Frequency-resolved microscopic second-order hyperpolarizability of azochromophores: a study on nonlinear all-optical switching applications.

Azochromophores present interesting optical properties for application in all-optical switches (AOSs), such as ultrafast photoisomerization and considerable nonlinear optical response. However, determining the frequency-resolved microscopic second-order hyperpolarizability (real and imaginary parts) related to the pure electronic effects of molecules in solution is a challenging task. In this context, we have used femtosecond-laser induced nonlinear ellipse rotation (NER) measurements to obtain the electronic nonlinear refraction (n2(ω)) and two-photon absorption spectra (α2PA(ω)) of four azochromophores dissolved in methanol and acetone. The measurements ranging from ∼600 up to ∼1300 nm were performed in Disperse Red 1 (DR1), Disperse Red 13 (DR13), Disperse Red 19 (DR19), and Disperse Orange 3 (DO3). Because we carried the solution in a silica cuvette and used a short focal length, we were able to measure the solution's nonlinearities with high precision, as the silica from the cuvette walls worked as a suitable reference medium. Consequently, we precisely determined n2(ω), α2PA(ω), and the second-order hyperpolarizability (γ(ω)) for all molecules and explained the different magnitudes based on the push-pull character. Furthermore, the solvation effect due to the change from methanol to acetone solvent on the n2(ω), α2PA(ω), and γ(ω) is also reported. The results were elucidated using the sum-over-states (SOS) approach within the few-energy-level model and the results were obtained via quantum-chemical calculations using the cubic response function formalism within the density functional theory framework. Finally, we used these results to determine the frequency-resolved figure-of-merit for all-optical switching applications. Our results suggest that chromophores have the potential for applications in AOS based on Fabry-Perot filters.

Absolute Nonlinear Refractive Index Spectra Determination of Organic Molecules in Solutions.

It has been a great challenge to measure the spectrum of pure bound-electronic third-order nonlinear refraction ( n2) of organic chromophores in solutions because of the spurious contribution from the solvent and cuvette walls. In order to circumvent this problem, we present here a new method to obtain a highly accurate absolute n2 value of organic molecules in solutions with a self-referenced nonlinear ellipse rotation (NER) technique. As a proof of concept, we measured n2 spectra of two well-known chromophores, rhodamines B and 6G dissolved in methanol, in the range from ∼600 to 1200 nm. Our results pointed out that these two dyes present similar dispersion curves with strong negative nonlinearities near the one-photon absorption band and small positive values at long wavelengths. Furthermore, the negative signal of the dyes can be strong enough to cancel and even invert the positive nonlinear refraction of the solvent (methanol) as the solution's concentration increases. To understand the n2 spectrum and its connection to molecular properties of organic chromophores, we employed the sum-over-states (SOS) approach within the few-energy-level model and observed an excellent agreement between the experimental and theoretical spectra. In this way, we believe that, employing our NER technique and the SOS model, it is possible to determine both experimentally and theoretically the absolute magnitude and spectra of pure electronic n2 for a large variety of other organic molecules.