…Hong recognized that biological tissues — including the brain and even the skull — are essentially transparent to infrared light, which could make it possible to deliver the light much deeper into the brain.

Since existing optogenetic tools don’t respond to infrared light, Hong’s team turned to a molecule that evolved to detect infrared’s other form: heat. By artificially outfitting specific neurons in the mouse brain with a heat-sensitive molecule called TRPV1, his team found that it was possible to stimulate the modified cells by shining infrared light through the skull and scalp from up to a meter away.

https://scienceblog.com/529268/dont-get-paranoid-researchers-control-brain-circuits-from-a-distance-with-light/