Features of spectra of intracenter optical transitions in quantum molecules with quasistationary $D_2^-$-states in the electric field

Background. The interest towards optical properties of tunnel-connected semiconducting nanostructures with impurity quasistationary states is caused by a possibility of creating new sources of radiation stimulation on the basis of intrecenter optical transitions. The aim of the work is to theoretically research features of spectra of intracenter optical transitions in quantum molecules with quasistationary $D_2^-$-states, associated with the presence of 1D-dissipative tunneling, taking into account the influence of two local phonon modes in the external electric field. Materials and methods. Calculations of average binding energies of impurity quasistationary states were carried out by the method of zero radius potential in effective mass approximation. Equations of radiation transition probability and $D_2^-$-center photoexcitation probability were obtained within the theory of dipole approximation perturbation. Results. The researchers obtained dispersion equations, determining the average binding energy and broadening of levels of quasistationary g- and u-states in the external electric field with tunnel decay. In dipole approximation the authors obtained an analytical formula of the probability of electron's radiative transition from the quasistationary u-state into the quasistationary g-state of the $D_2^-$-center in a quantum dot in the presence of the external electric field and 1D-dissipative tunneling with participation of two local phonon modes. In dipole approximation the researchers obtained an analytical formula of the probability of photoexcitation of the $D_2^-$-center, connected with electron's optical transition from the quasistationary g state into the quasistationary u-state of the $D_2^-$-center in the external electric field. Conclusions. The study shows that the dependencies of the average binding energy and the broadening of energy levels of quasistationary g- and u-states on the size of the external electric field have characteristic gaps, which appear in the field dependence of radiative transition probability in the form of resonance peaks. It has been revealed that the curve of the dependence of radiative transition probability on the external electril field strength includes three peaks. The leftmost peak appears, when the emitted photon's energy is comparable to the average energy of optical transition. The other two peaks are divided by the gap and are conditioned by the presence of two pnonon modes. It is shown that the photoexcitation spectral curve's position depends on the external electric field size and such parameters of 1D-dissipative tunneling, as temperature, phonon modes' frequencies and environment interaction constant, which significantly influence the distance between energy levels of quasistationary g- and u-states of the $D_2^-$-center in a quantum molecule. There occurs a possibility in the external electric field to efficiently control the life span of quasistationary $D_2^-$-states and, correspondingly, the intracenter optical transitions, which can be used in development of new sources of stimulated THz frequency radiation.