  Focusing light through dynamically varying heterogeneous media is a
sought-after goal with important applications ranging from free-space
communication to nano-surgery. The underlying challenge is to control the
optical wavefront with a large number of degrees-of-freedom (DOF) at timescales
shorter than the medium dynamics. Recently, many advancements have been
reported following the demonstration of focusing through turbid samples by
wavefront-shaping, using spatial light modulators (SLMs) having >1000 DOF.
Unfortunately, SLM-based wavefront-shaping requires feedback from a
detector/camera and is limited to slowly-varying samples. Here, we demonstrate
a novel approach for wavefront-shaping using all-optical feedback. We show that
the complex wavefront required to focus through highly scattering samples,
including thin biological tissues, can be generated at sub-microsecond
timescales by the process of field self-organization inside a multimode laser
cavity, without requiring electronic feedback or SLMs. This wavefront-shaping
mechanism is more than five orders of magnitude faster than state-of-the-art,
reaching the timescales required in many applications.
