Communications and Signal Processing Seminar
Opaque Lenses: Using Disorder to Bring Laser Light to a Focus
Materials such as white paint, skin or bone are opaque because they have a microscopic disordered structure that scatters light. Light impinging on such a material becomes diffuse and only a fraction of it is transmitted. By controlling the shape of the incoming wavefront, using a two-dimensional spatial phase modulator, we can cause coherent light to interfere constructively at a target point behind the sample. We find that at the target point, the light forms a tight focus that is up to 1000 times brighter than the diffuse background.[1,2]
Theorists have predicted that in any non-absorbing disordered sample there exist open eigenchannels: specific linear combinations of incoming waves which experience a transmittance of nearly one. Such eigenchannels have not yet been directly observed in condensed matter systems, however, they are the cause of phenomena such as universal conductance fluctuations. By carefully constructing a suitable wavefront we find we can selectively couple light to the open eigenchannels in a disordered optical material. As a result, the total diffuse transmittance increases. The magnitude of the increase is exactly as predicted by random matrix theory. (*) I will present work performed in cooperation with: D. Akbulut, F. van Beijnum, E.G. van Putten, I.M. Vellekoop, A. Lagendijk
 I.M. Vellekoop and A.P. Mosk, Focusing coherent light through opaque strongly scattering media, Opt. Lett. 32, 2309 (2007).
 I.M. Vellekoop, E.G. van Putten, A. Lagendijk and A.P. Mosk, Demixing light paths in disordered metamaterials, Opt. Express 16, 67 (2008).
 I.M. Vellekoop and A.P. Mosk, Universal optimal transmission of light through disordered materials, Phys. Rev. Lett. 101, 120601 (2008).
Allard Mosk (1970) received an M.Sc. in theoretical physics at the University of Amsterdam, and subsequently a Ph.D. in experimental physics at the same university, on optical experiments with ultracold Hydrogen gas. He worked as a postdoc with Rudi Grimm (Heidelberg., Innsbruck), with Michele Leduc (Paris), and with Gerard Meijer (Nieuwegein, Berlin), on experiments with ultracold atoms and molecules. Finally, he changed his area of work to photons, working with Ad Lagendijk and Willem Vos (Amsterdam, Twente) on photonic materials, light scattering and random lasers. Recently, he demonstrated wavefront shaping by a digital spatial light modulator as an effective way to control and focus light in strongly scattering materials. He is presently an associate professor at the Universiteit Twente in the Netherlands, and his work is made possible by a Vidi grant from the Netherlands Organization for Scientific Research NWO.