  Focusing light inside scattering media in a freely addressable fashion is
challenging, as the wavefront of the scattered light is highly disordered.
Recently developed ultrasound-guided wavefront shaping methods are addressing
this challenge, albeit with relatively low modulation efficiency and resolution
limitations. In this paper, we present a new technique, time-reversed
ultrasound microbubble encoded (TRUME) optical focusing, which is able to focus
light with improved efficiency and sub-ultrasound wavelength resolution. This
method ultrasonically destructs microbubbles, and measures the wavefront change
to compute and render a suitable time-reversed wavefront solution for focusing.
We demonstrate that the TRUME technique can create an optical focus at the site
of bubble destruction with a size of ~2 microns. Due to the nonlinear
pressure-to-destruction response, the TRUME technique can break the addressable
focus resolution barrier imposed by the ultrasound focus. We experimentally
demonstrate a 2-fold addressable focus resolution improvement in a microbubble
aggregate target.
