# High Resolution Imaging Using Micro-Objectives Fabricated by 2-Photon-Polymerization
## 1 Introduction  
The observation of minute objects and their details is an essential basis for research, development, and quality assurance in many areas of scientific investigation, including physics, chemistry, materials science, and life sciences. A robust and simultaneously simple method of observation of such minute objects is provided by light optical microscopy. This method allows for observation of the far field with a single aperture, without the use of additional effects such as fluorescence. However, the resolution limit, first described by [PERSON] in 1873 [1] and further specified by [PERSON] in 1879 [2], represents a physical limitation for the smallest object details that can be resolved. The resolution limit can be attributed to the absence of evanescent waves in the far field. Evanescent waves transmit the high spatial frequency components of the minute details of the object and exponentially decay with distance, rendering them invisible in the far field. Over the decades, various methods have been developed either in the field of near-field microscopy, e.g., scanning near-field optical microscopy (SNOM) [3, 4], or in far-field microscopy [5], e.g., stochastic optical reconstruction microscopy (STORM) [6], stimulated emission depletion microscopy (STED) [7], to overcome the classical resolution limit. In contrast to thesemethods, a combination of dielectric micro structures and a classic optical light microscope offers a much simpler and cost-effective method for high-resolution imaging [8; 9]. In this case, mainly solid immersion lenses (SIL) or transparent micro-spheres are used. In the case of SILs, half-spheres or super-half-spheres are applied directly to the object surface [8]. With transparent micro-spheres located directly on the object surface, a resolution of 50 nm was demonstrated [9]. Studies of the resolution characteristics of SILs have shown a strong correlation between imaging performance and the shape of the SIL [10]. Various fabrication methods, such as inkjet printing [11] and thermal deformation [12], can be used to modify the shape of SILs. However, these techniques are primarily suitable for producing SIL structures, such as half-spheres or hyper-hemi-micro spheres [13], and are not effective for producing micro lenses with complex geometries. [PERSON] et al. [14] reported about the shape adaptation of conventional micro-spheres and the influence on high-resolution properties. In their work, shape adjustments were performed using FIB (Focus Ion Beam) milling. Fabricating micro lenses with complex shapes and diameters of just a few micrometers remains a significant challenge. In contrast, two-photon polymerization lithography (2 PP) offers an highly versatile solution for the fabrication of complex optical micro structures, such as micro lenses [15], micro lens arrays [16] and even meta-surfaces [17]. Using the precision and flexibility of 2 PP lithography, it is possible to design and fabricate micro lenses tailored to specific applications, paving the way for improved imaging and optical performance in a wide range of fields. [PERSON] et al. [18] recognized the potential for application of 2 PP to create micro structures for use in high-resolution microscopy. The micro structures presented in this paper were limited to cylinder, truncated cone, hemisphere and protruding hemisphere, and all of these have a large contact area to the sample of interest and limited surface quality and shape accuracy. Furthermore, the micro structures did not provide a clear image of the used samples with details below the resolution limit. In this work, we present for the first time, experimentally a high-resolution imaging enabled by 2 PP fabricated micro spheres and bi-convex spherical micro lenses. To evaluate the high-resolution properties of 2 PP fabricated structures, the imaging quality of fabricated micro spheres is compared with conventionally fabricated micro spheres.