Scientists Found a New Way to Control the Brain with Light–No Surgery Required (singularityhub.com)

    At unprecedented depths of up to 7 mm with  millisecond  precision. 

That right there is the huge problem. Personal opinion only but using organoids [2] would permit that 7mm depth to become something useful. [1]

[1] The $100-million-plus effort aimed to develop new technologies that will produce a dynamic picture of the human brain, from the level of individual cells to complex circuits. (https://www.livescience.com/29365-human-brain.html)

[2] https://journalistsresource.org/tip-sheets/research/organoid...