EM wave on Em wave superposition

[iknowabsolutelynothing]

I was wondering if someone could superpose a wave say of 350 nm wavelength with one so as to just bounce it into the visible spectra.

[swansont]

You could combine the light via sum-frequency generation.

http://en.wikipedia.org/wiki/Sum-frequency_generation

[Enthalpy]

Frequency sum or difference, which would increase the wavelength as queried.

 

Difference is less commonly used because many sources of light, especially lasers, operate in the near IR or visible spectrum and users (DVD, lithography...) want visible or UV light, so frequency doublers or triplers fit.

 

Difference is used in some receivers (Lidar, a light Radar) to convert input light down to radio frequency signals for detailed processing. It was also used recently to produce permanent THz waves, from a frequency difference between far-IR light sources which were Quantum Cascade Lasers.

 

Sum, difference, multiplication need a non-linear material, which isn't very common at convenient light intensity. Some crystals behave like this, with the polarization varying as the field squared for instance (cubed is more common, as crystals are often symmetric). Then, if you input cos(w_A*t)+cos(w_B*t), the square introduces a term cos(w_A*t)*cos(w_B*t) which is mathematically cos[(w_A+w_B)*t] and cos[(w_A-w_B)*t], with the special case w_A=w_B being a frequency doubler. A cube would introduce 2*w_A+-w_B and triple frequencies.

 

This all needs fields not too small as compared with molecular polarization, far more than usual light intensity. So non-linear crystal are best placed within a fibre, or within a laser cavity. Because the effect sums up over many atoms, over a macroscopic length, all contributions must be in phase, which would require the same (simplifying) phase speed at all interesting frequencies: uncommon.

 

At even higher intensities, green or infra-red light can ionize nitrogen from air despite the photon energy is too small. The expression "multi-photon absorption" was coined for it, and people don't ask how this is compatible with quantum mechanics - just as I did with my cosines. You can also consider the Raman effect as an optical non-linearity.

 

To my knowledge, experiments searching for any tiny non-linearity of vacuum found none.