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.

† Electronic supplementary information (ESI) available: Materials and methods, including characterising data for new compounds, details of linear and nonlinear optical characterisation, and theoretical methodology; additional linear spectra, including those for neat chromophore lms; and representative Z-scan spectra. See

Considerable effort has been devoted to enabling all-optical signal processing (AOSP) for future communications systems with signifi cantly increased operational bandwidth. [ 1 ] Suitable materials for AOSP must exhibit ultrafast third-order nonlinear optical (NLO) response, as well as low linear and nonlinear optical loss, to achieve very high speed optical switching or modulation (100 Gbit/s or beyond). [ 1b ] While silicon is an attractive candidate material for AOSP due to its ease of chipscale photonic integration, its third-order NLO response is accompanied by parasitic non-linear absorption losses and longlived free carrier effects. [ 1c , 2 ] Recently, several groups have demonstrated silicon-organic hybrid (SOH) devices for AOSP with bandwidths exceeding 100 Gbit/s by integrating organic materials with silicon slot waveguides, [ 3 , 4 ] effectively combining the large non-linearities of organic materials with the extraordinary modal fi eld concentrations of silicon waveguides. Nonetheless, development of organic materials suitable for device integration which possess large non-linearities and small optical losses in the telecommunications region remains a challenge.Polymethine dyes [ 5 ] are an intriguing class of conjugated organic molecules for AOSP applications: the π -electrons along the conjugated backbone can be polarized easily, resulting in a large negative third-order molecular polarizability ( γ ) [ 6 ] and the electronic absorption bands are usually very narrow with sharp low energy band edges, which can be useful in reducing linear absorption losses at the desired operational wavelengths. Furthermore, polymethine dyes terminated with selenopyrylium end groups were recently found to exhibit large magnitudes of Re( γ ) at telecommunications wavelengths while also suppressing non-linear absorption such that excellent two-photon fi gures-of-merit (FOMs) (i.e., | Re( γ )/Im( γ ) | ) were achieved. [ 7 ] If the molecular third-order non-linearities of polymethine chromophores can be translated into macroscopic nonlinearities (e.g., χ (3) ) in high-number-density chromophoric materials, such materials could enable high-performance AOSP devices. However, there are several major challenges that need to be overcome in order to effectively translate the large γ into a large χ (3) . At high molecular number densities, the intrinsic optical properties of polymethine dyes can be deleteriously affected by intermolecular interactions resulting in, for example, aggregation [ 8 ] and ion-pairing effects, [ 9 ] both of which can lead to an increase in absorptive optical loss (linear and non-linear) at AOSP wavelengths. To mitigate the potential absorptive loss from these interactions, we have employed polymer guest-host approach to develop processable fi lms with high chromophore number density. However, with this approach, phase separation of polymethine salts in a host polymer can also lead to undesirable scattering and optical loss.Here, we report on the development of a miscible polymethine salt, polyme...

The aggregation of chalcogenopyrylium-terminated heptamethine dyes in high-chromophore density films can be substantially suppressed by attachment of rigid, bulky substituents that project out of the plane of the conjugated π-system of the dye, allowing the retention in the solid state of favorable linear and nonlinear optical properties relevant to all-optical signal processing (AOSP). Dye structures are systematically engineered with a range of out-of-plane substituents to investigate the impacts on the optical properties at 1550 nm in the solid state. Large magnitudes of the third-order susceptibility are obtained for solid films. Very high values of the two-photon figure-of-merit (2PA-FOM), an important parameter for AOSP, are observed in dilute solutions of seleno-and telluropyrylium dyes (in several cases >30). In the case of the telluropyrylium dye, the 2PA-FOM is not measureable for 50 wt% blends with polymers. Blends of the selenopyrylium examples exhibit 2PA-FOM values that, although decreased substantially relative to the solution values, in some cases meet the requirements for AOSP.

Rh(III) and Ir(III) can be complexed to bipyridine groups attached to the termini of bis(dioxaborine)-capped heptamethines; these chromophores exhibit large third-order polarisabilities at 1.55 μm, while retaining good film-forming properties and linear optical transparency in the near infrared.