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Organic Photonic Materials for All-Optical Signal Processing Irradiating a material with very intense pulses of light can actually modify the optical properties (i.e. refractive index, absorption) of that material. These modifications can, in turn, affect the pulses of light themselves (i.e. changing the phase, adjusting the amplitude, changing its direction). This ability to control light with light is a consequence of the third-order nonlinearity of the material. For some materials, this optical response can be quite fast (femtoseconds to picoseconds). Consequently, there is the potential to exploit these materials in all-optical signal processing applications (see below).
Accordingly, in our group we are investigating candidate organic materials that exhibit large third-order optical nonlinearities (i.e. large c(3)) with ultrafast temporal response (t < 10 ps). Any number of chemical structure variations can be used to optimize such organic systems: nature and length of conjugated bridges, electron affinity of terminal groups, and even intermolecular interactions. By determining the nonlinear optical (NLO) properties of molecules with known variations, we are able to establish appropriate pathways toward efficient third-order materials. This determination is done using a variety of NLO characterization techniques. A few important techniques we employ regularly are degenerate four-wave mixing, transient absorption, and Z-scan.
In addition to being able to determine the strength of c(3), these methods can also determine the constitution of the nonlinearity. For instance, the Rec(3) is associated with the nonlinear refractive index while the Imc(3) is associated with nonlinear absorption. This type of knowlege is critical in identifying potential applications for these materials. Listed below are a few selected organic systems we have investigated and the particular areas in which they could be utilized.
| | | Large Rec(3) --- all-optical switching
A bis-dioxaborine cyanine-proton sponge salt, was shown to exhibit large values of Rec(3) throughout the near infrared (i.e. -3.6´10-10 esu at 1.3 mm). This system also exhibited low nonlinear loss (small Imc(3)) and ultrafast response. The strongly delocalized p-electrons along the conjugated backbone are responsible for the large nonlinear response in such a small molecule . | 
| | | | Large |c(3)| --- optical correlation
Polyacetylene-based materials were synthesized by ring-opening metathesis polymerization of liquid phase precursors. This processing method allowed for facile processing and offers the possibility of in situ polymerization within a photonic device. These materials possess large |c(3)|, low transmission loss, and ultrafast response (< 100 fs) in the near infrared. | 
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Large Imc(3) --- dynamic noise suppression and optical limiting
An extended squaraine system consisting of a squarylium core and conjugated bridges consisting of vinylenes and electron-rich heterocycles showed strong nonlinear absorption (Imc(3) = 3.5´10-11 esu and a two-photon cross-section of 2100 GM, at 1.3 mm). The system exists as a neat liquid at room temperature suggesting facile processing of the material. | 
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