Re: passive frequency shifting material

Hi,
the answer to your question is actually a task that I perform on a daily
basis. There are many materials which can change the frequency of incident
light when it passes through it. The most useful one for me personally is
the non linear mixing crystal. The full explaination of this property is
quite involved but it has a fairly simple principle. 
Light is a combination of oscillating (vibrating) electric and magnetic
fields, hence the name the electromagnetic spectrum. When light enters one
of these non linear crystals, the electric part of the light causes the
electric charge in the crystal (electric dipoles= two charges separated by
a distance d) to vibrate at the same frequency as the light and they then
re-emit light of this frequency. These dipoles also vibrate partly with the
square of the frequency of the incident light and this causes them to
re-emit light at double the frequency of the incident light. The conversion
efficiency of this process depends on the input power and the focussing of
the light into the crystal, but I routinely get around 20% of the incident
light coming out at twice the frequency as it went in. 
If you have the right crystal you can do all sorts of mixing of light. In
our lab we double, triple and even quadruple the incoming light to get
higher and higher energy photons. 

For less specialist materials, the reason light changes colour when you
shine  it through something is because it loses energy. If you shine UV
light onto a piece of white paper, it glows blue. This is because the white
light absorbs the UV and re-emits it at a lower energy (blue). The
difference in energy between the incoming light and the outgoing light is
called the Stokes shift. This is especially important when you think about
light bulbs. On the Las Vegas strip there are many different coloured light
s and these are all made possible by the Stokes shift. Tubes are filled
with different gases depending on the colour you want. The inside of the
glass is coated with a material which will absorb one colour and re-emit
another. The gas inside the tube is heated up by electrons smashing into it
and it loses its energy by emitting light, usually of one specific energy.
The fluorescent material coated on the inside of the glass then absorbs
this light and re-emits it at a lower energy determined by the Stokes shift
of the fluorescent material.

If you'd like more information on the physics of frequency mixing try this
book (but its advanced and needs to be for the full explaination):

Quantum Electronics by Yariv (Published by Wiley, New York)

Regards

Ben