Sepehr Benis

Facile Incorporation of Pd(PPh3)2Hal Substituents into Polymethines, Merocyanines, and Perylene Diimides as a Means of Suppressing Intermolecular Interactions

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.

Effects of meso-M(PPh3)2Cl (M = Pd, Ni) substituents on the linear and third-order nonlinear optical properties of chalcogenopyrylium-terminated heptamethines in solution and solid states

Aggregation of cyanine-like dyes can significantly affect their optical properties. Here we report the effects of bulky meso-M(PPh3)2Cl (M = Pd, Ni) substitution on the molecular and solid-state optical characteristics of chalcogenopyrylium-terminated heptamethines. Metallated dyes were synthesised by reaction of the meso-chloro dyes with Pd(PPh3)4 or Ni(PPh3)4 at room temperature. The two PPh3 ligands are trans and the plane formed by the metal atom and its ligands is approximately orthogonal to that of the polymethine π-system. Replacement of Cl by M(PPh3)2Cl leads to a large blue shift of the solution absorption maximum and a decrease in the associated transition dipole moment, these effects being slightly more pronounced for Ni than for Pd. DFT calculations and electrochemical data suggest the blue shifts can largely be attributed to destabilisation of the LUMO by the more strongly π-donating M(PPh3)2Cl groups. The magnitude of the real part of the molecular third-order polarisability, Re(γ), decreases in the order Cl > Pd(PPh3)2Cl ≫ Ni(PPh3)2Cl. Within the framework of the sum-over-states expression for Re(γ), the difference between Cl and Pd(PPh3)2Cl examples can be rationalised considering the effects of the S0 → S1 transition energy and transition dipole moment on the two-state term associated with S0 → S1. On the other hand, the magnitude of Re(γ) for a Ni(PPh3)2Cl dye is anomalously low; SAC-CI/HF/cc-pVDZ excited-state calculations reveal this is due to a two-photon-allowed S2 state at unusually low energy for a cyanine-like dye, leading to a large positive three-state contribution to γ opposing the negative two-state S1 term. Thus, despite a cyanine-like molecular structure and linear absorption spectrum, this compound does not exhibit cyanine-like nonlinear optical behavior. Turning to the effects on aggregation, molecular dynamics simulations suggest that Pd(PPh3)2Cl substitution largely suppresses H- and J-aggregate formation; indeed experimental absorption spectra for neat films of Pd(PPh3)2Cl-substituted dyes are fairly similar to corresponding solution spectra. A 50 wt% blend of a Pd(PPh3)2Cl-substituted telluropyrylium-terminated dye with amorphous polycarbonate exhibits a third-order susceptibility of −3 × 10−11 esu, a two-photon figure-of-merit in excess of 10, and linear loss of 6.3 dB cm−1, which are close to the requirements for all-optical switching applications.

Measurement of the ultrafast dynamics of nonlinear refraction and absorption of highly doped semiconductors at epsilon-near-zero (Conference Presentation)

Materials exhibiting near zero refractive index are shown to have interesting nonlinear optical properties such as enhanced second and third harmonic generation, and large nonlinear refraction (NLR) due to their unique interplay between linear and nonlinear optical features. In particular, the NLR of highly doped semiconductors such as Indium Tin Oxide and Aluminum doped Zinc Oxide is enhanced in the near-infrared spectral regions, where the real part of the permittivity crosses zero with the advantage of having a tunable zero crossover frequency by controlling the doping level. This is also known as the epsilon near zero (ENZ) regime, where the refractive index is very small. We have used the Beam-Deflection (BD) method to directly characterize the temporal dynamics and polarization dependence of the nondegenerate (ND) NLR of doped semiconductors at ENZ. The origin of the nonlinear optical response of these materials is different than for the case of bound electronic nonlinearities which depend upon the third-order susceptibility. The ND BD technique has the potential to study the dependence on relative polarization of excitation and probe waves to accurately determine the instantaneous electronic nonlinearities separately from the non-instantaneous mechanisms such as carrier redistribution effects, however, the carrier nonlinearities are dominant in such materials. This method also reveals the effect of tuning the wavelength of excitation or probe waves through ENZ separately. BD has sensitivity to induced optical path length as small as 1/20,000 of a wavelength, which enables the possibility to resolve NLR in the presence of large nonlinear absorption backgrounds.

Ultrafast dynamics and spectral dependence of optical nonlinearities in doped semiconductors at epsilon-near-zero (Conference Presentation)

Materials where the real part of the permittivity is near zero are known to have interesting nonlinear optical properties such as enhanced harmonic generation and large nonlinear refraction (NLR). In particular, the NLR of highly doped semiconductors such as Indium Tin Oxide and Aluminum doped Zinc Oxide is enhanced in the near-infrared spectral regions, where the real part of the permittivity crosses zero, the precise wavelength of which can be tuned by controlling the doping level.. This is also known as the epsilon near zero (ENZ) regime, although the imaginary part of the permittivity is not necessarily small at this wavelength. In order to characterize these nonlinearities, we use the Beam-Deflection (BD) method to directly characterize the temporal dynamics and polarization dependence of the nondegenerate NLR and nonlinear absorption of doped semiconductors at ENZ. BD has sensitivity to induced optical path length as small as 1/20,000 of a wavelength, which enables us to resolve NLR in the presence of large nonlinear absorption backgrounds. The BD technique also allows separation of instantaneous bound electronic nonlinearities from non-instantaneous mechanisms such as the carrier redistribution effects that dominate in ENZ materials,. We can also study the dependence on relative polarization and incidence angle of excitation and probe waves. Our method also reveals the effect of tuning the wavelength of excitation or probe waves through ENZ separately and we find that that the strong wavelength dependence of nonlinearities around the ENZ point is quite different for pump and probe waves.

Cross-propagating beam-deflection measurements of third-order nonlinear optical susceptibility

We extend our recently developed Beam-Deflection (BD) method which we used to determine the sign, magnitude, and ultrafast dynamics of bound electronic and nuclear nonlinear optical responses, to measure elements of the third-order nonlinear optical susceptibility tensor with longitudinal field components. In these measurements, in contrast to the conventional BD technique where excitation and probe beams are nearly collinear, the interaction geometry involves orthogonal propagation of excitation and probe beams. This cross-propagating BD method enables probing with the electric field polarized parallel to the wavevector of the excitation beam as well as conventional parallel and perpendicular polarizations as in most other experimental methods. This technique may be of use in detecting possible magneto-electric contributions to the nonlinear susceptibility. The ratio between parallel and perpendicular bound-electronic responses in this method is shown to be larger than the factor of 3 predicted by theory and measured in the conventional configuration.

Nondegenerate, Transient Nonlinear Refraction of Indium Tin Oxide Excited at Epsilon-Near-Zero

We measure nondegenerate nonlinear refraction of ITO as we tune the excitation wavelength through Epsilon-Near-Zero (ENZ) for a fixed probing wavelength. Unlike the degenerate case, there is no observed peak in the nonlinearity at ENZ.

Measurement of the dynamics of nonlinear refraction and absorption via nonlinear beam deflection (Conference Presentation)

Many materials exhibit nonlinear refraction (NLR) and absorption (NLA) that has multifaceted temporal dynamics. As a result, measurements at one laser pulse width may not be fully predictive of the behavior at other pulse widths. We have recently developed a method, Nonlinear Beam Deflection, (BD) that allows sensitive time-resolved measurement of nonlinear refraction (NLR) and absorption (NLA) by using an excitation beam to create an index gradient deflecting a probe beam onto a quad-cell detector. The method has a demonstrated sensitivity to induced phase changes as small as 1/20,000 of a wavelength, which is sensitive enough to measure NLR in gases. By changing the relative polarization of the beams we can separate the bound-electronic response from the slower and different-symmetry nuclear contributions. In gases and liquids where reorientational nonlinearities are important, measurements at the magic angle allow isolation of the ultrafast nonlinearities. In isotropic solids the bound-electronic symmetry dictates a ratio of 1/3 for parallel to perpendicular polarizations which measurements confirm. This method also allows for measurements of nonlinearities using very different wavelengths for the excitation and probe. We have used this method to characterize the impulse response function for third-order nonlinearities in many transparent organic solvents. This allows accurate prediction of the nonlinear refraction for any pulse width longer than that used for the BD characterization. The method proves to be very useful in organic materials that may show strong nonlinear absorption, as it is able to resolve NLR in the presence of strong NLA better than other methods, such as Z-scan.