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Plasmon assisted two-photon direct laser writing of micro-structures composed of chiral Ag nanoparticles

X. Vidal, W.J. Kim, A. Baev, H. S. Jee, V. Tokar, M.T. Swihart and P.N. Prasad.
The Institute for Lasers, Photonics and Biophotonics, ES

Keywords: chirality, two-photon, nano-particles, plasmon


We present an approach to produce micropatterns of metallic nanoparticles (NPs) that preserve key optical properties of the individual NPs. The technique uses a photothermal reaction induced by two-photon direct laser writing. The studied NP property is plasmon chirality, which was obtained via chemical conjugation of Ag NPs with chiral ligands. This was achieved using a facile surface treatment of silver NPs functionalized with thermally cleavable chiral ligands: N-(tertbutoxycarbonyl)-L-cysteine methyl ester. The ligand cleavage initiated by a femtosecond pulsed laser induced thermal reaction results in a significant change in dispersiblility of the nanocrystals, thereby enabling a solvent selective development process after photo-patterning. We analyzed the optical chirality of the Ag NP films before and after micropatterning by two-photon lithography. We show that this patterning technique allows the patterned film to maintain this predefined optical property. In contrast, we show the disadvantage of the common use of a photo-acid generator for the cleavage of solubilizing groups via protonation. In this case, the ligand protonation results in the loss of the chiral behavior. In conclusion, we show both bottom-up and top-down techniques working simultaneously to obtain patterned structure with submicron resolution and optical properties conferred by the nanoparticle constituents. The two-photon absorption process occurs at the localized surface plasmon resonance of Ag NPs. It is the strong two-photon absorption of the Ag NPs that allows this highly localized photochemistry to be initiated in subwavelength domains without addition of any photosensitizer.
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