As soon as you have activated that diamond-laser-thingy you have to go outside. You may want to clear everything before you do that..
Anyway, after you pushed that lever to activate it, go outside through that long corridor.
Go through the door and climb the stairs you see infront of you. At this point, you're still in the building.
Now, take two steps outside it (you're walking on a sort of wall now) and just wait.
There is a good change that you will be attacked by one or two gassbags you didn't noticed before, but you can ignore them.
Look up at the sky (perhaps you'll also see that beam you've acivated, at this point) and after twenty seconds or so, you can see something flying down. Wait a little longer and it will land just infront of you. It is a flying elevator of some sort.
Get into it, quickly, and wait. After a few seconds it will go up again, taking you with it.