  Imaging through opaque, highly scattering walls is a long sought after
capability with potential applications in a variety of fields. The use of
wavefront shaping to compensate for scattering has brought a renewed interest
as a potential solution to this problem. A key to the practicality of any
imaging technique is the capability to focus light without direct access behind
the scattering wall. Here, we address this problem using photoacoustic feedback
for wavefront optimization. By combining the spatially non-uniform sensitivity
of the ultrasound transducer to the generated photoacoustic waves with an
evolutionary competition among optical modes, the speckle field develops a
single, high intensity focus significantly smaller than the acoustic focus used
for feedback. Notably, this method is not limited by the size of the absorber
to form a sub-acoustic optical focus. We demonstrate imaging behind a
scattering medium with up to ten times improvement in signal-to-noise ratio
(SNR) and five to six times sub-acoustic resolution. The results presented here
open up opportunities for 3D imaging applications in the diffusive regime
beyond the sound wave diffraction-limited resolution, including medical
imaging, photothermal therapy, neuroscience, and optogenetics.
