High-Resolution Reflection Microscopy via Absorbance Modulation
by Parul Jain
Date of Examination:2022-11-08
Date of issue:2023-01-11
Advisor:Prof. Dr. Alexander Egner
Referee:Prof. Dr. Alexander Egner
Referee:Prof. Dr. Jörg Enderlein
Persistent Address:
http://dx.doi.org/10.53846/goediss-9660
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Abstract
English
The properties of composite materials are primarily governed by their microstructural features, which can vary in size from a few nanometres to several micrometres. Optical microscopy is one of the primary tools for morphological characterisation in material science. However, due to the wave nature of light, the resolution of optical microscopes is limited by the diffraction limit [1], thereby limiting its capability to analyse these materials. Stimulated emission depletion (STED) microscopy, which is so far mostly used in life science imaging, circumvents the diffraction limit by exploiting the optical transitions of the fluorescent markers [2]. The concept of STED has been successfully applied in optical lithography (AMOL) and transmission microscopy as a technique called absorbance modulation imaging (AMI) [3, 4], where, instead of utilising the optical transitions of fluorophores, a layer of photochromic molecules, so called the absorbance modulation layer (AML), is used to achieve high resolution. The transparency of the AML, coated on the measurement surface, is rendered opaque and transparent when illuminated with light of different wavelengths. Using this wavelength-selective switching, a near-field aperture is generated in the AML to confine the light to the nanoscale dimension, resulting in sub-diffraction resolution. Despite the potential of reflection microscopy to analyse a much wider range of materials, AMI in reflection microscopy has not yet been demonstrated. Recently, Kowarsch et al. published a theoretical model on AMI in confocal reflection microscopy, predicting imaging beyond the diffraction limit is indeed possible [5]. In this thesis, for the first time, we experimentally validate this prediction by demonstrating one-dimensional AMI in reflection microscopy. A 2.4-fold resolution enhancement is achieved by this technique. The resolution is measured by imaging AML-coated gold nanoparticles. We further show the applicability of this technique on extended objects, an edge and a one-dimensional grating structure. The one-dimensional AMI that we demonstrate here can be extended to two dimensions which would facilitate high resolution optical imaging of microstructural features, in reflection.
Keywords: Absorbance Modulation Imaging; High-Resolution Reflection Microscopy