Periodic Reporting for period 3 - CHANSON (Chiral semiconductor nanophotonics for ultraresolved molecular sensing)
Berichtszeitraum: 2023-12-01 bis 2025-05-31
Just like our hands, chiral objects exist in two mirror forms that cannot be superimposed. The two chiral versions of an object are called enantiomers. Nature chose mostly chiral building blocks for biology using left-handed amino acids and right-handed sugars only. This fact trickles down to everyday chemicals in fragrances, flavors, or drugs. Detecting chirality in matter is thus of profound scientific and technological interest. The optical spectroscopy technique called circular dichroism (CD) can reveal chirality through a small difference in absorption for both circular polarizations of light, typically 3-5 orders of magnitude smaller than the measured light intensity. The application of CD spectroscopy is limited, however, by three challenges: 1) vanishing absorption for low molecular concentrations; 2) very small polarization contrast; 3) lack of spatial resolution.
This project tackles the challenge of increasing the sensitivity of chiral molecular detection using semiconductor nanophotonics through new contrast mechanisms relying on fluorescence instead of absorption. The project exploits two approaches based on nanophotonic and excitonic sensors around the following challenges:
A) Detecting the lowest possible concentration of chiral molecules, ultimately targeting the limit of a single molecule.
B) Exploiting excitons in atomically thin semiconductors as sensors for chiral sensing and nanoscale imaging.
This project tackles the challenge of increasing the sensitivity of chiral molecular detection using semiconductor nanophotonics through new contrast mechanisms relying on fluorescence instead of absorption. The project exploits two approaches based on nanophotonic and excitonic sensors around the following challenges:
A) Detecting the lowest possible concentration of chiral molecules, ultimately targeting the limit of a single molecule.
B) Exploiting excitons in atomically thin semiconductors as sensors for chiral sensing and nanoscale imaging.
Up to the end of the third period of the project, we have made progress on both sensing approaches. We have tailored nanophotonic structures for chiral molecular sensing at low molecular concentrations. We established the theory and carried out simulations to enhance fluorescence-based chiral sensing, providing us with the physical understanding to carefully carry out experiments. We have designed silicon metasurfaces for chiral sensing based on circularly polarized luminescence. Our realistic structures are capable of enhancing the fluorescence signal while enhancing the polarization contrast. We also proposed dual nanoresonators comprising hybrid structures for maximizing optical chirality in chiral hotspots. We exploited the strong magnetic field of silicon nanostructures together with the strong electric field created by a gold nanostructure. We have investigated how a chiral analyte modifies the response of a nanophotonic structure. This phenomenon is called chirality transfer because it imprints a chiral response on an otherwise achiral optical resonance in the nanophotonic structures.
We have developed nanofabrication recipes to produce monocrystalline silicon metasurfaces designed based on our theoretical predictions. We have characterized circular polarization in the near field of the nanostructures using polarized Raman spectroscopy. We have also constructed a polarization-resolved fluorescence microscope that allows us to do fine measurements of the degree of circular polarization of the emitted light. Finally, we have measured enhanced circularly polarized luminescence by chiral molecules using our silicon metasurfaces.
Regarding exciton-based nanoscale sensors, we observed localized fluorescence fluctuations in a monolayer semiconductor due to interaction with its environment. We adapted a fluctuation-based super-resolution technique to image localized exciton fluctuations in monolayer semiconductors and map disorder on the monolayer. We correlated the fluorescence fluctuations with features measured by atomic force microscopy and compared them with hyperspectral imaging data. We have combined monolayer semiconductors and chiral molecules towards the measurement of chirality through the polarization of the monolayer. Finally, we have fabricated and characterized WS2 and MoS2 nanostructures, both in the bulk and monolayer regimes.
We have developed nanofabrication recipes to produce monocrystalline silicon metasurfaces designed based on our theoretical predictions. We have characterized circular polarization in the near field of the nanostructures using polarized Raman spectroscopy. We have also constructed a polarization-resolved fluorescence microscope that allows us to do fine measurements of the degree of circular polarization of the emitted light. Finally, we have measured enhanced circularly polarized luminescence by chiral molecules using our silicon metasurfaces.
Regarding exciton-based nanoscale sensors, we observed localized fluorescence fluctuations in a monolayer semiconductor due to interaction with its environment. We adapted a fluctuation-based super-resolution technique to image localized exciton fluctuations in monolayer semiconductors and map disorder on the monolayer. We correlated the fluorescence fluctuations with features measured by atomic force microscopy and compared them with hyperspectral imaging data. We have combined monolayer semiconductors and chiral molecules towards the measurement of chirality through the polarization of the monolayer. Finally, we have fabricated and characterized WS2 and MoS2 nanostructures, both in the bulk and monolayer regimes.
The project results thus far have advanced the understanding and the methods for ultrasensitive chiral molecular detection. In particular, we have clearly established the boundaries between the chirality transfer and optical chirality mechanisms for nanophotonic chiral sensing. Using the polarization microscope developed in this project, we were able to enhance circularly polarized luminescence. We also plan on exploiting excitonic materials as nanoscale sensors for chiral molecules.