Beams of every type spread out over distance. It's what they do. A lens can focus a beam to a spot, but after that it diverges again.
Maybe that's OK, we still don't have any requirements from the original poster yet. If you shine a beam on a sensor and detect an interruption that would form the basis of an intrusion alarm. The problem is that you can see and avoid visible light. Every cheesy Holywood movie about art thieves will show you that. So what kind of radiation can be generated and detected which is not detectable with the eyes?