1. A method to depolarize the plasma membrane of a neural cell in an individual comprising: (a) placing a plurality of lanthanide-doped nanoparticles in proximity to the neural cell; and (b) exposing the plurality of nanoparticles to electromagnetic radiation in the infrared (IR) or near infrared (NIR) spectrum, wherein the electromagnetic radiation in the IR or NIR spectrum is upconverted into light in the visible spectrum by the nanoparticles, and wherein a light-responsive opsin is expressed on the plasma membrane of the neural cells and activation of the opsin by the light in the visible spectrum induces the depolarization of the plasma membrane.