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passive_fist comments on Shut up and do the impossible! - Less Wrong
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Maybe it's just that the word 'impossible' is overused. In my opinion, the word should only be reserved for cases where it is absolutely and without a doubt impossible due to well-understood and fundamental reasons. Trisecting angles with a straight edge and compass is impossible. Violating the law of conservation of energy by an arrangement of magnets is impossible. Building a useful radio transmitter that does not have sidebands is impossible. Often people use the word impossible to mean, "I can't see any way to do it, and if you don't agree with me you're stupid."
Am I mistaken, or are you using a definition of 'radio transmitter' that excludes a variable-intensity 640 kHz laser?
No. Anything which is not a constant-intensity sinusoidal wave in the time domain will have non-zero bandwidth in the frequency domain.
Varying the intensity of a laser will give its output sidebands. To transmit more data, you need to vary the intensity at a faster rate, which will make the sidebands wider.
Will varying the intensity of a constant wavelength of EMR produce radiation of a higher frequency? Solid red light e.g. can't provide the energy needed for a given photoelectric cell to function, regardless of the intensity of the light; but if the intensity of the red light varies fast enough, the higher frequency sideband radiation can?
Can this effect be duplicated with a fast enough shutter, if the required energy is close enough to the energy in a continuous beam?
Yes, although in the case of converting red light to e.g. blue light, the shutter frequency would have to be on the order of several hundred terahertz. Something capable of interacting with the EM field at several hundred terahertz, however, would need to have many unusual properties. It would not look like a conventional shutter in any sense.
This is the principle of operation of the optical frequency multiplier: http://en.wikipedia.org/wiki/Optical_frequency_multiplier
Basically, you use a nonlinear crystal that in essence lets through a varying amount of light based on the phase of the EM field. It is like an imperfect (in the sense of never completely 'closing'), very high-frequency shutter.