  Focusing light into opaque random or scattering media such as biological
tissue is a much sought-after goal for biomedical applications such as
photodynamic therapy, optical manipulation, and photostimulation. However,
focusing with conventional lenses is restricted to one transport mean free path
in scattering media, limiting both optical penetration depth and resolution.
Focusing deeper is possible by using optical phase conjugation or wavefront
shaping to compensate for the scattering. For practical applications, wavefront
shaping offers the advantage of a robust optical system that is less sensitive
to optical misalignment. Here, the phase of the incident light is spatially
tailored using a phase-shifting array to pre-compensate for scattering. The
challenge, then, is to determine the phase pattern which allows light to be
optimally delivered to the target region. Optimization algorithms are typically
employed for this purpose, with visible particles used as targets to generate
feedback. However, using these particles is invasive, and light delivery is
limited to fixed points. Here, we demonstrate a method for non-invasive and
dynamic focusing, by using ultrasound encoding as a virtual guide star for
feedback to an optimization algorithm. The light intensity at the acoustic
focus was increased by an order of magnitude. This technique has broad
biomedical applications, such as in optogenetics or photoactivation of drugs.
