Demonstrating the decoupling regime of the electron-phonon interaction in a quantum dot using chirped optical excitation

Abstract

Excitation of a semiconductor quantum dot with a chirped laser pulse allows excitons to be created by rapid adiabatic passage. In quantum dots this process can be greatly hindered by the coupling to phonons. Here we add a high chirp rate to ultrashort laser pulses and use these pulses to excite a single quantum dot. We demonstrate that we enter a regime where the exciton-phonon coupling is effective for small pulse areas, while for higher pulse areas a decoupling of the exciton from the phonons occurs. We thus discover a reappearance of rapid adiabatic passage, in analogy to the predicted reappearance of Rabi rotations at high pulse areas. The measured results are in good agreement with theoretical calculations. DOI: 10.1103/PhysRevB.95.241306 In semiconductors, a driven electron is damped by the interaction with phonons. In the context of quantum control, phonons lead to dephasing. The electron-phonon interaction is therefore important in the development of quantum technology with semiconductors. It is a rich and subtle subject.One possible way to suppress electron-phonon damping is to drive the electronic system so quickly that the relatively large inertia of the phonons prevents them from reacting to the driven electron. In the context of Rabi oscillations, the driven oscillations of a two-level system, a "reappearance" has been predicted [1]. As the drive is increased, the Rabi oscillations are initially damped more and more by the phonons, but then the damping decreases and is eventually suppressed. The reappearance regime represents phonon-free quantum control. It has, however, never been observed experimentally. Here, we demonstrate the experimental realization of the reappearance regime. Validation comes from a full microscopic theory.Our quantum system is a single self-assembled quantum dot (QD), an emitter of highly coherent single photons [2,3] and polarization-entangled photon pairs [4,5]. Quantum control of the exciton, an electron-hole pair, proceeds on picosecond timescales well before spontaneous emission takes place (timescale ∼1 ns). Phonons lead to a deterioration of the exciton preparation fidelity for schemes using resonant excitation [1,[6][7][8][9][10]. In fact, the interaction with the phonons is sufficiently strong that an exciton state can be prepared by relying on it (phonon-mediated relaxation following excitation with a detuned pulse) [11][12][13][14][15]. In a Rabi experiment, phonons lead to a clear damping [6,7]. The specific dephasing mechanism was identified as a coupling to longitudinal acoustic (LA) phonons. For higher pulse areas, theory predicts that the electronic oscillations become so fast such that the phonons decouple and the Rabi oscillations recover, the reappearance phenomenon. The existence of a pulse area for which the coupling to the phonons is maximal is a consequence of the nonmonotonic electron-phonon coupling [1,16].For the pulses used so far experimentally (pulses of 1-10 ps duration), the reappearance regime for Rabi oscillations can only be entered at ex...

Phonon impact on optical control schemes of quantum dots: Role of quantum dot geometry and symmetry

“…Hence, the restriction to a one-dimensional q goes along with an immense saving of computational cost. In the following we will employ a fourth-order correlation expansion, which has proven to simulate experimental results very well [1,5,6].…”

“…The dressed states are the eigenstates of the coupled QD-light system [1,35]. In the dressed state picture phonons give rise to transitions between the dressed states [6,35,46] if the actual splitting is comparable to the energy range covered by the phonon spectral density. The dressed states as function of time for the Gaussian excitation with a pulse area of 10π and 20π are presented in Fig.…”

“…The second example is an excitation using a chirped laser pulse [47,48], which turned out to be very sensitive to the phonon influence and for which excellent agreement between theory and experiment has recently been shown [5,6]. It is well established that at low temperatures only negatively chirped pulses exhibit a damping due to phonons [4][5][6]27], hence we concentrate on this case here. The theory for the description of the chirped pulses can be found, e.g., in [6,27].…”

“…It is well established that at low temperatures only negatively chirped pulses exhibit a damping due to phonons [4][5][6]27], hence we concentrate on this case here. The theory for the description of the chirped pulses can be found, e.g., in [6,27]. Following Ref.…”

“…Following Ref. [6], we take a resonant Gaussian pulse [Eq. (14)] with initial pulse length of τ 0 = 80 fs and apply a chirp filter with coefficient α = −0.5 ps 2 .…”