I dont like that one either , were looking for a single emitted photon or a line of emitted photons.
There really is no such thing as a solid line of emitted photons what process is ever going to do that ... you have to create the photons and that process is not instant I know you have looked at this
Light bulb, LED or laser there is a time and rate they can fire out photons because of the creation process do a read up on it again. In general you have to excite an atoms electron, it emits the photon and then drops back down and that all takes time there is a finite rate a chain of photons could be put out of the same atom.
That is what makes lights different intensity which is how fast they can spit out photons and how close together they are it's that whole particle nature behaviour of light thing you need how many photons per second thru a cross sectional area.
You can actually calculate how far apart each photon must be for a given light intensity see if you can work out the calculation. Google will help its a standard problem the sun for example emits 1E45 photons every second.
If you want to do an additional exercise look up the surface area of the sun and then work out on average how far apart are photons that are ejected per second on the suns surface.
I do like the phase and group velocity gif images you posted
but a 3d view would be better.
using circles instead of a line.
That is exactly what I am working on drawing and if that is as far as we can get you to that 3D point it is fine it is a lot closer to reality.
