Quantum dots (QD) have attracted great interest for applications in photonics, photovoltaics, sensors, and quantum information processing. For example, two photon biexciton-exciton cascade in a self-assembled QD is one of the most promising routes towards generation of on-demand polarization-entangled photon pairs at telecommunication relevant wavelength. Importantly, QD based sources do not rely on parametric down-conversion techniques and as such they enable generation of entangled pairs with sub-Poissonian statistics. Although two photon cascade from the QD biexciton state is a route to entangled photon generation in scalable devices, the efficiency of this process is limited by the fine-structure splitting (FSS) of exciton transitions. FSS is a many-body effect leading to a splitting of two bright exciton states into a linearly polarized doublet, thus it removes polarization entanglement. FSS originates from anisotropic electron-hole exchange interaction due to low QD symmetry. Understanding the microscopic origin of the electron-hole exchange interaction is therefore important for developing means of entangled photon pair generation. FSS can be manipulated by passive control of QDs which is obtained by tailoring dot size, shape, and composition via growth conditions. On the other hand, dynamical control of excitons in QDs is also possible and highly desirable in nanophotonics. This includes magnetic and electric fields applied to manipulate excitons, application of external strain or dressing excitons with optical fields; all of these can provide means to modify FSS. Recently we have proposed a scheme, in which an imposed nanomechanical strain is used to manipulate and control exciton FSS and emission polarization.
Continuous matter approaches like effective mass approximation or k*p method are limited by the fact that the resolution on the scale of a unit cell is lost. Proper description of true atomistic symmetry is of paramount importance if both qualitative and quantitative predictions are to be made. One of the most striking examples concerns the bright exciton splitting, which is usually observed for fully shape-symmetric dots; the k*p theory predicts no splitting in this case. Clearly, understanding atomistic details of exciton fine structure is crucial for developing schemes aiming to generate entangled photon pairs. In clear contrast, FSS is naturally emerging from atomistic theory based on tight-binding method developed by our group, which provides important predictive capability to search for systems of desired properties, before their experimental realization. This methodology enables us to include external magnetic, electric or strain fields and investigate specific field configuration for tailoring exciton states in QDs.
Development and application of atomistic models is crucial for proper description and prediction of the electronic properties of semiconductor nanostructures. In this project we intend to continue working on a theory of exciton fine structure in semiconductor QDs which accounts for electron-hole exchange including the crystal lattice symmetry, quantum dot anisotropy, external electric field, and imposed nanomechanical strain. The work will include joint effort in collaboration with Prof. P. Hawrylak and Quantum Theory Group at IMS/NRC. The work on imposed nanomechanical strain will be continued in collaboration with Dr. Garnett W. Bryant and his group from the National Institute of Standards and Technology (NIST), Gaithersburg, MD, USA.
In summary we aim to develop atomistic theory, implement high-performance computational tools and perform large scale, parallel calculation of fine structure splitting in semiconductor quantum dots. The project will be conducted in co-operation with leading international scientific groups. The project will result in development of control of exciton levels in quantum dot, aiming at efficient generation of entangled photon pairs.