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.

Measuring optical activity with the internal reflection in a glass prism.

A novel internal reflection-based polarimeter and measurement method to characterize optically active substances is presented. The working principle relies on the phase difference acquired by s- and p-polarized light undergoing internal reflection: they interfere after passing through an analyzer and produce a minimum near the critical angle, whose position depends on the rotation angle imposed by the optically active sample. Since the location of the minimum does not depend on the laser power, the measurement is nearly insensitive to any power fluctuation. Furthermore, this low-cost device is rugged, very compact and stable, and practically immune to mechanical vibrations. It was used to measure concentrations of sucrose and fructose dissolved in distilled water and presented a resolution better than 0.04°.

Simultaneous measurement of thermo-optic and thermal expansion coefficients with a single arm double interferometer.

A low-cost single arm double interferometer was developed for the concurrent measurement of linear thermal expansion (α) and thermo-optic (dn/dT) coefficients of transparent samples with plane and parallel surfaces. Owing to its common-path optical arrangement, the device is compact and stable, and allows the simultaneous measurement of interferences arising from a low-finesse Fabry-Perot etalon and from a Mach-Zehnder-type interferometer. The method was demonstrated with measurements of solid (silica, BK7, SF6) and liquid (water, ethanol and acetone) samples.