It is maintained in the simple e=mc^2 way. Photons have varying energy depending on wavelength.
c = 299792458
wave length of blue photon = 400nm
mass of electron = 9.11E-31kg
planck's constant h = 6.63E-34
m*c^2*2/(c/w*h) = 330000 blue photons to make 1 electron/positron pair.
Have to address this...
A photon is a quantum of energy. It cannot interact with anything but as an indivisible packet. To create a positron electron pair (which needs to be in the region of a nucleus to preserve conservation of momentum) a single photon must have at least the energy equivalent to the rest mass of BOTH the positron and an electron. This falls well within gamma frequency and could not possibly occur from any number of blue photons. Intensity has no bearing on this sort of reaction.
EDIT:
What they use with the Titan laser in Lawrence is likely what the article is trying to talk about. Though with some different details in the laser target.
A high power laser is used to focus on a high Z material (such as gold foil). This laser (billions of joules) heats the target over a few picoseconds imparting a huge kinetic energy boost to the electrons. The electrons in the presence of the massive nucleus will spontaneously produce a "virtual" photon to take away some of this kinetic energy (the field accelerates the electron, it is known as bremsstrahlung radiation if it is a real photon, it can be virtual as well though) which will can become a positron electron pair if it is of high enough energy and still in the presence of an electric field.
The amount of positrons produced will depend on the heating (the power of the laser). But note that the laser photons just heat (increase kinetic energy).. it is bremsstrahlung photons or virtual photons produced from the accelerating electrons that become the positron electron pair.
This is known as the trident process and has been well understood for years. Mind you the ability to produce the high power lasers to use it at these levels is very new. It is simply the production of positrons from bremsstrahlung like virtual radiation, whether in a particle accelerator or laser target. Generally if it is a real photon it is merely called the cascade process.
Edit 2: To add to the trident process.. it can be a bit tricky...
The virtual particles in this process are the photons, not the positrons or electrons. They are called virtual because they are not actually seen as it appears the electron upon interaction with a nucleus simply becomes a lower energy electron plus a positron and electron pair. In a traditional bremsstrahlung case the photon exists, moves around a touch, and then upon interaction with an electric field becomes a pair (cascade process). The word virtual is a bit strange really.. Whether they exist or not is moot. They are used as a construct to explain interactions and since they 'exist' for such a small amount of time and space uncertainty prevails and we cannot really know much (anything) about them. We thus call them virtual and use them to explain other interactions. Almost all standard model interactions involve them in one way or another.
