  Deep levels control the space charge in electrically compensated
semi-insulating materials. They limit the performance of radiation detectors
but their interaction with free carriers can be favorably exploited in these
devices to manipulate the spatial distribution of the electric field by optical
beams. By using semi-insulating CdTe diodes as a case study, our results show
that optical doping functionalities are achieved. As such, a highly stable,
flux-dependent, reversible and spatially localized space charge is induced by a
line-shaped optical beam focused on the cathode contact area. Real-time
non-invasive imaging of the electric field is obtained through the Pockels
effect. A simple and convenient method to retrieve the two-dimensional electric
field components is presented. Numerical simulations involving just one deep
level responsible for the electrical compensation confirm the experimental
findings and help to identify the underlying mechanism and critical parameters
enabling the optical writing functionalities.
