  Biomedical applications requiring tissue diagnosis, activation, and treatment
could be substantially leveraged by optical methods, owing to their unique
feature set. However, their widespread application is severely limited by the
strong light scattering that occurs in many tissues of interest, which
dramatically limits achievable penetration depths. Here we demonstrate a new
method to solve this issue by utilizing free-running ultrasound waves,
transversal to the light propagation direction, to guide light into deeper
tissue regions. We study the formation of the ultrasound-induced refractive
index structures and waveguides using simple ultrasound field configurations
and analyze their effects on the propagation of short light pulses. Our results
show waveguide support and associated light intensity increase up to the depths
of at least 20 mm in Intralipid-20% phantoms with a reduced scattering
coeffcient close to real tissue. Thus we present an important milestone towards
low-loss light delivery, focusing and manipulation in deep tissue.
