Measuring the transmission matrix of a complex medium: focusing and beyond
Institut Langevin, CNRS and ESPCI ParisTech
Mon, Nov. 19th 2012, 14:30
Salle Claude Itzykson, Bât. 774, Orme des Merisiers
Scattering of coherent light in heterogeneous biological media and tissues , leads to strong scattering and interferences phenomena which destroy both the spatial amplitude and phase information of any laser illumination. At the spatial level, it gives rise to the well-known ``speckle'' interference patterns. At the temporal (or spectral) level, a short pulse entering a scattering medium will be stretched due to the multiplicity of path lengths in the propagating medium. Consequently this greatly limits the imaging of an object through a scattering medium. Multiple scattering is a highly complex but nonetheless deterministic process: it is therefore reversible, in the absence of absorption. Speckle can be coherently controlled. By ``shaping'' or ``adapting'' the incident wavefront, it is in principle possible to control the propagation and to overcome the scattering process. Liquid crystal spatial light modulator (SLM) is a tool of choice to shape a laser beam over a very large number of modes, in order to match the high complexity of a multiple scattering medium. I will show how, using a SLM, one can measure the transmission matrix which links the input - output modes of the scattering medium. We present our original approach of solving the inverse problem for the reconstruction of an arbitrary image through the scattering media. I will detail our recent experiments with phase SLMs applied to, spatial focusing, imaging and phase conjugation through a thick opaque multiple scattering media. Beyond the spatial aspects of wave control, I will also review the recent progress on temporal control in complex media and its interests for mesoscopic physics studies.