# High Resolution Imaging Using Micro-Objectives Fabricated by 2-Photon-Polymerization
## 2 Fabrication  
To fabricate the polymer micro structures used in this study, we employ the commercially available 2 PP-lithography system, Photonic Professional+ from Nanoscribe GmbH (Eggenstein-Leopoldshafen, Germany). This system uses a fs-laser with a pulse duration of 120 fs, a repetition rate of 80 MHz, and a central wavelength of 780 nm. The laser beam is tightly focused within the UV-sensitive photoresist by a 100\(\times\) high numerical aperture (NA = 1.4) oil immersion objective (Zeiss Plan-Apochromat DIC) using the dip-in technique to achieve optimum resolution and precision. The photoresist chosen was Nanoscribe's IP-Dip, a material specifically designed for high-resolution 2 PP lithography. The 2 PP process relies on the non-linear absorption of photons in the photoresist, allowing polymer ization to occur only within a highly localized region known as a voxel (volume pixel). The dimensions of a typical voxel are about 150 nm in diameter and about 1000 nm in height, depending on system parameters and material properties. In order to create complex structures, the polymerization zone is moved 3-dimensionally inside the photoresist with highly precise xyz-piezo axes. Further important process parameters are a laser power of 9 mW and a maximum writing speed of 190 \(\upmu\)m/s. In the fabrication of micro structures for high-resolution applications, such as microspheres or bi-convex spherical micro-lenses, the stretched shape of the voxels presents a significant challenge during the design and fabrication process. Therefore, a customized geometry was developed specifically for the manufacturing process. This tailored design ensures for example the production of micro-spheres of fidelity in shape with diameters greater than 5 \(\upmu\)m [20]. Figure 1 demonstrates the process of fabricating transparent micro structures with complex geometry. With the process parameters described above the fabrication of such a structure requires about 45 min with a typical surface roughness of 10 nm [21]. A test structure consisting of four interconnected micro-spheres with a diameter of 10 \(\upmu\)m was designed using a Computer-aided design (CAD) program and the optical properties were simulated using the Finite Refractive Index Selective Propagation method [19]. In this structure, the central micro-sphere acts as the imaging optics, while the three surrounding micro-spheres, connected to the central one by thin connecting rods, are used for positioning and stabilization. In this work, the outer microspheres served only to stabilize the central imaging optics on the sample surface. Based on the work of [PERSON] et al., this design could potentially be utilized for precise control of the imaging optics relative to the surface of the object, possibly employing techniques such as optical tweezers [22]. Figure 1a shows the CAD model of the designed geometry. The fabricated micro structure, produced directly from the CAD data using 2 PP without further adjustment, is shown in the scanning electron microscope (SEM) image in Figure 1b. The top right inset shows a schematic cross-section of one of the micro-spheres, overlaying the actual fabricated shape with the intended design contour (dashed line). Although the lateral diameter of the fabricated micro-spheres matches the desired 10 \(\upmu\)m, significant deviations are observed in the vertical direction, resulting in a noticeable oval deformation. In addition, the contact area between the micro structure and the substrate is significantly larger than designed. These deformations are also visible in the connecting rods, which show irregularities compared to the intended geometry. The observed discrepancies are due to the high aspect ratio of the voxel in the 2 PP process, which contributes to an expansion of the structure in the vertical direction. This demonstrates the challenge of reproducing the desired geometry in the production of micro-spheres and bi-convex micro structure using 2 PP lithography.