Polarized Third-harmonic Generation Imaging of Two-photon Photopolymerized Microstructures

Two-photon photopolymerization (TPP) has been shown to be a versatile tool for fabricating subwavelength features that are localized in three dimensions (3D) [1]. However, detection of the photopolymerized structures after polymerization is difficult as the polymerized regions exhibit poor contrast against the background when viewed under a conventional light microscope. Furthermore, the traditional optical techniques used to visualize such structures are invasive, often requiring mechanical sectioning, use of contrast agents and utilization of complicated sample preparation and instrumentation. Thus, there is an obvious need to develop simple and nondestructive characterization tools for photopolymerized structures. Here, we show that polarization-dependent third-harmonic generation (THG) microscopy [2] can be used for visualizing photopolymerized microstructures. We demonstrate the applicability of the technique by imaging grating-like microstructures on a photoresist film. The structures were fabricated using a custom-built direct-laser-writing setup equipped with a femtosecond laser (780 nm, 290 fs, 80 MHz), a three-axis piezo-scanner and an objective (NA = 0.75 [3]. The sample used was SU-8 5 (Microchem) photoresist that was pre-baked and spin-coated on cover glass. For nonlinear imaging, we used a customized point-scanning THG microscope equipped with a femtosecond laser (1060 nm, 200 fs, 82 MHz), a three-axis piezo-scanner, a strain-free objective (NA = 0.8) and a photon-counting system. We found that THG allows high contrast imaging of photopolymerized structures, which is further improved when circular polarization is used. We also found that our technique is nondestructive as expected from a coherent optical process. Finally, the technique permits optical sectioning of 3D photopolymerized microstructures (Fig. 1a-d). Overall, our results suggest that polarized THG microscopy has potential in diagnostics of 3D photopolymerized devices.