A simple solution that may meet your need:
The area where the light is placed is surrounded by a big box (selected below) with an open face where the portal is placed.
The node tree for this surrounding box is simply a "glass" with IOR set to one (flat transparency).
The light source (an area) also has a node tree:
It is emission shader except when the transmission depth (what comes from glass/transmissive materials) is at least 1.
Additionally, the surrounding box has a solidify modifier, as if not the light may go through it at the intersection with the floor plane.
There is some limitations, depending on the shape of the surrounding box (from which points of view you want to catch that). For instance the cube's shadow is visible outside of the portal.
