The wavefunction of a particle confined within a quantum dot molecule is distributed over the entire molecular structure, which includes the barrier between the dots. Recently we discovered that the g-factor for a charge carrier confined in the molecular orbital changes with the application of an electric field. This effect occurs because the amplitude of the wavefunction within the barrier is tuned by the application of the electric field, which varies the relative energies of the ground states of each dot. Because the barrier has different material properties than the quantum dots, a change in the amplitude of the wavefunction in the barrier results in a change in the overall properties of the confined carrier. This technique could provide the basis of new electrical or optical techniques to control spins and spin interactions. We are studying the detailed physical mechanisms that create this effect, including the effect of spin-orbit interactions on hole states.

STAFF
Graduate Student:
Weiwen Liu

COLLABORATORS
Dan Gammon, Naval Research Laboratory
Allan Bracker, Naval Research Laboratory

PUBLICATIONS

M.F. Doty, M. Scheibner, I.V. Ponomarev, E.A. Stinaff, A.S. Bracker, V.L. Korenev, T.L. Reinecke, D. Gammon. Electrically tunable g-factors in quantum dot molecular spin states. Phys. Rev. Lett. 97 197202 (2006)

A.S. Bracker, M. Scheibner, M.F. Doty, E.A. Stinaff, I.V. Ponomarev, J.C. Kim, L.J. Whitman, T.L. Reinecke, D. Gammon. Engineering electron and hole tunneling with asymmetric InAs quantum dot molecules. Appl. Phys. Lett. 89, 233110 (2006)

M.F. Doty, M. Scheibner, A.S. Bracker, I.V. Ponomarev, T.L. Reinecke, D. Gammon. Optical spectra of doubly charged quantum dot molecules in electric and magnetic fields Phys. Rev. B. 78 115316 (2008)

M.F. Doty, J.I. Climente, M. Korkusinski, M. Scheibner, A.S. Bracker, P. Hawrylak, D. Gammon. Antibonding molecular ground states in semiconductor artificial molecules. in review, cond-mat arXiv:0804.3097v1 (2008).

In a Quantum Dot Molecule, electrons and holes can be spatially distributed into two separate quantum dots. This “indirect ” exciton has a recombination energy that depends strongly on applied electric field and an optical dipole that is significantly reduced because of the reduced overlap of electron and hole wavefunctions. This reduced overlap should lead to changes in the excitonic lifetime, which is typically on the order of 1 ns for a “direct” exciton. Longer excitonic lifetimes could result in longer carrier or spin storage times. Control of exciton lifetimes is also critical to the design and performance of intermediate band solar cells. In this project we study the lifetimes of indirect excitons using time-correlated single photon counting.

STAFF
Graduate Student:
Weiwen Liu

COLLABORATORS
Dan Gammon, Naval Research Laboratory
Allan Bracker, Naval Research Laboratory

PUBLICATIONS

M. Scheibner, M.F. Doty, I.V. Ponomarev, A.S. Bracker, E.A. Stinaff, V.L. Korenev, T.L. Reinecke, D. Gammon. Spin Fine-Structure in Optically Excited Quantum Dot Molecules. Phys. Rev. B 75, 245318 (2007)

A.S. Bracker, M. Scheibner, M.F. Doty, E.A. Stinaff, I.V. Ponomarev, J.C. Kim, L.J. Whitman, T.L. Reinecke, D. Gammon. Engineering electron and hole tunneling with asymmetric InAs quantum dot molecules. Appl. Phys. Lett. 89, 233110 (2006)

E.A. Stinaff, M. Scheibner, A.S. Bracker, I.V. Ponomarev, V.L. Korenev, M.E. Ware, M.F. Doty, T.L. Reinecke, D. Gammon. Optical signatures of coupled quantum dots. Science 311 636 (2006)

As electronics get increasingly small it becomes critically important to control processes and interactions on the level of single electrons and holes. Ultimately, future generations of electronics may store information in the spins of single confined electrons and holes. Quantum dots provide an ideal material for locally confining these single charges. To control interactions between spins in separate quantum dots, the dots will need to be located close to one another and the coupling between the dots must be turned on and off in situ. Vertically stacked quantum dots provide an ideal test bed for studying these interactions because molecular beam epitaxy techniques allow for precise control over the dimensions and spatial proximity of the dots. Ultimately, however, it will be impossible to access individual QDs in a vertically stacked column. Laterally ordered dots will enable the deposition of electrical gates that can individually control each dot and the coupling between them. In this project we are looking at the fundamental mechanisms of coupling between laterally ordered dots and how the coupling depends on material structure.

STAFF
Graduate Student
Shilpa Sawlani

Nanocomposite and polymer based solar cells require an interface between the matrix material (TiO2 or an organic polymer) and QDs in order to dissociate the photogenerated excitons. After dissociation, one carrier conducts through the matrix, while the other carrier conducts by hopping between the QDs. We are investigating whether the efficiency of exciton dissociation and electrical conduction can be enhanced with ordered networks of strongly coupled QDs. Techniques include absorption and photoluminescence spectroscopy and time-correlated single photon counting. This work is highly interdisciplinary: we are investigating methods to form these ordered networks of QD that range from co-sputtering of matrix and QD material to the synthesis of self-organizing biopolymers.