Title: Photon Transport in Disordered Photonic Crystals
Description: One of the daunting challenges in wave physics is to accurately control the flow of light at the subwavelength scale. By patterning the optical medium one can design anisotropic artificial medium, this engineering method is commonly known as photonic crystals or metamaterials. Negative or zero index of refraction, slow-light propagation, cloaking with transformation optics material, and beam collimation are only a few such unique functionalities that can be achieved in engineered media at the subwavelength scale. Another interesting phenomenon in wave physics, Anderson localization, which suggests electron localization inside a semiconductor, has been intensely investigated over the past years, including transverse localization in bulk and waveguide arrays periodic in one and two dimensions. Here we report the photon transport and collimation enhanced by transverse Anderson localization in chip-scale anisotropic artificial medium, a similar physical model to doping the impurity in insulator and turning it into a semiconductor.

First, by engineering the photonic crystal, we demonstrate a new type of anisotropic artificial medium for diffraction-free transport through cascaded tunneling of guided resonances. High-resolution near-field measurements demonstrate the coupling of transverse guided resonances, supported by large-scale numerical modeling. Second, with the disordered scattering sites in this superlattices, we uncover the mechanism of disorder-induced transverse localization in chip-scale. Arrested spatial divergence is captured in the power-law scaling, along with the exponential asymmetric mode profiles and enhanced collimation bandwidth for increasing disorder, over 4,000 scattering sites. With increasing disorder, we observe the crossover from cascaded guided resonances into transverse localization regimes, beyond the ballistic and diffusive transport of photons. As disorder is ubiquitous in natural and artificial materials, the transport through random media is of great importance. It also leads to various interesting optical phenomena, of which the most surprising one is Anderson localization of light.

However, not all the states in disordered system are localized. Nonlocalized modes that extend over the whole sample via coupling between multiple local cavities with similar resonance frequencies are also present in disordered systems. These extended modes are called necklace states. Here, we also show that long-distance beam collimation can be witnessed in millimeter-scale photonic crystals that were fabricated lithographically with ultrahigh resolutions. By precisely controlling the disorder level of three million scattering sites in photonic crystals, we uncovered the transformation of light flows from the propagation of regular Bloch modes to necklace states.