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Direct measurement of circular dichroism by evanescent wave cavity ring-down spectroscopy using rationally designed plasmonic structures

Subject Area Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Analytical Chemistry
Term from 2019 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 423427290
 
Final Report Year 2023

Final Report Abstract

Circular dichroism (CD) is the difference in the absorption of light by the two enantiomers of a chiral species. The difference is quite small compared to the overall absorption (< 10^-5 of the absorption), rendering the sensitivity of CD low and limiting the application of Cd. This project intends to combine two enhancement strategies to enhance the sensitivity of CD. The first enhancement strategy is to enhance the near-field optical chirality (OC) provided by rationally plasmonic nanostructures. The second strategy is to enhance the sensitivity of absorption spectroscopy by evanescent-wave cavity ring-down spectroscopy (CRDS). The project thus aimed at designing a smart plasmonic substrate that enhances the near-field OC and introduces the plasmonic substrate into an EW-CRDS cavity in order to ultimately maximize the sensitivity of chiroptical analysis of chiral molecules based on CD. Following the plan, in the first half of the project, smart gold plasmonic grating nanostructures were successfully designed and fabricated. EW-CRDS cavity with a customized prism for ppolarized light to enter the prism at the Brewster angle was also successfully developed and tested. The construction of the optical setup was unfortunately delayed because the purchase of the required instrument was not possible within the first year of the project due to the outbreak of covid-19 in 2020. Experiments were also limited due to the covid-19 measures implemented in 2020 and 2021. Nevertheless, during the covid-19 pandemic, the focus of the project turned into the theoretical design of various plasmonic and dielectric substrates that can serve as effective interfaces for chiroptical sensing, either with EW-CRDS as originally planned or with optical trapping. It was found that the loss (absorption and scattering) of the gold plasmonic nanostructures is too high to be used in the EW-CRDS cavity. Therefore, plasmonic nanostructures were concluded to be not suitable for CD based on EW-CRDS. In the 2nd half of the project, the research team turned the direction to dielectric metasurfaces in order to replace the lossy plasmonic nanostructures. Theoretical design of various dielectric nanostructures was carried out and the team finally obtained an effective design of a dielectric metasurface, which is made of GaP and offers a high reflectivity of up to 99.999% at the operational wavelength (800 nm). The dielectric nanostructure is simple and provides finite non-zero optical chirality upon linearly polarized excitation. The idea and theoretical study on using dielectric metasurfaces in EW-CRDS for CD have been reported in a paper recently accepted for publication by Nanoscale. Experimental work, especially the fabrication of the designed dielectric metasurface, was not possible during the funding period mainly due to the restricted output of the fabrication facility during covid-19 pandemic. Nevertheless, recently collaborations with experts in nanofabrication and theoretical design of dielectric metasurfaces have been established. Currently, the fabrication of similar metasurfaces made of InGaP has been achieved. In the future, a nanofabrication route for GaP structures will also be developed. New dielectric nanostructures will be designed with the help of artificial intelligence (collaborate with experts at Purdue University). Once positive preliminary experimental results of dielectric metasurface-assisted EW-CRDS are obtained, a proposal will be prepared in order to get research funds to follow up on this research. Overall, the originally proposed plan has been partially followed. However, due to the high loss of plasmonic structures, the research has turned to dielectric metasurfaces, which show much more promising results. New dielectric metasurfaces have been designed and will be fabricated. In the future, the experimental implementation of dielectric metasurface-assisted EW-CRDS for CD will be carried out.

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