  Manipulating and focusing light deep inside biological tissue and tissue-like
complex media has been desired for long yet considered challenging. One
feasible strategy is through optical wavefront engineering, where the optical
scattering-induced phase distortions are time reversed or pre-compensated so
that photons travel along different optical paths interfere constructively at
the targeted position within a scattering medium. To define the targeted
position, an internal guidestar is needed to guide or provide a feedback for
wavefront engineering. It could be injected or embedded probes such as
fluorescence or nonlinear microspheres, ultrasonic modulation, as well as
absorption perturbation. Here we propose to use a magnetically controlled
optical absorbing microsphere as the internal guidestar. Using a digital
optical phase conjugation system, we obtained sharp optical focusing within
scattering media through time-reversing the scattered light perturbed by the
magnetic microshpere. Since the object is magnetically controlled, dynamic
optical focusing is allowed with a relatively large field-of-view by scanning
the magnetic field externally. Moreover, the magnetic microsphere can be
packaged with an organic membrane, using biological or chemical means to serve
as a carrier. Therefore the technique may find particular applications for
enhanced targeted drug delivery, and imaging and photoablation of angiogenic
vessels in tumours.
