sticking fluorescence (and luminescence) inside an organism of choice is not particularly hard! just add a GFP gene and you're done. if you can do any genetics at all, this is one of the first things to try.
reply"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.
replyCalcium 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.
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