When an ultrashort pulse of light propagates in a scattering medium, its spatial and temporal properties get mixed and distorted because of the scattering process. Spatially, the output pattern is the result of the multiple interference between the scattered photons. Temporally, light gets stretched within the medium due to its characteristic confinement time, thus the output pulse is broadened in the time domain. Nonetheless, as the scattering process is linear and deterministic, the spatio-temporal profile of light at the output can be controlled by shaping the input light using a single spatial light modulator (SLM).
We report the first experimental measurement of the Time-Resolved Transmission Matrix of a multiple scattering medium using a coherent time-gated detection system. This operator contains the relationship between the input field, controllable with a SLM, and the output field accessible with a CCD camera for a given arrival time of photons at the output of medium. The delay line of the time-gated detection system sets the arrival time at will within the time of flight distribution of photons of the output pulse.
We exploit this time-resolved matrix to achieve spatio-temporal focusing of the output pulse at any arbitrary space and time position. The pulse is recompressed in time to its original Fourier-limited temporal width and spatially to the diffraction-limited size defined by the speckle grain size. We also generate more sophisticated spatio-temporal profiles such as pump-probe like pulse, thus opening interesting perspectives in coherent control, light-matter interaction and imaging in disordered media.
Mickael Mounaix, Hugo Defienne, and Sylvain Gigan, "Deterministic light focusing in space and time through multiple scattering media with a Time-Resolved Transmission Matrix approach (Conference Presentation)," Proc. SPIE 10073, Adaptive Optics and Wavefront Control for Biological Systems III, 100730S (Presented at SPIE BiOS: January 29, 2017; Published: 24 April 2017); https://doi.org/10.1117/12.2251820.5380600099001.
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