On page 348 of the geant4 User’s guide and applications manual (refer to following link) http://ftp.tku.edu.tw/Linux/Gentoo/distfiles/BookForAppliDev-4.10.03.pdf
it states that "Pol01 - interaction of polarized beam (e.g. circularly polarized photons) with polarized target"
On lines 25 and 26 in the histo.mac file of the Pol01 example it has the following two lines of instructions...
/gun/polarization 0. 0. -1.
/gun/particle gamma
The direction of this gamma beam is along the z-axis, and so, assuming the code is correct, the first line of code cannot be describing the polarization state of the electric field. Am I to take it then that, in this context, the first line defines the photon spin projection, and therefore it is defining a circularly polarized photon, either left or right depending on which convention geant4 uses?
Reading the code, Geant4 treats the three vector you use in /gun/polarization for a photon as the S1,S2,S3 components of a Stokes vector in calculations described in the Wikipedia article below.
https://en.wikipedia.org/wiki/Stokes_parameters
A (0,0,1) vector will represent circularly polarized light.