Super-resolution optical microscopy using transmissive micro structures

• Applicant: Professor Dr. Ralf Bernhard Bergmann
• Subject Area: Measurement Systems
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 431605610

The examination of object details with high resolution is essential for many areas of research, development and fabrication. A simple option is the classical light microscopy. The resolution of this approach is, however, restricted by the resolution limit described by Abbe. Therefore, approaches have been investigated again and again in the past to overcome this limit. One way to increase the resolution is the application of microspheres to the surface of the object. The achievable resolution essentially depends on the diameter of the microsphere and the refractive index difference between the microsphere and the surrounding medium. In the literature, objects with a separation <100 nm were resolved with a standard light microscope, which corresponds to a resolution much smaller than the wavelength. Presently, no general model is available to explain this experimentally observed effect. However, a broad understanding is essential for the evaluation of the limits and potential of the method as well as for optimizing the imaging properties of the microstructures used. In the literature, experimental data are limited to studies on spherical microstructures, which neither show distortion-free images nor larger fields of view. Investigations of microstructures that deviate from a spherical shape are limited to simulations of their focussing properties; experimental results are not available.Therefore, the proposed project has two objectives: 1. Development of a theoretical model to explain the effect of a resolution below the Abbe diffraction limit (in the following termed super-resolution): In contrast to the literature, the model proposed here is not based on the solution of Maxwell's equations, but on the simpler principles of scalar diffraction theory, specifically an extended Huygens-Fresnel approach in order to understand the effects of super-resolution. This approach will be employed in the project to describe the coupling of evanescent fields into the microstructure in a simplified way. The procedure outlined above leads to a reduction of the computation time and allows an iterative optimization of the microstructures in term of imaging properties. 2. Experimental implementation of the theoretical findings: While in the literature so far only spherical structures were investigated experimentally and aspheric structures were only described by simulations, in our proposal spheres and freeform-microstructures are fabricated by direct laser writing, optically characterized and in this way the effect of the super-resolution will be verified. The proposed project thus makes it possible to obtain a well-grounded theoretical understanding to explain the effect of super-resolution. It also brings the super-resolution optical light microscopy using spherical and aspherical microstructures fabricated by direct laser writing to a practical application.

• DFG Programme: Research Grants