Thermal Discouragement Beam
Thermal Discouragement Beams or lasers are a puzzle element in 
Portal 2. Lasers emanate from emitters and can be used to trigger sockets and destroy turrets. They can be extended through portals, just like other testing elements. When players touch a laser, they receive a small amount of damage and are pushed away from the beam. Prolonged contact kills the player. Normal cubes block the laser, but redirection cubes point it in a different direction (usually parallel, but occasionally perpendicular to the floor).
Tip:It can sometimes be annoying to be damaged by lasers, especially when travelling through a portal with a laser in it. Consider using a point_servercommand to set sv_player_collide_with_laser 1, so the player does not take damage from lasers.Creation
The two most important elements of any laser setup are env_portal_laser and prop_laser_catcher, which create and "catch" the laser, respectively. Two optional elements are the prop_weighted_cube with CubeType set to 2 (redirection cube) and the prop_laser_relay.
External Links
Example map using this tutorial. VMF included.