When the excited electron emits a photon, it is moving. Therefore, the motion of the photon is affected by the motion of its source, Electron. So the photons have 3-Dimentional motion, helical motion. To have the tight spot beam of laser we should consider this motion. The front view of helical motion is a circle with the radius of R, so it is not possible to have a tight spot with radius less than the R.
