Charles Figura

Thickness dependence of second-harmonic generation in thin films fabricated from ionically self-assembled monolayers

An ionically self-assembled monolayer (ISAM) technique for thin-film deposition has been employed to fabricate materials possessing the noncentrosymmetry that is requisite for a second-order, χ(2), nonlinear optical response. As a result of the ionic attraction between successive layers, the ISAM χ(2) films self-assemble into a noncentrosymmetric structure that has exhibited no measurable decay of χ(2) at room temperature over a period of more than one year. The second-harmonic intensity of the films exhibits the expected quadratic dependence on film thickness up to at least 100 bilayers, corresponding to a film thickness of 120 nm. The polarization dependence of the second-harmonic generation yields a value of 35° for the average tilt angle of the nonlinear optical chromophores away from the surface normal.

Enhanced Nonlinear Optical Response Of An Endohedral Metallofullerene Through Metal-to-cage Charge Transfer

A new mechanism for increasing the third-order nonlinear optical susceptibility, χ(3), is described for endohedral metallofullerenes. A two to three orders of magnitude increase in the nonlinear response is reported for degenerate four-wave mixing experiments conducted with solutions of Er2@C82 (isomer III) relative to empty-cage fullerenes. A value of −8.7×10−32 esu is found for the molecular susceptibility, γxyyx, of Er2@C82 compared to previously reported values of γxxxx=3×10−34 esu and γxyyx=4×10−35 esu for C60. The results confirm the importance of the metal-to-cage charge-transfer mechanism for enhancing the nonlinear optical response in endohedral metallofullerenes.

Second-order nonlinear optical thin films fabricated from ionically self-assembled monolayers

We demonstrate, for the first time, that a new ionically self-assembled monolayer (ISAM) technique for thin film deposition can be employed to fabricate materials possessing the noncentrosymmetry that is requisite for a second order, nonlinear optical response. Using two different commercially-available polyelectrolytes, we have produced ISAM nonlinear optical thin films with values comparable to that of quartz. As a result of the ionic attraction between successive layers, the ISAM films self-assemble into a noncentrosymmetric structure that has exhibited no measureable decay of at room temperature over a period of more than four months. The x2 of ISAM films has been examined by second harmonic generation using a fundamental wavelength of 1200 nm. The second harmonic intensity of the films exhibits the expected quadratic dependences on fundamental intensity and film thickness while the polarization dependence is consistent with orientation of the chromophore dipole moment perpendicular to the substrate.

Thermal stability and immersion solution dependence of second-order nonlinear optical ionically self-assembled films

Polymeric films fabricated from ionically self-assembled monolayers (ISAMs) spontaneously from in a noncentrosymmetric structure requisite for a nonzero second order nonlinear optical (NLO) susceptibility, (chi) (2), without the need for electric field poling. ISAM NLO films exhibit excellent long-term temporal stability of (chi) (2), having shown no decay over a period of nearly three years. They are also remarkably stable at elevated temperatures. While (chi) (2) decreases by 20 percent as the temperature is raised to 150 degrees C, total recovery of the susceptibility is observed upon cooling, demonstrating that the decrease is not due to an irreversible randomization of the chromophore alignment. The thickness, orientational order, and NLO response are found to be strongly dependent on the pH and ionic strength of the solutions form which the films are deposited. The largest (chi) (2) values are observed in films with the smallest bilayer thickness. This suggests that polar orientation is obtained primarily at the interfaces between adjacent layers rather than throughout a full monolayer.

Optical limiting of C60, C60 charge-transfer complexes, and higher fullerenes from 532 to 750 nm

The wavelength-dependence of optical limiting in the reverse saturable absorber C60 has been studied in detail over a large spectral range and compared to that of two phthalocyanines. The reverse saturable absorption of C60 is found to increase at wavelengths longer than the standardly measured 532 nm. The optical limiting data have been analyzed in terms of the dynamical population rate equations to obtain the excited state absorption cross-sections. The computational effort of the analysis is greatly reduced by a semi-analytic solution to the rate equations that we have developed. We have also demonstrated that the spectral window for optical limiting of fullerenes can be increased to include longer wavelengths at which C60 is transparent. Both a C60 charge-transfer complex and C76 have been shown to exhibit strong reverse saturable absorption at long wavelengths.

Optical limiting over an extended spectral region by derivatization of C60

The optical limiting performance of C60 is shown to be extended to longer wavelengths by derivatization of the fullerene. While the reverse saturable absorption behavior of C60 generally improves at wavelengths longer than 532 nm due to decreased ground state absorption and increased excited state absorption, the peak of the triplet-triplet excited state absorption at 750 nm is inaccessible as a result of the near complete transparency of C60 beyond 650 nm. Partial breaking of the spherical it-conjugation by attached substituents leads to increased ground state absorption cross-sections in the appropriate wavelength range. The peak of the triplet-triplet absorption is shifted to 700 nm in the C60 derivatives, resulting in exceptionally strong optical limiting in this spectral region.

Degenerate four-wave mixing in endohedral metallofullerenes and optical limiting of C60 derivatives and higher fullerenes

We report two separate studies of the nonlinear optical properties of unique fullerene materials. Degenerate four- wave mixing measurements of the endohedral metallofullerene Er2 at C82 at 1064 nm show that the third order susceptibility is increased by more than two orders of magnitude relative to the empty cage fullerenes as a result of the metal-to-cage charge transfer. Optical limiting studies of C60 derivatives and higher fullerenes in the 680 to 880 nm spectral region demonstrate that these materials are strong reverse saturable absorbers at wavelengths where C60 itself is transparent. The tremendous variety of available fullerene-derived structures provides important opportunities for optimization of nonlinear optical responses.

Second Order Nonlinear Optical Thin Films Fabricated from Electric Field-Assisted Electrostatic Self-Assembled Monolayer Method

A new method for the build-up of non-centrosymmetric multilayer thin films has been developed for the first time using an electric field-assisted electrostatic self-assembled monolayer (EF-ESAM) technique. An increase by 116% of the second-harmonic intensity of the films has been observed in comparison with that of ESAM film.