2 J/cm(2)), and this value is comparable with the benchmark OL materials like C-60. The mechanism behind the NLA is found to be the combination of excited state absorption (ESA) and two-photon absorption (TPA). The enhanced NLA and OL action of C-Ag/Au nanohybrids are attributed to the synergetic effect among the two parent components as well as the local field effects of NPs. Enhancement in non-linear optical property is found to be stronger in hybrid system with Au NPs and is due to the resonant charge transfer and intense local field effect on exciting with 532-nm pulse compared with that of Ag NPs. Both the parent compounds and the nanohybrids exhibit negative non-linearity, and the non-linear refraction is also found to be enhanced. The improved NLA and OL property of hybrid system guarantees successful usage of the strategy in OL applications.