Q:   Are there any instances in which a beam of light travelling
               through a slab of material can emerge with a
               LARGER intensity?


          Yes!  If one chooses the material properly, and 
               pumps energy of just the right type into it,
               then when the light beam enters, it may 
               Stimulate the Emission of Radiation.

          Surely you've heard of LASERs and MASERs?