When I first heard of optogenetics, I figured they had the idea completely backwards. optogenetics uses light from outside the organism to trigger actions inside cells in the organism. I thought the idea would be to put light-emitting genes inside organisms, and then "read out" the underlying biology. I learned only recently that somebody is working on adding fluorescence to cuttlefish to see "when neurons fire" (with spatial info), among other thiings: https://www.science.org/content/article/scientists-engineere...

Of course my long game is to add cuttlefish-style "pulsing, glowing" tattoos to humans.

"using light to read out from organisms" has been around since basically GFP. If you are looking for dynamics, some early work was done by, e.g. Alice Ting.

https://www.pnas.org/doi/10.1073/pnas.211564598 (2001)

Calcium imaging of neuronal activity (see, e.g., https://alleninstitute.org/education-resources/how-scientist...) is a thing, and has been for a while. It does something like what you said: neurons that have been genetically modified glow briefly, which can be picked up by microscopy. It has many advantages (and some drawbacks) compared to more traditional electrical recording techniques.