X-ray microscopy of polymer photonic structures

Over the last two decades, direct laser writing (DLW) has developed into one of the most versatile techniques for the fabrication of nanophotonic structures [1]. DLW employs an ultrafast optical pulse, which is focused tightly in a transparent photoresist. This focus locally induces non-linear absorption that initiates polymerization in the 3D volume of the resist. Continuous development has led to commercial nanofabrication systems with around 100 nm lateral resolution. Inspection of the fabrication results, however, poses a challenge: Readily available diagnostic methods such as scanning electron microscope (SEM) are limited to the exterior of the fabricated structure. A method like Focused Ion Beam (FIB) milling gives access to the interior. However, this method requires physically slicing the structure and hence inhibits subsequent optical measurements on the same structures. Here we argue that X-ray tomography offers a solution. It can be used to quantify the effect of nanofabrication disorder on the optical response of a DLW-written photonic medium. The relation between the nanostructure of a medium and its optical response is of critical importance for security applications that utilize complex media as authentication tokens [2, 3].