Kushal C. Wijesundara

Quantum-confined Stark effects in coupled InAs/GaAs quantum dots

We report the effects of tunnel coupling on the Quantum-Confined Stark Effect (QCSE) for excitons in InAs/GaAs coupled quantum dots (CQDs). As the barrier separating the dots is reduced, the zero-field dipole moment and the polarizability are both found to increase. This systematic variation as a function of barrier thickness is due to factors including the formation of molecular wavefunctions, the electron/hole effective masses, and the CQD structural properties. The dipole moment for the interdot exciton is found to be up to 100 times larger than that of the intradot exciton resulting in a predominantly linear shift with field. The ability to control the QCSE of the exciton in a single CQD could be useful for a new class of single photon optical switches and tunable emitters.

Electric field control of a quantum dot molecule through optical excitation

Nonresonant optical excitation of a coupled quantum dot system was seen to generate a shift in the electric-field-dependent photoluminescence spectra. By monitoring the interdot recombination associated with an electron and hole in different dots we were able to precisely monitor the internal electric field generated. Power, wavelength, and applied field dependence of the charging was studied. Such an optically generated electric field may provide a means for applying local oscillating voltages, allowing for optical tuning of the device parameters.

Electric Field Tunable Exchange Interaction in InAs/GaAs Coupled Quantum Dots

Spin manipulation in coupled quantum dots is of interest for quantum information applications. Control of the exchange interaction between electrons and holes via an applied electric field may provide a promising technique for such spin control. Polarization dependent photoluminescence (PL) spectra were used to investigate the spin dependent interactions in coupled quantum dot systems and by varying an electric field, the ground state hole energy levels are brought into resonance, resulting in the formation of molecular orbitals observed as anticrossings between the direct and indirect transitions in the spectra. The indirect and direct transitions of the neutral exciton demonstrate high and low circular polarization memory respectively due to variation in the exchange interaction. The ratio between the polarization values as a function of electric field, and the barrier height was measured. These results indicate a possible method of tuning between indirect and direct configurations to control the degree of exchange interaction.

Oscillatory acoustic phonon relaxation of excitons in quantum dot molecules

We study electrically tunable self-assembled InAs quantum dot molecules through photoluminescence (PL) and time-resolved PL measurements. For the model we assume quantum dots with cylindrical symmetry, for which the confinement potentials have been modeled as narrow quantum wells in the growth and in-plane directions matched to parabolic potentials. We focus on the hole scattering rates by bulk acoustic phonons, as these rates are the leading contribution for the neutral indirect exciton relaxation rate when the electron localizes primarily on one dot. The hole–phonon scattering structure factor for acoustic phonons is found to contain a phase relationship between the phonon wave and the hole wave function, which can be tuned by an external electric field. The phase relationship leads to interference effects and tunable oscillatory relaxation rates of indirect excitons, in agreement with experiments.

Ultra-compact low-power ICO/VCO circuits with double gate MOSFETs

As the CMOS device scaling reaches to sub 20 nm scale by 2016 [1], multi-gate MOSFET architectures are poised to replace the conventional bulk devices [2]. Although non-planar multi-gate architectures can come in various forms including double-gate (DG) MOSFETs, FinFET, MIGFET, Π-gate or Ω-gate versions, they operate on the same premise of increasing current drive via multiple channels and better electro-static gate control. As a result, these compact devices can not only maximize the ION/IOFF ratio for logic switching, but also possess significant potential for analog/RF applications such as tunable circuits, mixers and current/voltage controlled oscillators, facilitated by cross modulation between multiple gates, high cutoff frequency, high intrinsic gain and near ideal gm/Id ratios [3].

Stark Effect and the Measurement of Electric Fields with Quantum Dot Molecules

Using the physically separated electron and hole of an interdot exciton in a quantum dot molecule we have studied local electric fields with extremely high resolution. By monitoring the interdot exciton energy we have measured an electric field generated through non-resonant excitation in a Schottky device. A maximum optically generated field of ∼3.25 kV/cm was observed which corresponds to 5.04% of the total applied field. The time decay of the field was found to be in the range of 110–140 μs while the onset of the field was shorter than our experimental resolution (7–8 μs).

Spin effects of charged exciton states in electric field tunable quantum dot molecules

Tuning the relative energy levels in coupled quantum dots with an applied electric field results in controllable spin interactions of bound carriers. These interactions may provide new directions in engineering these systems for optical and spintronic applications. Using polarization resolved photoluminescence experiments we observe spin dependent effects in the spectra which vary as a function of applied field. As we tune the exciton emission we observe variations in circular polarization memory in the neutral exciton and in the singly charged exciton states. We propose possible mechanisms for externally manipulating the spin coupling.

Characterization of the Shell Structure in Coupled Quantum Dots through Resonant Optical Probing

Excited states in single quantum dots (QDs) have been shown to be useful for spin state initialization and manipulation. For scalable quantum information processing it is necessary to have multiple spins interacting. Therefore, we present initial results from photoluminescence excitation studies of excited states in coupled quantum dots (CQDs). Due to the rich set of possible excitation and recombination possibilities, a technique for visualizing photoluminescence excitation in coupled quantum dots is discussed, by which both the interaction between the dots and the type of absorption and emission that generated the photoluminescence is easily and clearly revealed. As an example, this technique is applied to characterize the shell structure of the hole in the top dot and the results are compared with those using Level Anti-Crossing Spectroscopy (LACS).