Esmaspäeval, 17. veebruaril 2020 kell 16:15 Physicumi auditooriumis B103
Hasan Yılmaz (Yale University, Department of Applied Physics)
Coherent control of light transport and imaging through scattering opaque media
Materials such as white paint, fog, 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 spatial information. The transmittance can be enhanced by utilizing the interference of scattered coherent waves-a striking phenomenon first theoretically predicted in the context of mesoscopic electron transport through conducting wires [1-4]. In recent years, spatial light modulators (SLMs) have been used to control the transmittance of light by finding transmission eigenchannels of the scattering system [5,6]. By coupling light into transmission eigenchannels, transmittance through the medium can be controlled between 0 (closed channels) and 1 (open channels). Furthermore, not only the transmittance but also the imaging resolution can be enhanced through opaque scattering media by exploiting angular correlations “angular memory effect” .
First, the recent discovery on transmission eigenchannels through a layer of white paint is presented. It was discovered that transmission eigenchannels are exponentially localized in the transverse directions, even in the diffusive regime far from Anderson localization . The results show that open channels not only enhance total transmitted power, but also energy density inside and on the back surface of a scattering opaque medium, which is important for applications such as optogenetics and multiphoton imaging that aim for enhancement of light-matter interactions in complex optical systems. It is further demonstrated that selective coupling of light into a single transmission eigenchannel 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.
Next, the speckle correlation resolution enhancement (SCORE) imaging that simultaneously produces wide-field and high-resolution fluorescence images is introduced . SCORE is a scanning optical microscopy method that benefits from the angular memory effect through a scattering opaque layer. The high-resolution of SCORE is due to very fine speckle grains that are generated by a solid immersion medium which is made of a gallium phosphide (GaP) substrate with an opaque layer. Using SCORE, we demonstrated a deconvolved Abbe resolution of 116 nm with a field-of-view of 10 µm × 10 µm.
Figure 1. (a) When a beam of a flashlight shines through an opaque medium such as white paint or a fog, the light spreads in both longitudinal and lateral directions. Consequently, the transmitted beam becomes wider and the transmitted light intensity is lower. (b) By shaping its wavefront using a SLM, a laser beam propagates through the opaque medium without lateral spread. Moreover, the transmitted light intensity is enhanced. The enhancement of light transmittance and the suppression of lateral beam spreading keep the optical energy density high throughout the opaque medium.
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 an Associate Research Scientist at Yale University, Department of Applied Physics, working on physics and applications of complex optical systems with Prof. Hui Cao.
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