X-ray imaging non-destructively identifies functional 3D photonic nanostructures

To investigate the performance of three-dimensional (3D) nanostructures, it is vital to study in situ their internal structure non-destructively. Hence, we perform synchrotron X-ray holographic tomography on exemplary 3D silicon photonic band gap crystals without irreversible preparation steps. Here, we obtain real space 3D density distributions of whole crystals buried on 2 mm^2 beams with 20 nanometer resolution. Our X-ray results identify why structures that look similar in scanning electron microscopy have vastly different nanophotonic functionality: One crystal with a broad photonic gap reveals 3D periodicity as designed ("Good"), a second structure without gap reveals a buried void ("Bad"), a third one without gap is shallow due to fabrication errors ("Ugly"). We conclude that X-ray tomography is a crucial tool to critically assess 3D functional nanostructures.