Dissipative preparation of the exciton and biexciton in self-assembled quantum dots on picosecond time scales

Abstract

Pulsed resonant fluorescence is used to probe ultrafast phonon-assisted exciton and biexciton preparation in individual self-assembled InGaAs quantum dots. By driving the system using large area (≥ 10π) near resonant optical pulses, we experimentally demonstrate how phonon mediated dissipation within the manifold of dressed excitonic states can be used to prepare the neutral exciton with a fidelity ≥ 70%. By comparing the phonon-assisted preparation with resonant Rabi oscillations we show that the phonon-mediated process provides the higher fidelity preparation for large pulse areas and is less sensitive to pulse area variations. Moreover, by detuning the laser with respect to the exciton transition we map out the spectral density for exciton coupling to the bulk LA-phonon continuum. Similar phonon mediated processes are shown to facilitate direct biexciton preparation via two photon biexciton absorption, with fidelities > 80%. Our results are found to be in very good quantitative agreement with simulations that model the quantum dot-phonon bath interactions with Bloch-Redfield theory.Due to their discrete electronic structure and strong interaction with light, self-assembled quantum dots (QDs) are often described as artificial atoms in the solid state. Indeed, many quantum optical experiments have recently been performed that exploit these atom-like properties; specific examples including deterministic single [11][12][13]. Moreover, in cavity QED experiments [14] remarkable effects such as photonblockade or photon-tunneling have opened the way to exploit QDs to generate novel quantum states of light [15]. In all these experiments, the solid state environment of the QDs manifests itself primarily in coupling to acoustic phonons -an effect that is unwanted since it results in decoherence of the quantum state, particular examples include incoherent population transfer between a QD and a detuned microcavity mode [16] or intra-molecular tunneling in vertically stacked QD-molecules [17,18]. Moreover, in coherent optical exciton control experiments, coupling to acoustic phonons dominates the damping of Rabi oscillations [19,20], limiting state preparation and control fidelities and the scope for possible applications. However, in other cases the coupling to acoustic phonons was exploited instead, e.g. for the high-fidelity spin initialisation by tunnel ionisation [21] or for achieving population inversion in electrically driven quantum dots [22]. Recently, Glässl et al. [23] proposed that high-fidelity preparation of excitonic states could be achieved by exploiting the coupling of the dressed excitonic states to a quasi continuum of vibrational modes. Their approach involves to combining the relative advantages of both Rabi oscillations and rapid adiabatic passage [24,25] by making use of phonon-mediated relaxation in the presence of a strong, near resonant pulsed optical field.In this paper we investigate the phonon-assisted preparation of neutral exciton (X 0 ) and biexciton (2X) states in QDs using pulsed resonant...

Coherent and robust high-fidelity generation of a biexciton in a quantum dot by rapid adiabatic passage

“…Resonant two-photon schemes involving Rabi rotations [4,17,18] are sensitive to fluctuations in both laser power and QD optical frequency. They are likely to suffer from an imperfect biexciton preparation resulting in undesired exciton photons unrelated to the cascade process.A more robust scheme using phonon-assisted excitation was reported by several groups recently [18][19][20][21][22]. An impressively high biexciton occupation of up to 95% was demonstrated using this quasiresonant scheme [20].…”

Optical control of nonlinearly dressed states in an individual quantum dot

“…The detuning dependent shape of the phonon assisted excitation process results from the asymmetric LA -phonon QD interaction spectrum J(∆ω exc ) discussed in the previous section and Ref. [25,[40][41][42][43]. Note, that for the two-photon excitation scheme presented in the letter, we are blue detuned from the X 0 → 2X 0 transition by ∆ b = 0.92 meV leading to an effective LA -phonon interaction.…”

“…Here, the excitation laser is positioned exactly on resonance at E exc = 1 2 (E X 0 + E 2X 0 ). The simultaneous emission of fluorescence results from the cascaded recombination of 2X 0 via the two single-photon transitions 2X 0 → X 0 → cgs [25] indicated in Fig. 1d and is a clear signature of resonant two-photon excitation of the biexciton cgs →→ 2X [23][24][25].…”

“…To characterize the two-photon excitation of individual QDs, we operate the Schottky diode on the neutral charge stability plateau at a lattice temperature of T = 4.2 K [25]. The energy level structure of the QD is presented in Fig.…”

“…While cgs represents the crystal ground state of the system, the first optical excitation corresponds to a charge neutral exciton X 0 (a single excited electron-hole pair e − h) and the second optical excitation to a charge neutral biexciton 2X 0 (two e − h pairs). The optically active one-photon transitions from 2X 0 → X 0 and X 0 → cgs are detuned by 2∆ b with respect to each other due to the different attractive Coulomb interactions in the X 0 and 2X 0 states respectively [25,34]. This detuning 2∆ b allows to directly address the biexciton state 2X 0 from the cgs via a resonant two-photon excitation process cgs →→ E 2X 0 by red detuning the excitation laser from the X 0 → cgs transition by half the binding energy ∆ b = 1 2 (E X 0 − E 2X 0 ).…”