Is it possible to build a transmitter that would be able to emit electromagnetic waves of any possible frequency?

[Racer]

If you build a transmitter with a frequency of about 700 THz

will its antenna start emitting visible violet light?

[swansont]

If you could build one, then yes. 700 THz has a wavelength of just over 400 nm, so it would be blue. The trick is making such a device.

[Ronald Hyde]

Nature seems to able to do that, but we don't know how she does.

 

http://en.wikipedia.org/wiki/BL_Lac_object

[Enthalpy]

At least antennas are easily made for 700 THz with present semiconductor technology. They can be metallic (losses are high) or rather dielectric. Many design attempts at photovoltaic cells embed such antennas in order to concentrate light's electric field. Others make metamaterials with arrays of small antennas.

 

The generator is a completely different worry. To my knowledge, nothing has ever been made at 700 THz that looks like MHz or GHz technology. Semiconductor amplifiers stop to amplify at very few 100 GHz (clean operation at 94 GHz is getting usual), vacuum tubes provide higher power, varactors can double or triple the frequency with losses (astronomers made sensitive receivers at 300 GHz some 20 years ago to detect remote molecules).

 

So-called THz waves use to be under 1 THz (yes) or very little above. Their technology is immature: essentially short pulses of undefined frequency, like spark transmitters did before the vacuum valve was invented. This is still far from visible light, which starts at 370 THz. I believe to have ideas for CW 30 THz but need to clarify my texts and make drawings.

 

Everything above is optical technology, understand: lasers. Including down to 30 THz, which uses quantum cascade lasers or heterodyne of two IR lasers. They work up to 151nm with excimers for semiconductor manufacture, which develops technology for 13.5nm and 11nm presently. Lack of a light source, optics... has stopped semiconductor progress in the last years.

 

These lasers let atoms or molecules radiate directly, though radiation is aided by a resonant cavity (usually much bigger than the wavelength, but tuned to it) and by collective behaviour of the lasing centres.

 

Though, at least one example exists in Switzerland (ETH Zürich I believe) where the lasing medium is tiny (a minute diode if memory serves) and coupled with an L/2 metallic antenna which resonates and radiates as a bigger cavity would have.

 

An other example are simply nanocrystals, whose dimensions resonate as a dielectric antenna, which tunes the colour of their fluorescence.

 

So: resonators and antennas have already joined radio waves and optics, while amplifiers have not.