Eric W. Van Stryland

Investigation of an optical limiting mechanism in multiwalled carbon nanotubes

We report our investigation of the mechanism that is responsible for the optical limiting behavior in multiwalled carbon nanotubes. We conducted energy-dependent transmission measurements, picosecond time-resolved pump–probe experiment, and nonlinear scattering experiments at 532-nm wavelength on multiwalled carbon nanotube suspension. For comparison, C60–toluene solutions and carbon black suspensions were also studied in the same experiments. The similarities that we observed between the multiwalled carbon nanotubes and carbon black suspension suggest that nonlinear scattering, which is known to be responsible for the limiting action in carbon black suspension, should play an important role in the limiting effect in multiwalled carbon nanotubes.

Energy Band-Gap Dependence of Three-Photon Absorption in Semiconductors

The bandgap and wavelength scaling of three-photon absorption (3PA) is studied in several semiconductors by the Z-scan technique. The 3PA coefficient is found to vary as Eg-7as predicted by theory.

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.

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.

Comparison of Linear and Nonlinear Absorption in Three Series of Similar Cyanine Dyes: D-?-D, A-?-A and D-?-A

We report the linear and nonlinear spectra of three series of cyanine dyes (donor-?-acceptor, acceptor-?-acceptor, and donor-?-donor) including effects of conjugation length and terminal groups. Two-photon-absorption up to 16,000 GM is observed in acceptor-?-acceptor structures.

Nonlinear Absorption Spectroscopy of a Bis(Porphyrin)-Substituted Squaraine

The nonlinear absorption mechanisms of a bis(porphyrin)-substituted squaraine have been studied with femtosecond, picosecond, and nanosecond pulsewidths. The two-photon absorption is ~10× larger than those of the constituents and is explained by intra-molecular charge transfer.

Temporal and Spectral Nonlinear Absorption Characterization of a Hybrid Porphyrin-Squaraine-Porphyrin Macromolecule

The nonlinear absorption mechanisms of a porphyrin-squaraine-porphyrin macromolecule have been studied with femto/pico/nanosecond pulsewidths. Two-photon absorption of the macromolecule is ~10× larger than the constituents and is explained by intra-molecular charge transfer.