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P. Ambichl, A. Brandstötter, J. Böhm, M. Kühmayer, U. Kuhl, S. Rotter:
"Focusing inside Disordered Media with the Generalized Wigner-Smith Operator";
Talk: Complex Nanophotonics Science Camp, London; 2017-07-25 - 2017-07-28.

In the emerging field of "wavefront shaping" spectacular advances have recently been made, such as to focus light behind an opaque layer, or to send and retrieve images across it. Thanks to these advances also the focusing of light inside highly disordered media could recently be demonstrated using embedded fluorescent probes and nano-crystals or with digital optical phase conjugation to focus light onto a target moving inside an otherwise static environment. Here we present a new approach for focusing inside a disordered material that has the considerable advantage of working without the requirement to implant a fluorescent body at the focus or to phase-conjugate a wave scattered at the focus position [1]. Our technique is based on a generalization of the established Wigner-Smith time-delay operator [2,3]. The key ingredient for our approach is the scattering (or transmission) matrix of the medium [4,5] and its derivative with respect to the position of the target one aims to focus on. A specific experimental realization in the microwave regime is presented showing that the eigenstates of a corresponding operator are sorted by their focusing strength - ranging from strongly focusing on the designated target to completely bypassing it. Our protocol works without optimization or phase-conjugation and we expect it to be particularly attractive for optical imaging in disordered media.

[1] P. Ambichl et al., Phys. Rev. Lett. 119, 033903 (2017).

[2] E. P. Wigner, Phys. Rev. 98, 145 (1955).

[3] F. T. Smith, Phys. Rev. 118, 349 (1960).

[4] S. M. Popoff et al., Phys. Rev. Lett. 104, 100601 (2010).

[5] H. Yu et al., Phys. Rev. Lett. 111, 153902 (2013).

Wavefront Shaping, Disordered Media, Focusing