  While controlling particle diffusion in a confined geometry is a popular
approach taken by both natural and artificial systems, it has not been widely
adopted for controlling light transport in random media, where wave
interference effects play a critical role. The transmission eigenchannels
determine not only light propagation through the disordered system but also the
energy concentrated inside. Here we propose and demonstrate an effective
approach to modify these channels, whose structures are considered to be
universal in conventional diffusive waveguides. By adjusting the waveguide
geometry, we are able to alter the spatial profiles of the transmission
eigenchannels significantly and deterministically from the universal ones. In
addition, propagating channels may be converted to evanescent channels or vice
versa by tapering the waveguide cross-section. Our approach allows to control
not only the transmitted and reflected light, but also the depth profile of
energy density inside the scattering system. In particular geometries perfect
reflection channels are created, and their large penetration depth into the
turbid medium as well as the complete return of probe light to the input end
would greatly benefit sensing and imaging applications. Absorption along with
geometry can be further employed for tuning the decay length of energy flux
inside the random system, which cannot be achieved in a common waveguide with
uniform cross-section. Our approach relies solely on confined geometry and does
not require any modification of intrinsic disorder, thus it is applicable to a
variety of systems and also to other types of waves.
