Spatially shaping waves deep inside a forbidden gap

Wave propagation inside crystals can be controlled through bandgap formation and engineering of resonant and functional features within (e.g. cavities). However, the Bragg interference of waves exponentially attenuates the waves from penetrating deep into the crystal, thereby disallowing access to these functional features and reducing the accessible functional volume of the crystal. Breaking this convention, here we demonstrate the ability to send waves much deeper into crystals. We study light in exemplary two-dimensional silicon photonic crystals. By spatially shaping the optical wavefronts, the intensity of laterally scattered light, that probes the internal energy density, is observed to enhance at a tunable depth within the crystal. We measured intensity enhancement factors of >100x and wave penetration to >8x the Bragg length. Our novel steering of waves inside a forbidden gap exploits the transport channels induced by unavoidable deviations from perfect periodicity, here unavoidable fabrication deviations. Importantly, the reconfigurability of the wavefronts enables programmable wave control, which was hitherto impossible, thereby opening up new avenues, for e.g. reconfigurable resonant photonic circuits in 3D photonic crystals.