Spatially Shaping Waves to Penetrate Deep inside a Forbidden Gap

It is well known that waves with frequencies within the forbidden gap inside a crystal are transported only over a limited distance—the Bragg length—before being reflected by Bragg interference. Here, we demonstrate how to send waves much deeper into crystals in an exemplary study of light in two-dimensional silicon photonic crystals. By spatially shaping the wave fronts, the internal energy density—probed via the laterally scattered intensity—is enhanced at a tunable distance away from the front surface. The intensity is up to 100× enhanced compared to random wave fronts, and extends as far as 8× the Bragg length, which agrees with an extended mesoscopic model. We thus report a novel control knob for mesoscopic wave transport that pertains to any kind of waves.