It has long been known that nonlinear refraction in solvents can depend on pulse width, and this along with experimental uncertainties has led to orders-of-magnitude disagreements in nonlinear refractive coefficients reported in the literature. To resolve this issue, we perform beam-deflection (BD) measurements of the rigorously defined nonlinear impulse response function for 24 commonly used solvents selected from various classes of molecules. Using this polarization-resolved BD, the bound-electronic and the three major nuclear contributions are separately measured by determining the magnitudes, symmetry, and temporal dynamics of each mechanism. This allows us to construct the response functions that we use to accurately establish self-consistent references for predicting and interpreting the outcomes of other experiments performed on these materials over the temporal range from 10 fs to 1 ns. The results also provide insight into relating solvent nonlinearities with their molecular structures and exploring the effects of the Lorentz-Lorenz local field. We find that nonconjugated molecules with small polarizability anisotropy exhibit negligible reorientational response, and hence the nonlinear refraction is almost independent of pulse width. Knowledge of the response functions also allows engineering the transient nonlinear refractive properties of solutions of organic dyes, for example, materials with effectively zero nonlinear refraction.

The two-photon absorption (2PA) properties are investigated for two series of organic, π-conjugated, fused-ring, quadrupolar A-π-D-π-A chromophores of the type originally developed as nonfullerene acceptors for organic photovoltaics. These molecules are found to exhibit large nondegenerate two-photon absorption (ND2PA) cross-sections (ca. 6−27 × 10 3 GM) in the near-infrared (NIR). In the first series, involving molecules of varying core size, ND2PA spectra and cross-sections characterized by femtosecond ND2PA spectroscopy in chloroform solutions reveal that increases in core size, and thus conjugation length, leads to substantially redshifted and enhanced 2PA. In a second series, variation of the strength of the terminal acceptor (A) with constant core size (seven rings, indacene-based) led to less dramatic variation in the 2PA properties. Among the two core types studied, compounds in which the donor has a thieno[3,2-b]thiophene center demonstrate larger 2PA cross-sections than their indacene-centered counterparts, due to the greater electron-richness of their cores amplifying intramolecular charge transfer. Excited-state absorption (ESA) contributions to nonlinear absorption measured by open-aperture Z-scans are deduced for some of the compounds by analyzing the spectral overlap between 2PA bands and NIR ESA transitions obtained by ND2PA and transient absorption measurements, respectively. ESA cross-sections extracted from transient absorption and irradiance-dependent open-aperture Z-scans are in reasonable agreement, and their moderate magnitudes (ca. 10 −21 m 2 ) suggest that, although ESA contributions are non-negligible, the effective response is predominantly instantaneous 2PA.

Multimode optical fibers have recently reemerged as a viable platform for addressing a number of long-standing issues associated with information bandwidth requirements and power-handling capabilities. As shown in recent studies, the complex nature of such heavily multimoded systems can be effectively exploited to observe altogether novel physical effects arising from spatiotemporal and intermodal linear and nonlinear processes. Here, we study for the first time, accelerated nonlinear intermodal interactions in core-diameter decreasing multimode fibers. We demonstrate that in the anomalous dispersion region, this spatiotemporal acceleration can lead to relatively blue-shifted multimode solitons and blue-drifting dispersive wave combs, while in the normal domain, to a notably flat and uniform supercontinuum, extending over 2.5 octaves. Our results pave the way towards a deeper understanding of the physics and complexity of nonlinear, heavily multimoded optical systems, and could lead to highly tunable optical sources with very high spectral densities.

Compounds with polarizable π systems that are susceptible to attack with nucleophiles at C-Hal (Hal = Cl, Br) bonds react with Pd(PPh3)4 to yield net oxidative addition. X-ray structures show that the resulting Pd(PPh3)2Hal groups greatly reduce intermolecular π-π interactions. The Pd-functionalized dyes generally exhibit solution-like absorption spectra in films, whereas their Hal analogues exhibit features attributable to aggregation.

Pd(PPh3)2Cl groups hinder aggregation and afford promising solid-state NLO properties.