I work with medical lasers. One variety is a frequency-doubled NdYAG laser. The non-doubled laser emits light at 1064nm whereas the frequency doubled instrument emiits at 532nm (ie half the original wavelength=2x original frequency).
My understanding is that a KTP filter (K=potassium, TP=titanyl phosphate) produces this effect but how does it do it?
I’m used to the idea of lower energy photons being radiated after a photon capture event but how can higher energy photons be created?
This is called parametric up-conversion, or frequency doubling. Part of your question may be made more obvious if you know that it’s accomplished with one pump laser, but split into two parts that shine on to the KTP crystal at different angles. The short version of the explanation is that two pump photons at the lower frequency are absorbed to emit one photon at the higher output frequency, thus conserving energy.
Longer version of the explanation coming up - stop listening now if you don’t like maths and physics! KTP responds in non-intuitive ways to the electrical field incident on it from the Nd-YAG laser beam: where most substances respond linearly to electric fields, this sort of crystal responds both linearly and quadratically (it has an internal field that’s partially dependent on the square of the external field).
If we consider an input field varying E = V sin (Wt), the squared component of the internal field will vary as sin^2 (Wt). From trig identities:
sin^2 (Wt) + cos^2 (Wt) = 1
cos^2 (Wt) - sin^2 (Wt) = cos (2Wt)
sin^2 (Wt) = (1 - cos (2Wt))/2
Thus, two beams with single frequency input field will generate within the crystal fields at frequencies of 0, W and 2W; you observe a frequency doubled output when the field oscillating at 2W couples to a photon.
If this explanation leaves you wanting more, any textbook on non-linear optics will have more details, and many introductory optics textbooks will probably have a brief mention of the phenomenon for a friendlier intro.