Nonlinear absorption, refraction, and optical limiting by a series of monoaxially chloro- and aryl-substituted indium(III) phthalocyanines are described. The absorption cross sections and temporal evolution of the low-lying excited states are also reported. A large nonlinear absorption that increased with wavelength between 500 and 590 nm was observed in each material. The nanosecond nonlinear absorption and the optical limiting are shown to be dominated by a strong excited state absorption from an orientationally averaged triplet state. We derive and experimentally confirm the relation between the molecular absorption cross sections and the fluence-dependent nonlinear absorption coefficients. The effective nonlinear refraction on the nanosecond time scale was reduced because the electronic contribution to the nonlinear refractive index was of the opposite sign from the thermal contribution. An optical limiter using the new material, p-(trifluoromethyl)phenylindium(III) tetra-tert-butylphthalocyanine [(t-Bu)4PcIn(p-TMP)], showed a much lower threshold for optical limiting and a much lower transmission at high fluences than previously reported indium phthalocyanine limiters. This improved optical limiting was due both to the larger nonlinear absorption coefficient and to the design of the limiter device. The optical properties of the In phthalocyanine moiety were found to be surprisingly robust to structural changes in the axial position.

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The ability to tune the resonant frequency in plasmonic nanostructures is fundamental to developing novel optical properties and ensuing materials. Recent theoretical insights have shown through the conductive concatenation of plasmonic nanoparticles that the effective depolarization factor of the nanostructure, and subsequent charge transfer plasmon (CTP) resonance, can be intricately controlled [Appl. Phys. Lett.2014105011107]. However, translating these charge transfer properties from proof-of-principle experiments to high-quality, macroscale quantities for material applications remains challenging. Here, we experimentally demonstrate by using an electrostatic-based molecular assembly approach how to controllably concatenate gold nanorods end-to-end into discrete dimers, preventing unwanted longer structures and forming a capacitively coupled plasmon (CCP) resonance along the long axis of the dimer. Irradiating these suspensions with femtosecond laser pulses at the CCP dimer resonance wavelength selectively welds only the CCP dimers together, bridging the nanorods with gold nanojunctions and producing large, high-quality yields of welded dimers. Macroscale (∼1012 dimers) absorbance measurements reveal a CTP resonance arising from these welded dimers with absorbance peak magnitudes as large as 0.5 and full-width-at-half-maximum of 274 nm. We show by controlling the aspect ratio of the welded dimers that the CTP absorbance wavelength can differ significantly (∼20%) from a single nanorod with a similar aspect ratio, demonstrating the ability to modulate the effective depolarization factor of the dimer structures and resulting CTP resonance. We also carried out three-dimensional finite element simulations showing less than a 5% shift in the CTP absorbance wavelength as a function of the contact point connecting the nanorods and relative orientation, in agreement with our experiments.

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