New technique for the reduction of helicity-correlated instrumental asymmetries in photoemitted beams of spin-polarized electrons.

We present a new optical system that significantly reduces helicity-dependent instrumental intensity asymmetries. It is an extension of a previous scheme [Appl. Opt.47, 2465 (2008)], where one laser beam is split using a polarizing beam splitter into two with orthogonal linear polarizations. The beams are sent through a chopper, allowing only one to pass at a time. The two temporally separated beams are then spatially recombined using a second beam splitter. A liquid crystal retarder preceding the first beam splitter controls the relative intensity of the two oppositely polarized beams, allowing reduction of instrumental asymmetries. This system has been modified to include a spatial filter and a Pockels cell placed after the second beam splitter to act as a second active polarization element. Using this method, we can control instrumental asymmetries to ∼5×10(-7) in 1 h of data taking, which is comparable to the precision achieved in "second-generation" high energy electron-nuclear scattering parity violation experiments.