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Physical Review B

Coherent control in semiconductor quantum dots: Reduced optical gain thresholds via biexciton control

David Gerbasi, Gregory D. Scholes, Paul W. Brumer

Published Sep 21, 2010• 5 citations
Publisher

Abstract

Coherent control theory is used to show that one can manipulate biexciton vs exciton populations in semiconductor quantum dots (QDs) in a typical dissipative environment. Both aligned and randomly oriented quantum dots are considered, and population control is shown to be considerable. An application to CdSe QD shows that the proposed coherent control scenario results in a significant reduction in pump intensity for the onset of optical gain.

DOI
10.1103/physrevb.82.125321

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Coherent control theory is used to show that one can manipulate biexciton vs exciton populations in semiconductor quantum dots (QDs) in a typical dissipative environment. Both aligned and randomly oriented quantum dots are considered, and population control is shown to be considerable. An application to CdSe QD shows that the proposed coherent control scenario results in a significant reduction in pump intensity for the onset of optical gain.
Exciton-biexciton coherent coupling effects are examined in semiconductor quantum dots. The exciton absorption spectrum is measured with the microphotoluminescence excitation technique in a single InGaAs quantum dot. The spectrum changes from a Lorenztian-type line shape to an unusual dip-shaped line shape with increasing excitation intensities in a higher exciton state where there is a large oscillator strength between the exciton and biexciton states. The intensity dependence of the dip energy width clearly indicates that coherent Rabi oscillation occurs between the exciton and biexciton states. The absorption properties with excitation light of different polarizations show that the dip-shaped spectra only appear when there is a large biexciton state population with linear polarization. A theoretical analysis undertaken with the density matrix method agrees well with experimental results. This agreement reveals that exciton-biexciton coherent interactions lead to unusual absorption spectra and contribute crucially to the optical properties of quantum dots. The exciton-biexciton coherent effects provide a scheme for controlling four distinguishable states, which can be applied to a demonstration of quantum gate operations.
This paper discusses optical nonlinear functions in semiconductor quantum dots (QDs) and their underlying physics and possible applications. The optical functions involve controlling light with no energy dissipation. The applications include conventional devices as well as future devices based on quantum phenomena, so they will be very useful in various technological areas. Theoretical results explain the physical principle of the nonlinear functions and the concept of the coherent slowdown of light. Experiments with actual QDs show that their optical responses agree well with the theoretical results. Light absorption was successfully controlled in a QD. These results clearly reveal that QDs provide intriguing optical functions and possible innovative applications.
Coherent control of a strongly inhomogeneously broadened system, namely, InAs self-assembled quantum dots, is demonstrated. To circumvent the deleterious effects of the inhomogeneous broadening, which usually masks the results of coherent manipulation, we use prepulse two-dimensional coherent spectroscopy to provide a size-selective readout of the ground, exciton, and biexciton states. The dependence on the timing of the prepulse is due to the dynamics of the coherently generated populations. To further validate the results, we performed prepulse polarization dependent measurements and confirmed the behavior expected from selection rules. All measured spectra can be excellently reproduced by solving the optical Bloch equations for a 4-level system.
Optical techniques for the quantum control of the dynamics of multiexciton states in a semiconductor quantum dot are explored in theory. Composite bichromatic phase-locked pulses are shown to reduce the time of elementary quantum operations on excitons and biexcitons by an order of magnitude or more. Analytic and numerical methods of designing the pulse sequences are investigated. Fidelity of the operation is used to gauge its quality. A modified quantum Fourier transform algorithm is constructed with only Rabi rotations and is shown to reduce the number of operations. Application of the designed pulses to the algorithm is tested by a numerical simulation.
The focus of the thesis is nonlinear transient optical properties and coherent control of single interface fluctuation quantum dots (QD's). When single exciton and biexciton resonances were driven by strong resonant laser pulses, Rabi oscillations of these transitions were observed in the differential transmission (DT) geometry. These results extend the concept of coherent control to the limit of single quantum confined systems in solids and have important implications for the feasibility of building quantum information processing devices using semiconductor QD's. Exciton Rabi oscillations are equivalent to arbitrary single-bit operations. The pulsed control of the exciton-biexciton transition is utilized as the optical operation for building a two-bit controlled rotational gates (CROT) in a single QD. A fidelity of 0.79 was obtained from numerical evaluation of the gate operation itself assuming typical decay parameters measured from these dots. Next, the nonradiative quantum coherence between two orthogonally polarized excitons confined in a single QD was studied. By using pulses with large enough bandwidth and the proper polarization, two exciton states are simultaneously excited. The quantum beats between the two exciton states were temporally resolved as the relative phase between the two dipole oscillations changed. It was found that the nonradiative coherence time is limited by the lifetimes of the excitons involved even in the strong field regime. In the basis of the pseudo-spin Bloch vectors of exciton transitions, the total wavefunction following the pulsed excitation is an entangled state, i.e. the wavefunction is non-factorizable. Finally, the quantum coherence between the biexciton, exciton, and the crystal ground states was studied in a novel quantum beat experiment, where the oscillations only existed when the pump and probe pulses overlapped in time. Laser pulses generated after a passive pulse shaper with just enough bandwidth to cover both the exciton and biexciton transitions were used in this experiment. The appearance of the beats is indicative of a reasonably long coherence time between the biexciton and other states even in the presence of strong pulsed excitations.
Summary form only given. The optical properties of semiconductor quantum dots have been studied intensively in the past few years. Recently the experiment has demonstrated not only the ability to probe a single QD but also how to coherently control the exciton in a single QD on a time scale that is short compared with the decoherence time. We had proposed a scheme to use the excitons in QD as qubits and to use strong ultra-fast laser pulses to manipulate the qubits via Rabi flopping. As a result a full understanding of the multi-exciton coherent nonlinear spectroscopy with strong pump field is needed. In this work we present a non-perturbative calculation of the nonlinear CW pump-probe spectra.