# High Resolution Imaging Using Micro-Objectives Fabricated by 2-Photon-Polymerization
## 4 High Resolution Imaging: Results and Discussions
This demonstration shows the usability of biconvex micro lenses for high-resolution imaging of structures below the diffraction limit and opens up new degrees of freedom for the design of the micro-optics used. By using the biconvex microlens as the central imaging optics, we were able to achieve the high resolution properties of the microspheres used in Figure 4 with a significant expansion of the field-of-view (FOV). The previously mentioned extension of the FRISP method presented in Section 3 will be used to evaluate the additional degrees of freedom in the design opened up by the use of 2 PP lithography for the fabrication of optical micro structures. In order to demonstrate the potential of this easily accessible simulation method, the experimental figures are compared with the predictions of the simulation in the following. Figure 4: Demonstration of the high-resolution properties for different materials with various refractive indices on the basis of micro-spheres with a diameter of 10 \(\upmu\)m. As test object the surface of a BluRay and as illumination a halogen lamp with a bandpass filter with a central wavelength of 450 nm \(\pm\) 10 nm is used. Real high-resolution images of the test object with (**a**) commercially available polystyrene micro spheres (n = 1.59 [26]) and (**b**) micro-objectives made with 2 PP using IP-Dip consisting of 4 micro spheres with connecting rods (n = 1.54 [25]) (see Figure 1). In all images, the fine surface features of the BluRay disc, including the individual vertical tracks of the BluRay surface and the characteristic pits and lands (see Figure 3b), are resolved. The red lines are guidelines for a better visualization of the direction and the distance of the BluRay tracks is 320 nm. Figure 6 shows the comparison of the simulated intensity distributions of the central imaging element with the experimental images for the micro-objective made of micro spheres shown in Figure 4 and the biconvex micro lens shown in Figure 5a,b show the comparison of the intensity distributions for a micro sphere with a diameter of 10 \(\upmu\)m. The observed intensity peaks occurring at the edges of the micro sphere in the simulation in Figure 6a result from simulation artifacts. These artifacts are caused by the low amount of illumination directions, in this case 25, and the intense focusing of the micro sphere (photonic nanojet). Figure 5b,c shows the corresponding images for the micro lens with a radius of curvature of 12.5 \(\upmu\)m, an aperture diameter of 20 \(\upmu\)m. In this case, we can't recognize the artefacts observed in (a), because the focus position is located in a wider distance from the object surface due to the smaller radius of curvature. The bright areas inside the high resolved area are the result of the higher contrast of the object surface during the simulation compared to the original BluRay in the experiment. The test object was a BluRay disk, which was also modeled in the simulation environment with its typical pits and lands structure. All four intensity distributions show clearly resolved traces in the vertical direction. Therefore, the simulation allows a reliable prediction of the imaging properties for different geometries of the central imaging elements of the investigated micro-objectives. In addition, the simulation also allows a prediction of the magnification through the micro-objective. In the simulation, the magnification is 1.63 for the micro spheres and 1.24 for the biconvex micro lenses. The magnification values match very well the values of 1.64 for the spheres and 1.21 for the biconvex lenses observed in the experiment. The deviation is mainly caused by slight shape deviation and differences in the refractive index between the parameters used in the simulation and the micro structures used in the experiment. Regarding the size of the FOV, there are slightly larger deviations between the simulation and the experimental observations.