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physica status solidi (b)

Transient absorption spectroscopy of a single lateral InGaAs quantum dot molecule

Christian Wolpert, Lijuan Wang, Armando Rastelli, Oliver G. Schmidt, Harald W. Giessen, Markus Lippitz

Published Feb 16, 2012• 3 citations
Publisher

Abstract

Abstract Lateral quantum dot molecules promise a scalable approach to quantum registers. The individual neutral exciton states can be controlled by a lateral bias voltage. In this way radiative emission of localized excitons can be switched between the two quantum dots in the molecule. Here, we report on coherent absorption spectroscopy of a single quantum dot molecule. By transient reflection spectroscopy we demonstrate that not only the emission but also the absorption switches from one dot to the other when varying the applied electric field. This result is consistent with Rabi flopping showing a radically suppressed dipole moment for the non‐emitting exciton transition

DOI
10.1002/pssb.201100783

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Abstract Lateral quantum dot molecules promise a scalable approach to quantum registers. The individual neutral exciton states can be controlled by a lateral bias voltage. In this way radiative emission of localized excitons can be switched between the two quantum dots in the molecule. Here, we report on coherent absorption spectroscopy of a single quantum dot molecule. By transient reflection spectroscopy we demonstrate that not only the emission but also the absorption switches from one dot to the other when varying the applied electric field. This result is consistent with Rabi flopping showing a radically suppressed dipole moment for the non‐emitting exciton transition
Transient nonlinear optical spectroscopy, performed on excitons confined to single GaAs quantum dots, shows oscillations that are analogous to Rabi oscillations in two-level atomic systems. This demonstration corresponds to a one-qubit rotation in a single quantum dot which is important for proposals using quantum dot excitons for quantum computing. The dipole moment inferred from the data is consistent with that directly obtained from linear absorption studies. The measurement extends the artificial atom model of quantum dot excitonic transitions into the strong-field limit, and makes possible full coherent optical control of the quantum state of single excitons using optical pi pulses.
Abstract In a single self‐assembled InGaAs quantum dot, the one‐exciton ground‐state transition defines a two‐level system, which appears as an extremely narrow resonance of only a few μeV width. The resonant interaction of this two‐level system with cw laser fields can be studied in detail by photocurrent spectroscopy, revealing the fine structure of the excitonic ground state as well as the effects of nonlinear absorption and power broadening. For the case of pulsed laser fields and in the absence of decoherence, the two‐level system represents a qubit. Excitations with ps laser pulses result in qubit rotations, which appear as Rabi oscillations in photocurrent experiments. Double pulse experiments further allow us to infer the decoherence time and to perform coherent control on a two level system.
In this thesis, the coherent interaction of single semiconductor quantum dots and ultrafast optical pulses is studied. Under certain conditions, localized exciton transitions in quantum dots can be seen as semi-isolated two-level systems. While this description is sufficient for the explanation of some observations in coherent experiments, it is sometimes necessary to explicitly consider coupling of the discreet quantum states confined to the dot with the environment. We start out from simple, classical examples of coherent spectroscopy and then turn towards experiments where the interaction with the vicinity of the dot becomes an important factor. First, a novel method for transient differential reflectivity spectroscopy of single quantum systems is introduced. It is a pure far-field optical technique which does not require any sophisticated sample preparation steps which makes it applicable to a broad range of structures. Pump pulses excite the sample structure and probe pulses read out the pump-induced changes in the system after a variable delay time. In the case of a single dipole, the signal is given in the form of the spectral inteferogram between the backscattered wave from the particle and the probe light which is reflected at the sample surface. This form of homodyne detection amplifies the weak scattered wave from the particle and thus makes this kind of spectroscopy for single quantum dots feasible. In the remainder of this thesis our spectroscopic method is applied to either characterize the coherent properties of single quantum dots, to prepare and read-out a desired quantum state or to deliberately manipulate them. Coherence times and oscillator strengths are determined for localized exciton transitions. Arbitrary population states can be written by driving coherent population oscillations using resonant pulses, while entangled superpositions of two exciton states in a single dot are investigated by quantum beats on transient differential spectra. We finally exploit the interaction between the dot and a nearby absorbing layer to switch the dot's absorption spectrum on ultrafast timescales via light-induced transient electric fields.
We review our recent advances in four-wave mixing spectroscopy of single semiconductor quantum dots using heterodyne spectral interferometry, a novel implementation of transient nonlinear spectroscopy allowing the study of the transient nonlinear polarization emitted from individual electronic transitions in both amplitude and phase. We present experiments on individual excitonic transitions localized in monolayer islands of GaAs/AlAs quantum wells and in self-assembled CdTe/ZnTe quantum dots. We investigate the formation of the photon echo from individual transitions, both with increasing number of transitions in the ensemble, and in the presence of temporal jitter of the energy of a single transition. The detection of amplitude and phase of the signal allows the implementation of a two-dimensional femtosecond spectroscopy, in which mutual coherent coupling of single quantum dot states can observed and quantified.
This thesis presents experiments carried out into the coherent properties of charge carriers confined within a single InGaAs/GaAs quantum dot. Specifically the ground state - single exciton qubit system is controlled with ultrafast optical pulses and then the population of the dot is measured by a high-sensitivity photocurrent detection technique. There are two principal results of these experiments, firstly by modulating the bias applied to the quantum dot diode, the detection efficiency of the photocurrent measurement technique is enhanced. The enhancement is possible as the modulated bias allows fast switching of the voltage-dependent carrier tunneling rates. Secondly, when the transition is addressed by an intense, positively detuned laser, the qubit system can be driven to a population inversion. This occurs due to phonon-assisted relaxation of the optical dressed states of the system. Here, the rapid thermalization due to the exciton-phonon interaction actually enables the population inversion, instead of simply being a source of decoherence.
We report on observation of spontaneous emissions from a single quantum dot after resonant excitation. To capture weak emissions from a single quantum dot, reflection of an excitation laser was reduced by applying obliquely incident geometry and crossed polarization configuration. In addition, collected emissions were temporally resolved to be separated from residual reflection. These allowed us to realize a simple manipulation and read out of an exciton qubit. We observed an exciton Rabi oscillation in excitation amplitude dependence of the emission intensity. We determined an exciton dipole moment from the Rabi oscillation and from the emission decay time, independently. Comparison between the two values shows reasonable agreement.