There is no generally established sign for "laser."
I have seen and occasionally used a sign for "LASIK" wherein an "L" hand is aimed at first one eye and then the other, which (in the context of improving one's vision) seems readily understood by most people. (However I've also seen the sign for SURGERY done as two quick, small slashes (one in front of each eye) to mean "LASIK." So, in context we have a sign that means "beam a laser into the eye as a type of surgery for the purpose of improving one's vision" -- but we don't have a single specific sign for just "laser." (Go figure.)
If you have established your context well (perhaps through a combination of fingerspelling and signs), you can use the ZAP sign to indicate a laser. The ZAP sign starts as a "cocked-X" wherein the index finger is cocked on the thumb-tip. I suppose you could call that a "modified X" or a "closed-X." Then you "aim" your hand at whatever it is you want zapped and you do a "plosive" with your mouth and lips as you flick your index finger out toward whatever it is you are zapping. That "plosive" I mentioned looks sort of like a combination between spitting, and mouthing as if you were saying "ptboo." (Remember -- you don't actually have to make any noise here. Don't do any spitting either.)
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