Nanooptik

The first part of this project addressed a fundamental gap in interpreting vibrational circular dichroism (VCD) spectra by bridging the Bouguer–Beer–Lambert approximation with quan-tum mechanical methods for determining absolute molecular configuration. A wave-optics-based approach was developed to extract both the dielectric function and the chirality admit-tance function from infrared and VCD spectra. VCD is defined as the difference in negative decadic transmittance between left- and right-circularly polarized light, preserving linear de-pendence on optical path length even within wave optics. For classical interpretation, Max Born’s oscillator model—featuring magnetically coupled, perpendicularly arranged damped oscillators—was applied. Using this model, infrared and VCD spectra could be simultaneous-ly fitted, enabling the extraction of oscillator strengths, damping constants, transition frequen-cies, coupling constants, and intermolecular distances. The resulting methodology extends dispersion analysis to chiral materials and enhances our understanding of their optical behav-ior. The second part of the project aims to develop an achiral substrate, such as elliptical na-nopores in a gold film, that can be used to measure the chiral response, mainly VCD of chiral molecules under the illumination of linearly polarized light. This structure should generate chiral optical near fields locally in the proximity of the nanostructures under linear polarized excitation. This avoids most of the noises from chiral light and chiral structures and also would enable circular dichroism measurement on an optical microscope, which is not friendly for circularly polarized light. We have studied two different structures. The first structure is the originally proposed elliptical nanopores on a gold film. We optimized and characterized the chiral optical field generated in the nanohole and measured the transmission spectrum of R6G dye molecules and found clear Raman signals. We have also explored the possibility of using the elliptical nanopores for enantioselective optical trapping. We theoretically showed that polymer nanobeads coated with densely packed chiral surfactants can be trapped or re-pelled by the elliptical nanoholes according to the handedness of the near field relative to the handedness of the surfactants, making the elliptical nanohole illuminated by linearly polarized light an enantioselective optical trap. This part has been published. In the second structure, we show a single achiral dielectric disc array covered with diagonally aligned stripes can be used as a simple scheme for circular dichroism with linearly polarized light. This approach is particularly useful for using cavity-ring down spectroscopy for circular dichroism measure-ment because in cavity-ring down spectroscopy, circularly polarized light cannot survive. Therefore, our approach of using well-designed dielectric nanostructures to locally covert linearly polarized light with extremely low scattering, low absorption and low depolarization becomes very attractive. This work was published.