Energy of photoluminescence emitted by a Quantum Dot Molecule in an applied electric field

Quantum dot molecules are formed by coherent tunneling between two individual quantum dots. The coherent tunneling leads to the formation of delocalized states that are truly molecular in nature, with bonding and antibonding orbital states. Quantum dot molecules are analogous to the hydrogen molecule, with two “artificial atoms” (individual quantum dots) coupled together to form delocalized orbitals. Unlike traditional molecules, however, the degree of energy confinement in each quantum dot (analogous to electronegativity) and the degree of coupling (analogous to the separation between atoms) can each be individually tuned. Our prototype material is two InAs quantum dots stacked on top of each other and embedded in a GaAs matrix. By growing a Schottky diode structure with a doped substrate, we can apply electric fields along the growth direction that control both the total charge state of the quantum dot molecule and the relative energy levels of the two dots. The amount of energy level offset controls the degree of coupling and the formation of molecular states.

Quantum dot molecules were originally discovered in an attempt to understand how two separate quantum dots interact. Since their discovery we have learned that quantum dot molecules have a number of fascinating properties on their own. For example, the formation of delocalized states of single holes leads to tunable g factors for the holes [Doty, PRL, 2006]. The g factor determines the energy splitting between opposite spin projections as a function of magnetic field and is critically important for the development of new information processing devices that operate with single spin. Such information processing devices are particularly exciting because they could provide a mechanism for implementing novel computing architectures like quantum computing that promise dramatic improvements in processing power.

Experimental and computational data demonstrating that hole spin mixing explains complex photoluminescence spectra from a quantum dot molecule in a 6 T magnetic field.

Changes to the structure of the quantum dot molecule can lead to surprising properties. For example, changing the spacing between the two quantum dots can lead to a surprising reversal of the orbital character of the ground state – an antibonding ground state that is never found in natural molecules. This effect originates in the spin-orbit interaction and the complex light and heavy hole character of holes confined in quantum dots [Dopty, PRL, 2009]. Changing the lateral offset between the dots turns on additional apin-orbit interactions and results in a novel spin-flip tunneling mechanism that we expect will provide new tools for controlling single spins with optical pulses [Doty, PRB, 2010].

We work on the design and characterization of new quantum dot molecules with tailored properties. For example, we have demonstrated that the addition of aluminum to the barrier between the quantum dots creates an electrically tunable g factor for single electrons.  Other ongoing projects include engineering hole spin mixing and designing molecular structures that can be easily integrated into photonic cavities for all-optical control of single spins.These projects are facilitated by a strong collaboration with Dan Gammon and Allan Bracker of the Naval Research Lab.

Although vertical quantum dot molecules create a remarkable opportunity for engineering and controlling spin and photonic properties, they also face a serious limitation: because the quantum dots are stacked vertically and separated by only a few nm, it is impossible to develop devices that individually contact each quantum dot. As a result, it is challenging to use vertically-stacked quantum dot molecules as a template for scalable devices in which each quantum dot defines a logical bit. We are investigating alternative systems that could provide a scalable architecture in which individual quantum dots can be used as a logical bit, with interactions between nearest neighbors that can be turned on an off in situ. Our model systems for this investigation are lateral quantum dot molecules, produced in the group of Greg Salamo at the University of Arkansas. In lateral quantum dot molecules a single InAs quantum dot serves as a nucleation site from which two closely-spaced laterally-separated dots are grown. InAs quantum dots are typically pancake shaped, about 3-5 nm in height and about 20 nm in diameter. In a vertically-stacked geometry the centers of the pancakes are separated by only a few nm (2-20). In the lateral geometry the centers are a minimum of 40 nm apart. Because tunneling probability falls exponentially with barrier thickness, this large separation between dots leads to dramatically weaker tunnel coupling, and the spectral signatures of coupling in lateral quantum dots look different, as shown in the figure.

(a) AFM image of a lateral quantum dot molecule. (b) The device structure used to apply electric fields to the lateral quantum dot molecules. (c) Photoluminescence spectra of the ground state transitions of a single lateral quantum dot molecule as a function of voltage applied along the growth direction. (d) Electrons are controllably added to the lateral quantum dot molecules as the applied voltage increases.

We are developing a three-terminal electrical device to study coupling between individual laterally-separated quantum dots as a function of fields applied along both the growth and lateral axes. The lateral electric field controls the degree of coupling between the two dots, while a vertical field (along the growth direction) controls the charge state of the dots. This unique design gives us an unprecedented level of control and provides a critical step towards charging individual dots with a single spin, whose spin projection can serve as the logical basis, while preserving the ability to turn on and off the quantum mechanical coupling with the neighboring dots. Initial results, showing controllable charging of a single lateral quantum dot molecule, are shown in the figure.