# High Resolution Imaging Using Micro-Objectives Fabricated by 2-Photon-Polymerization
## 4 High Resolution Imaging: Results and Discussions  
Figure 3 shows the experimental setup schematically and provides an SEM image of the surface structure of a BluRay disc. Imaging experiments were performed using a Zeiss AXIO Imager.A1m light microscope with a Zeiss microscope objective _Epiplan-Neofluar HD 50x_ with a numerical aperture (NA) of 0.75. The illumination was provided by the halogen source built into the microscope and a passband filter with a central wavelength of \(450\) nm \(\pm\) 10 nm. In particular, this microscope configuration makes it impossible to resolve the pits and lands of the BluRay disk without the aid of micro spheres, as the resolution limit according to the Rayleigh criterion [2] for \(\lambda\) = 450 nm is 366 nm and the distance between the tracks of the BluRay is 320 nm. Figure 4 shows a side-by-side comparison of the imaging performance of conventional commercially available micro-spheres made and a micro-objective consiting of 4 microspheres fabricated by 2 PP-lithography using IP-Dip (refractive index \(n=1.54\)) [25]. Both types of micro-spheres have a diameter of 10 \(\upmu\)m to ensure consistency in the comparison. In both cases, the fine surface features of the BluRay disc, including the individual horizontal tracks of the BluRay surface, with a track distance of 320 nm and the characteristic pits and lands, with lateral dimensions s small as 130 nm, are resolved, demonstrating the high-resolution imaging capabilities of these types of micro-spheres. The comparison highlights that 2 PP micro-spheres can achieve imaging performance comparable to that of conventional commercially available micro-spheres. This result is significant as it confirms that 2 PP lithography can fabricate optical micro structures suitable for high resolution imaging applications. Such verification via a comparison with conventional micro-spheres or reference literature has not been performed in previous publications [18]. To demonstrate the potential of 2 PP technology for the fabrication of refractive micro structures to be used in high-resolution microscopy, micro-objectives with biconvex micro lenses as central imaging element were fabricated using IP-Dip. The imaging properties of the biconvex micro lenses are compared with those of conventionally fabricated micro spheres and micro spheres fabricated using 2 PP, as well as with simulation results based on the FRISP extension presented in Section 3. Figure 3: (**a**) Schematic diagram of the experimental setup for high-resolution imaging using transparent micro-optical structures. The setup includes a light microscope equipped with a high numerical aperture objective, a halogen illumination source with a bandpass filter (centered at 450 nm), and a BluRay disc as the test sample. The micro-optical element is positioned on the sample surface, immersed in ethanol to facilitate manipulation and imaging. (**b**) SEM image of the BluRay disc surface, showing the characteristic pits and lands that serve as a challenging test pattern for evaluating the resolution capabilities of the fabricated micro-optics. Figure 5a shows an SEM image of a micro-objective, as introduced in Figure 1, with a biconvex micro lens with a radius of curvature of 12.5 \(\upmu\)m and an aperture diameter of 20 \(\upmu\)m as the central imaging element. The three micro spheres at the end of the connecting rods serve to stabilize the biconvex micro lens on the object surface and can be used for positioning on the object surface. The positioning of such a micro-objective can, for example, be done without contact using optical tweezers [22]. Figure 5b shows an optical microscope image with the experimental configuration shown in Figure 3. The central biconvex micro lens enables the resolution of the structures on the BluRay surface, with all surface details including the pit and lands structure. The zoom in of the area marked in (b) shown in (c) shows the clearly resolved pits and lands structure of the BluRay surface.