Recent breakthroughs in optical wavefront engineering have opened the possibility of controlling light intensity distribution inside highly scattering medium, but their success is limited by the open geometry of the sample and the difficulty of covering all input modes. Here we demonstrate experimentally an efficient control of energy density distribution inside a strong scattering medium. Instead of the open slab geometry, we fabricate a silicon waveguide that contains scatterers and has reflecting sidewalls. The intensity distribution inside the 2D waveguide is probed from the third dimension. With a careful design of the on-chip coupling waveguide, we can access all the input modes. Such unprecedented control of incident wavefront leads to 10 times enhancement of the total transmission or 50 times suppression. A direct probe of light intensity distribution inside the disordered structure reveals that selective excitation of open channels leads to an energy buildup deep inside the scattering medium, while the excitation of closed channels greatly reduces the penetration depth. Compared to the linear decay for random input fields, the optimized wavefront can produce an intensity profile that is either peaked near the center of the waveguide or decay exponentially with depth. The total energy stored inside the waveguide is increased 3.7 times or decreased 2 times. Since the energy density dictates light-matter interactions inside a scattering system, our results demonstrate the possibility of tailoring optical excitations as well as linear and nonlinear optical processes inside the turbid medium in an on-chip platform.
Raktim Sarma, Alexey Yamilov, Sasha Petrenko, Yaron Bromberg, and Hui Cao, "Control of energy density inside turbid medium (Conference Presentation)," Proc. SPIE 10073, Adaptive Optics and Wavefront Control for Biological Systems III, 100730Q (Presented at SPIE BiOS: January 29, 2017; Published: 24 April 2017); https://doi.org/10.1117/12.2251457.5380600097001.
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