By opening a sodium ion channel in the neuron you can cause an action potential. The problem is triggering the channel to open, so how do you go about this? By charging the ion channel above 70mv you can trigger it to open. To do this it is theoretically possible to do it with an IR Laser. The idea being the laser will focus on the sodium ion channel and then charge it by 70mv then the chain reaction known as an action potential will do the rest for you. This will not work though because you can’t get a laser to focus 70mv on 0.0003µm or 0.3nm. The other option is the Neurotransmitters, by giving that area of the neuron a charge of 70mv the chemical process will be triggered and will fire the next neuron. This area is significantly larger.

If I were to use voltage-sensitive fluorescence or intracellular fluorescent dye-marking to show the firing of neurons and aim the laser at the axon bouton then my prediction is the neuron will light up showing it has fired.

The uses of this are endless in BCI. This would be a whole new way of outputting directly into the brain. This would also have many medical applications as well. This although very exciting and possible is also incredibly dangerous. We are still only scratching the surface when it comes to understanding the brain.

References 

BOISVERT, M. (2013, December 4). CONTROLLING THE  BRAIN WITH LASERS! Retrieved from Neuwrite:  https://neuwritesd.org/2013/12/04/controlling-the-brain-with-lasers/

Millisecond infrared laser pulses depolarize and  elicit action potentials on in-vitro dorsal root ganglion neurons. (2017,  September 1). Biomedical Optics Express, 8(10), 4568–4578.  doi:10.1364/BOE.8.004568

O'DONNELL, E. (2012, March). Light-Up Neurons.  Retrieved from Harvard Magazine:  https://harvardmagazine.com/2012/03/light-up-neurons

Richter, C.-P., & XiaodongTan. (2014, May).  Photons and neurons. Hearing Research, 311, 72-88.  doi:10.1016/j.heares.2014.03.008