All-optical switching applications require materials with large third-order nonlinearities and low nonlinear optical losses. We present a design approach that involves enhancing the real part of the third-order polarizability (gamma) of cyanine-like molecules through incorporation of polarizable chalcogen atoms into terminal groups, while controlling the molecular length to obtain favorable one- and two-photon absorption resonances that lead to suitably low optical loss and appreciable dispersion enhancement of the real part of gamma. We implemented this strategy in a soluble bis(selenopyrylium) heptamethine dye that exhibits a real part of gamma that is exceptionally large throughout the wavelength range used for telecommunications, and an imaginary part of gamma, a measure of nonlinear loss, that is smaller by two orders of magnitude. This combination is critical in enabling low-power, high-contrast optical switching.

Novel alkene and alkyne branched structures have been synthesized, and their two-photon absorption (2PA) properties are reported. This series of alkene and alkyne trimer systems tests the mechanistic approach for enhancing the 2PA process which is usually dictated by the pi-bridging, delocalization length, and corresponding charge transfer on the 2PA cross sections. The results suggest that alkene branched systems have higher 2PA cross sections. While steady-state absorption and emission measurements were not successful in predicting the observed trend of 2PA cross sections, time-resolved measurements have explained the trends observed. It was found that, upon photoexcitation, there is an ultrafast charge localization to an intramolecular charge-transfer (ICT) state, followed by the presence of a solvent and conformationally relaxed ICT state in these branched systems.

Nanoscale features as small as 65 +/- 5 nm have been formed reproducibly by using 520 nm femtosecond pulsed excitation of a 4,4'-bis(di-n-butylamino)biphenyl chromophore to initiate crosslinking in a triacrylate blend. Dosimetry studies of the photoinduced polymerization were performed on chromophores with sizable two-photon absorption cross-sections at 520 and 730 nm. These studies show that sub-diffraction limited line widths are obtained in both cases with the lines written at 520 nm being smaller. Three-dimensional multiphoton lithography at 520 nm has been used to fabricate polymeric woodpile photonic crystal structures that show stop bands in the near-infrared spectral region.

Various single-standed DNA-encapsulated Ag nanoclusters (nanodots) exhibit strong, discrete fluorescence with solvent polarity-dependent absorption and emission throughout the visible and near-IR. All species examined, regardless of their excitation and emission energies, show similar µs single-molecule blinking dynamics and near IR transient absorptions. The polarity dependence, µsec blinking, and indistinguishable µsec-decaying transient absorption spectra for multiple nanodots suggest a common charge transfer-based mechanism that gives rise to nanodot fluorescence intermittency. Photoinduced charge transfer that is common to all nanodot emitters is proposed to occur from the Ag cluster into the nearby DNA bases to yield a long-lived charge-separated trap state that results in blinking on the single molecule level.

Extended bis(donor)-substituted squaraine chromophores exhibit very high two-photon cross-sections (as high as 33 000 GM) in the near-IR; these can be attributed to the combination of large transition dipoles with small detuning energies. The modulus of the third-order nonlinear optical susceptibility at 1.3 mum has been found to be 7.0 x 10-11 esu for one of these chromophores.

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Organic materials with large third-order nonlinearities in the near-infrared spectral regime are critical in the development of photonic devices to be utilized in all-optical signal processing. We have developed polymethine materials, specifically bisdioxaborine-terminated polymethine dyes, which possess large ultrafast third-order nonlinearities and low nonlinear loss all in the near-infrared spectral regime. An extended bisdioxaborine polymethine anion exhibited the largest value of gamma (third-order microscopic nonlinearity) at 1.3 mum (|gamma| = 5.7 x 10-32 esu) and showed no characteristics of symmetry breaking, unlike other polymethines of similar lengths. A neat film of this molecule maintained relatively low linear loss in the near-infrared and showed a large third-order macroscopic nonlinearity at 1.3 mum (|chi(3)| = 3.6 x 10-10 esu), with a temporal response of less than 8 ps. Furthermore, the real part of chi(3) was nearly an order-of-magnitude larger than the imaginary component. Consequently, this material exhibited good figures of merit for all-optical signal processing throughout the entire telecommunications band.

Organic materials possess many key attributes that make them suitable for exploitation in all-optical signal processing applications including facile tunability of their optical properties, strong and ultrafast nonlinear optical response, and potential for integration into device structures. In this perspective, we present molecular design guidelines for organic chromophores that could serve as the active constituents for such materials. Using a relatively simple model, a candidate class of chromophores, namely cyanine-like polymethines, is identified based on promising microscopic nonlinear optical properties in the near-IR spectral region. The challenges associated with translating these microscopic properties into materials with macroscopic properties suitable for device applications are presented along with molecular engineering approaches for overcoming these hurdles.