研究目的

The understanding of light polarization from materials absorption, emission, and reflection can be addressed by matching one or more polarizers in the optical path of conventional photoluminescence and UV-vis spectrometers, called polarized photoluminescence and polarized absorbance, respectively. These methodologies have been applied with the aim of acquiring information on the molecular arrangement (anisotropy and order factor). Besides that, Circular Dichroism spectroscopy has also been widely used to understand molecular organization based on their chirality. Nevertheless, these techniques fail to identify all possible polarization states when applied separately. In the case of photoluminescence, for example, it is possible to get information from linear polarization states, but not from circular ones. The polarized absorption, on the other hand, can elucidate the molecular order, but does not provide information on circular birefringence effects and Circular Dichroism. The last one consists in the difference between left and right circularly polarized light, thus resulting in the molecular asymmetry. For instance, the study of energy transfer processes by means of the investigation of emitted polarized light can be very complicated when the polarized state is not completely described. In this sense, Emission Ellipsometry (EE) is a technique that, combined with the Stokes theory, allows the complete description of the emitted light polarization states being, in this manner, an excellent alternative in the study of energy transfer processes, mainly in polymeric materials. In addition, it is possible to obtain full information regarding molecular organization, anisotropy, and asymmetry factors in photoluminescent materials. All these studies and several applications in diverse materials lead to the development of a new method for Raman Optical Activity studies, named ellipsometric Raman spectroscopy (ERS). In this new methodology, Basilio et al. have been shown that EE combined with the Stokes theory can also be applied, not only for emitted but also for the scattered light. From this experiment, one can obtain meaningful information about materials optical activity even for easily photodegradable ones (both biological and polymeric) due to the limited exposition to exciting light. This chapter aims a detailed review of Achromatic Ellipsometry from the appliance of Stokes theory on the determination of polarization parameters to its applications on materials studies.

研究成果

In this work, we illustrated a new methodology for measuring Raman Optical Activity and the application of achromatic Ellipsometry in the study of benzotiadiazole derivative dye doped in the E7 nematic liquid crystal. Emission and transmittance Ellipsometry experiments were used to verify its molecular ordering within a liquid crystal cell. The obtained results showed that chromophores were ordered in the same direction of the E7 molecules. In addition, due to the change in the excitation wavelength, penetration depth in the sample could be verified, enabling the analysis of molecules ordering within a liquid crystal cell. Ellipsometry experiments also indicate that the molecular orientation inside the cells somehow changes the polarization of transmitted light. This effect can be due to system birefringence (both liquid crystal and dye) or due to combined process between dyes optical absorption and system birefringence. It is important to emphasize that changes in light polarization occur differently for one process (birefringence) or two processes simultaneously (birefringence and optical absorption). Finally, achromatic Ellipsometry was demonstrated to be a complete spectroscopic technique that enables the analysis of bulk and interfacial molecular ordering.

研究不足

The techniques fail to identify all possible polarization states when applied separately. In the case of photoluminescence, for example, it is possible to get information from linear polarization states, but not from circular ones. The polarized absorption, on the other hand, can elucidate the molecular order, but does not provide information on circular birefringence effects and Circular Dichroism.