Could an oscillator at a high enough frequency produce light instead of radio waves? Considering that light is in the 400-800 THz range, if you had an electrical oscillator that ran at that frequency connected to an aerial of some sort, would the antenna emit visible light, in the same manner that radio waves emit radio "light"?
We have specialized transistors that can run at many hundreds of gigahertz, and even some that work up to the few-terahertz regime. I can fairly easily see further advances perhaps producing oscillators at a hundred THz, which would be well into the infrared range, if not visible light.
 A: If you mean traditional circuitry then answer is likely no. At such high frequencies there are no good conductors so it would be very hard to drive such an antenna. You probably would need some waveguides rather than wires. And antenna size should be comparable to (or even slightly less than) wavelength, which further complicates things.
But such oscillators do exist. Consider LED diode, or still better - laser. Both rely on electronic transitions. So we probably can't build oscillator in question, but we can use atoms, quantum wells and dots and many others acting as such oscillator.
A: In short, no. It's not possible to generate a 600THz electrical signal. The sole reason for this is that electrons could not physically oscillate (vibrate) fast enough using the same methods as when generating radio waves (100MHz.) 
One way to have electrons (which make up electricity) have that high of a frequency, is to accelerate them to high speeds and stop them. This sudden change in motion can theoretically (stretching it here) create 600THz electrical signals.
