Advanced near-field microscopic studies of nanophotonic and complex biological samples

Gölz, Thorsten Nicolas (2025): Advanced near-field microscopic studies of nanophotonic and complex biological samples. Dissertation, LMU München: Fakultät für Physik

Abstract

Infrared microscopy is commonly referred to as chemical microscopy, as it enables the deciphering of the chemical structure of examined materials based on their intrinsic, materialspecific infrared absorption. The detected infrared absorption serves as a spectroscopic fingerprint that is unique to the chemical nature of the material. Scattering scanning near-field optical microscopy (s-SNOM) is a technique that extends infrared microscopy and spectroscopy to the nanometer level, far below the diffraction limit of light, making the nanoworld accessible to chemical microscopy, previously restricted to several micrometers due to the diffraction limit. This work serves as a milestone in expanding the application range of s-SNOM across multiple fields, in combination with classical diffraction-limited far-field infrared microscopy methods. In the first section of this work, s-SNOM is applied to optical all-dielectric metasurfaces whose ultra-sharp resonances are based on the physics of bound-states in the continuum (BIC). It is shown that s-SNOM can resolve the optical near-fields of the individual metasurface resonators. Furthermore, using a newly introduced image-processing method for s-SNOM images, it is demonstrated that the finite array size effect, directional coupling of the resonators, defects, and edge effects in optical metasurfaces can be decoded within the near field. The insights gained trough this method can be used, among other things, to reduce the geometric footprint of metasurfaces for applications in catalysis or biosensor technology. In the second part of this work, a study oriented towards a biomedical application is presented in which a dental filling is characterized using various infrared microscopy techniques. It is shown that the infrared images and spectra can distinguish the chemical nature of the composite filling and the dental resin. In addition, the infrared images can be used to determine the porosity of dental fillings. Finally, s-SNOM is employed to examine the composite material of the dental filling at the nanoscale, revealing its chemical heterogeneities. The demonstrated infrared microscopy-driven characterization of the dental filling has great potential to optimize the increasingly complex composite materials through a better understanding of their chemical composition down to the nanoscale. In the final part of this work, the functional range of s-SNOM, which has so far been almost exclusively applied to dry samples, is extended to aqueous solutions. For this purpose, a method is presented that uses ultra-thin silicon nitride membranes to protect the delicate sSNOM tip from the aqueous solution. As a highly relevant model system, the method is applied to investigate photoswitchable lipid vesicles in aqueous solution, which are potential lightdriven drug delivery systems. The study demonstrates that lipid vesicles can be characterized by near-field microscopy and spectroscopy far below the diffraction limit of infrared light. In addition, a transient infrared spectroscopic method based on s-SNOM is introduced that resolves the millisecond switching dynamics of single lipid vesicles. The presented concept of s-SNOM measurements in liquids holds great potential for future studies on e.g. the degradation of catalysts or the assembly of neurotoxic protein fibrils. The studies presented in this thesis demonstrate the versatility of s-SNOM with investigations ranging from photonic metasurfaces and dental fillings to photoswitchable lipids in aqueous solution. The findings and concepts will allow further optimization of different material systems based on the attained near-field microscopic and spectroscopic results.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: s-SNOM, near-field microscopy, photoswitchable lipids, BIC, dental material
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 530 Physik
• Fakultäten: Fakultät für Physik
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 6. Februar 2025
• 1. Bericht­erstatter:in: Tittl, Andreas