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Physical review. B./Physical review. B

Detuning dependence of Rabi oscillations in an InAs self-assembled quantum dot ensemble

Takeshi Suzuki, Rohan Singh, M. Bayer, Arne Ludwig, Andreas Dirk Wieck, Steven T. Cundiff

Published Apr 4, 2018• 30 citations
Publisher

Abstract

Inhomogeneous broadening in ensembles of semiconductor quantum dots (QDs) has hindered coherent operations due to the detuning effects, caused by the large fluctuation in the QD transition energy due to size dispersion. This difficulty is especially evident when using femtosecond laser pulses for excitation. Here, the authors successfully measure detuning-dependent coherent evolution for a QD ensemble by employing prepulse two-dimensional coherent spectroscopy. The dephasing mechanism is found to be significantly different from that observed using picosecond lasers. This work presents a novel method to explore collective phenomena, such as superfluorescence, as well as motivating future studies of coherent interactions in the femtosecond regime.

DOI
10.1103/physrevb.97.161301

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Inhomogeneous broadening in ensembles of semiconductor quantum dots (QDs) has hindered coherent operations due to the detuning effects, caused by the large fluctuation in the QD transition energy due to size dispersion. This difficulty is especially evident when using femtosecond laser pulses for excitation. Here, the authors successfully measure detuning-dependent coherent evolution for a QD ensemble by employing prepulse two-dimensional coherent spectroscopy. The dephasing mechanism is found to be significantly different from that observed using picosecond lasers. This work presents a novel method to explore collective phenomena, such as superfluorescence, as well as motivating future studies of coherent interactions in the femtosecond regime.
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.
Light matter coherent interactions require that the coherent state induced in the matter be maintained for the duration of the observation. The only way to induce and observe such interactions in room temperature semiconductors, where the coherence time is of the order of a few hundred femtoseconds, is to use ultrashort pulse excitations and an ultrafast characterization technique. For media comprising an ensemble of nanostructure semiconductors such as self-assembled quantum dots, the gain broadening inhomogeneity also affects the interaction. Moreover, when gain media in the form of an active waveguide, such as optical amplifiers, are used, the interaction is distributed and includes nonresonant incoherent phenomena that occur simultaneously with the coherent effects. Such a complex system can exhibit, nevertheless, clear coherent interactions even at room temperature. Using InAs/InP quantum dot and wirelike quantum dash amplifiers, Rabi oscillations, self-induced transparency, coherent control using spectral pulse shaping, Ramsey interference, and photon echo have been demonstrated. The characterization employed cross frequency resolved optical gating, and the experiments were accompanied by a comprehensive finite difference time domain model that solves the Maxwell and Lindblad equations. This work has major implications on the understanding of the details of dynamical processes in active semiconductor devices, on short pulse generation from semiconductor lasers, and on various future quantum devices.
Interaction of light with atomic-like solid-state systems, such as semiconductor quantum dots (QDs) with discrete energy levels, is key for the implementation of high-speed quantum gates and information processing in semiconductor-based devices. Of particular interest are degenerate multiwave-mixing processes under resonant conditions that establish strong light–matter interactions beyond the limits of perturbative nonlinear optics. Here, we exploit a multitude of picosecond optical pulses to manipulate the unitary evolution of excitons in (In,Ga)As QDs and address the role of resonance detuning in an inhomogeneously broadened ensemble. In particular, we use the two-pulse photon echo sequence to uncover the coherent dynamics of charged excitons, whereas the areas of two additional control pulses serve as tuning knobs for adjusting the magnitude and timing of the coherent emission in a wide range up to several hundred of picoseconds. Furthermore, we make use of the spin degeneracy of the ground and excited states of charged QDs to control the polarization state of the emitted light. We reveal that control pulses, whose durations are comparable to the dephasing time of the ensemble, lead to a temporal sorting of optical multiwave-mixing processes, which is in accordance with the developed theoretical model. Lifting the temporal degeneracy allows us to smoothly trace the transition from the perturbative to the regime of Rabi rotations and opens up new possibilities for the optical investigation of complex energy-level structures in so far unexplored material systems.
We present measurements of the dephasing time and of optical Rabi oscillations in the excitonic ground-state transition of an InGaAs quantum dot ensemble at low temperature, using an heterodyne pump–probe technique. Dephasing times of several hundred picoseconds are measured, enabling coherent light–matter interaction with picosecond optical pulses. Optical Rabi oscillations which are damped versus pulse area and change period when changing pulse duration are observed. Comparison with calculations indicates that the observed damping is not intrinsic to a single dot. Dephasing processes and the biexciton resonance change the amplitude and the period of the oscillations, respectively, while the damping versus pulse area is due to a distribution of transition dipole moments in the ensemble.
We demonstrate the hallmark concept of periodic collapse and revival of coherence in a room-temperature ensemble of quantum dots (QDs) in the form of a 1.5-mm-long optical amplifier. Femtosecond excitation pulses induce coherent interactions with a number of discrete homogeneous QD subgroups within an inhomogeneously broadened ensemble, which interfere constructively to induce coherent revival (CR). The amplitude decay of CR is dictated by the QD homogeneous linewidth, thus enabling its extraction in a double-pulse Ramsey-type experiment. The more common photon echo technique is also invoked and yields the same linewidth provided that both experiments use the same bias. A dephasing time T2{T}_{2}T2​ longer than 5 ps is extracted. This is a record long T2{T}_{2}T2​ for a room-temperature QD ensemble and testifies to the high quality of the InAs/InP QDs used in the experiments. Measured electrical bias and temperature dependencies of the transverse relaxation times (T2{T}_{2}T2​ and T2∗{T}_{2}^{*}T2∗​) enable the determination of the two main decoherence mechanisms: carrier-carrier and carrier-phonon scatterings.