Harnessing multiple scattering for universal photonic applications

Abstract

Scattering, be it elastic or inelastic, is the reason why we can see the world around us. Ironically, it is also the main phenomenon that impedes clear imaging using traditional optical systems. This is due to the fact that conventional refraction based lenses are designed to focus an image using ballistic light components. However, in real life, the path of light is always obstructed, for instance, through the refractive index variant atmosphere, the skin covering internal organs, or even a well-designed cover slip of a microscope. Due to this reason, multiple scattering has always been considered as an obstacle that was best to be avoided. In this thesis, we control multiple scattering and demonstrate that the extremely high degree of freedom brought on by multiple scattering can, on the contrary, be used to our advantage for various photonic applications. The thesis consists of two main branches: (1) taking advantage of multiple scattering to enhance photonic properties of a turbid medium and (2) utilizing time reversal to maximize energy delivery through turbid media. In the first part of the story, the large degree of freedom is shown to be sufficient to control the paths of different wavelengths and polarization, as well as steer light to focuses that are smaller than described by Abbe’s diffraction limit. The first demonstration of widefield super-resolution imaging is also achieved using the ‘scattering superlens’. These examples demonstrate that a thin turbid medium can be effectively used as a powerful photonic device with multiple functionalities that are impossible to obtain with conventional lenses. In the second part of the story, to maximize energy delivery through turbid media in in-vivo situations, we demonstrate a novel phase conjugation scheme allowing the measurement and control of a complex 2D field using a single point detector and an SLM placed in reflection geometry. We hope that the findings of this thesis can be further improved to realize powerful ‘turbid lenses’ that can be widely used in the community and ultimately help in finding the best solution for imaging through turbid media.