There are plenty of circuits around for photo-electric break beam applications, most use infra red light rather than a laser but the principal is the same.
A lot of people are going to point out the obvious flaw with using a laser beam that can be seen as 'trip-wire' alarm, if the would be intruder can see it , what stops them from just stepping over it?
https://www.nlectc.org/perimetr/Full2.htm
All is not lost however, have a read of the above link and see if this gives any further ideas...
Must admit, though it's fiddly to set up, a Laser Fence is a damm good visible deterrent.
Add a couple of loudspeakers to provide that low hum to suggest in the mind the hidden slicing power of the lasers for added effect