  Waves incident to a highly scattering medium are incapable of penetrating
deep into the medium due to the diffusion process induced by multiple
scattering. This poses a fundamental limitation to optically imaging, sensing,
and manipulating targets embedded in opaque scattering layers such as
biological tissues. One strategy for mitigating the shallow wave penetration is
to exploit eigenmodes with anomalously high transmittance existing in any
disordered medium. When waves are coupled to these eigenmodes, strong
constructive wave interference enhances deeply penetrating waves. However,
finding such eigenmodes has been a challenging task due to the complexity of
disordered media. In this Letter, we present an iterative wavefront control
method that selectively enriches the coupling of incident beam to
high-transmission eigenmodes. Specifically, we refined the high-transmission
eigenmodes from an arbitrary initial wave by either maximizing transmitted wave
intensity or minimizing reflected wave intensity. Using the proposed method, we
achieved more than a factor of 3 increases in light transmission through a
scattering medium exhibiting hundreds of scattering events. Our approach is
readily applicable to in vivo applications in which only the detection of
reflected waves is available. Enhancing light penetration will lead to
improving the working depth of optical imaging and treatment techniques.
