What would I use to laser etch the back of my aluminum laptop?
Google Cermark and Cerakote
+1 to Cermark,
however if your laptop is anodized or has an intentionally built up oxide layer you can laser that directly off just like other materials.
Search on Talk for Cerakote and Cermark, there are a few threads.
You can also etch a design on some materials with Molybdenum spray (“Moly”) as in https://www.evilmadscientist.com/2013/laser-moly/
I strongly recommend testing anything that is potentially destructive on scrap material before going live. With pretty much anything - painting, dyeing, cutting with a new tool . . .