Coherent control of light in opaque media
Opaque materials such as white paint, milk, paper or biological tissue have spatial inhomogeneities in the refractive index which cause multiple scattering of light. In such opaque materials, most of the light reflects in the backward direction, hindering the transport of optical energy and information. However, by utilizing the interference of scattered coherent waves, it is possible to prepare optimized wavefronts (transmission channels) that completely suppress reflection-a striking phenomenon first theoretically predicted in the context of electron transport through conducting wires [1-4]. In recent years, spatial light modulators (SLMs) have been used to shape light into transmission channels [5,6]. By coupling light into transmission channels, transmittance through the medium can be adjusted from 0 (closed channels) to 1 (open channels).
In the first part of my talk, I will present our recent experimental and numerical results on transmission channels of white paint made of an ensemble of zinc oxide (ZnO) nanoparticles in wide slab geometry. We discover that the transmission channels of a wide diffusive slab exhibit transversely localized incident and outgoing intensity profiles, even in the diffusive regime far from Anderson localization . Our results show that coupling light into open channels not only enhances the total transmitted power, but also the energy density inside and on the back surface of an opaque medium that is important for applications such as optogenetics and deep-tissue multiphoton imaging that aim for the enhancement of light-matter interactions in complex optical systems. We further demonstrate that selective coupling of light into a single transmission channel modifies the angular memory effect correlation range . Open channels have a wider memory effect range than a plane wave or a Gaussian beam, thus will provide a wider field of view for memory-effect-based imaging through opaque media.
In the last part of my talk, I will introduce the research direction I plan to establish. The main questions of my research interest are “how does light behave in complex photonic media” and “can we manipulate the behavior of light in such complex media to design optical devices that outperform the conventional ones?” First, I will describe the concept of spatial degrees of freedom and channel capacity in a complex photonic medium. Second, I will introduce the problem of gathering high-density optical information deep inside such complex photonic media. Finally, I will propose and describe the idea of realizing a universal all-optical hyperspectral analyzer that can gather spatial information from deep inside complex photonic media independent of their light-scattering properties.
In summary, my research activities will focus not only on the exploration of new mesoscopic light transport phenomena in complex systems but also on the development of new spatial and spectral measurement techniques, algorithms, and devices that exploit new physical phenomena to overperform the current state-of-the-art techniques.
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 H. Yılmaz, C. W. Hsu, A. Yamilov, and H. Cao, Nat. Photonics 13, 352-358 (2019).
 H. Yılmaz, et al., Phys. Rev. Lett. 123, 203901 (2019).
About The Speaker
Hasan Yılmaz received his B.Sc. degree in Physics Engineering from İstanbul Technical University in 2008. He received his M.Sc. degree in Materials Science and Engineering at Koç University in 2011, where he worked with Prof. Ali Serpengüzel at the Microphotonics Research Laboratory. In 2015, he received his Ph.D. degree from University of Twente in The Netherlands for his work on “Advanced Optical Imaging with Scattering Lenses,” with Prof. Allard Mosk. He is currently a Postdoctoral Research Associate at Yale University, Department of Applied Physics, working on physics and applications of complex optical systems with Prof. Hui Cao.