🚀 go-pugleaf

>Frequency multipliers do exist, though.

>One application of their use is to convert  the output of a high power
>laser; eg: a Nd-YAG laser (IR light); to higher frequency, eg, visible light.

>The doubler is grossly inefficient, expensive, and is usable because
>the laser gives off lots of watts (large photon flux so multi photon aborption
>is more probable).  

Doublers are not inefficient. 5 mm long potassium titanyl phosphate

( K TiO PO ) crytsals will double 1.064 micron light with typical
          4

efficiencies of 60 to 75%. With two in series you can get total
90% efficiency. They are expensive ($2700 for a 5x3x3mm crystal).
They do need large powers (say 50 watts to get 60% efficiency).

Multiphoton absorption is to  be avoided like the plague - if it happens,
the crystal EXPLODES! Whoops, $2700 down the drain! 
These things work on nonlinear index of refraction, not absorption.

Doug McDonald