Optical excitation of spin-triplet states of two-electron donors in silicon

In this paper, we propose a method for resonant optical excitation of ortho states of two-electron donors in silicon, direct transitions to which from the ground state are extremely suppressed in case of a weak spin-orbit coupling. Excitation is proposed to be carried out using the points of anti-crossing of ortho and para states under conditions of uniaxial stress of the crystal. In these points the states cannot be unambiguously assigned to any group of states with a certain spin, as a result of which the optical transition becomes allowed. The structure of the energy levels of two-electron impurities is such that the excitation of such state almost unambiguously leads to the population of the underlying ortho-type state, which is expected to be very long-lived in the case of weak spin-orbit coupling. In the present work, theoretical estimates of the cross sections for optical transitions in the vicinity of the level anticrossing point as a function of strain for strong and weak spin-orbit coupling are made.