Tomasz Jakubczyk

Radiatively Limited Dephasing and Exciton Dynamics in MoSe2 Monolayers Revealed with Four-Wave Mixing Microscopy

By implementing four-wave mixing (FWM) microspectroscopy, we measure coherence and population dynamics of the exciton transitions in monolayers of MoSe2. We reveal their dephasing times T2 and radiative lifetime T1 in a subpicosecond (ps) range, approaching T2 = 2T1 and thus indicating radiatively limited dephasing at a temperature of 6 K. We elucidate the dephasing mechanisms by varying the temperature and by probing various locations on the flake exhibiting a different local disorder. At the nanosecond range, we observe the residual FWM produced by the incoherent excitons, which initially disperse toward the dark states but then relax back to the optically active states within the light cone. By introducing polarization-resolved excitation, we infer intervalley exciton dynamics, revealing an initial polarization degree of around 30%, constant during the initial subpicosecond decay, followed by the depolarization on a picosecond time scale. The FWM hyperspectral imaging reveals the doped and undoped areas of the sample, allowing us to investigate the neutral exciton, the charged one, or both transitions at the same time. In the latter, we observe the exciton–trion beating in the coherence evolution indicating their coherent coupling.

Pronounced Purcell enhancement of spontaneous emission in CdTe/ZnTe quantum dots embedded in micropillar cavities

The coupling of CdTe/ZnTe quantum dot(QD) emission to micropillar cavityeigenmodes in the weak coupling regime is demonstrated. We analyze photoluminescencespectra of QDs embedded in monolithic micropillar cavities based on Bragg mirrors which contain MgSe/ZnTe/MgTe superlattices as low-index material. The pillar emission shows pronounced cavityeigenmodes, and their spectral shape is in good agreement with simulations. QD emission in resonance with the cavity mode is shown to be efficiently guided toward the detector, and an experimental Purcell enhancement by a factor of 5.7 is determined, confirming theoretical expectations.

Monolithic ZnTe-based pillar microcavities containing CdTe quantum dots

Micropillars of different diameters have been prepared by focused ion beam milling out of a planar ZnTe-based cavity. The monolithic epitaxial structure, deposited on a GaAs substrate, contains CdTe quantum dots embedded in a ZnTe λ-cavity delimited by two distributed Bragg reflectors (DBRs). The high refractive index material of the DBR structure is ZnTe, while for the low index material a short-period triple MgTe/ZnTe/MgSe superlattice is used. The CdTe quantum dots are formed by a novel Zn-induced formation process and are investigated by scanning transmission electron microscopy. Micro-photoluminescence measurements show discrete optical modes for the pillars, in good agreement with calculations based on a vectorial transfer matrix method. The measured quality factor reaches a value of 3100.

Inhibition and enhancement of the spontaneous emission of quantum dots in micropillar cavities with radial-distributed Bragg reflectors.

We present a micropillar cavity where nondesired radial emission is inhibited. The photonic confinement in such a structure is improved by implementation of an additional concentric radial-distributed Bragg reflector. Such a reflector increases the reflectivity in all directions perpendicular to the micropillar axis from a typical value of 15-31% to above 98%. An inhibition of the spontaneous emission of off-resonant excitonic states of quantum dots embedded in the microcavity is revealed by time-resolved experiments. It proves a decreased density of photonic states related to unwanted radial leakage of photons out of the micropillar. For on-resonance conditions, we find that the dot emission rate is increased, evidencing the Purcell enhancement of spontaneous emission. The proposed design can increase the efficiency of single-photon sources and bring to micropillar cavities the functionalities based on lengthened decay times.

In-plane radiative recombination channel of a dark exciton in self-assembled quantum dots

We demonstrate evidence for a radiative recombination channel of dark excitons in self-assembled quantum dots. This channel is due to a light hole admixture in the excitonic ground state. Its presence was experimentally confirmed by a direct observation of the dark exciton photoluminescence from a cleaved edge of the sample. The polarization resolved measurements revealed that a photon created from the dark exciton recombination is emitted only in the direction perpendicular to the growth axis. Strong correlation between the dark exciton lifetime and the in-plane hole g-factor enabled us to show that the radiative recombination is a dominant decay channel of the dark excitons in CdTe/ZnTe quantum dots.

Light-matter coupling in ZnTe-based micropillar cavities containing CdTe quantum dots

We study the coupling of CdTe quantum dots emission with ZnTe-based micropillar cavity modes. Nonresonant cavity mode feeding is reported together with an enhancement of the emission of a quantum dot thanks to resonant coupling with the cavity mode. The coupling is evidenced both in experiments with continuous and pulsed excitation. A theoretical Purcell factor is calculated and an experimental Purcell factor 5.7 is determined confirming the theoretical predictions. Additionally, we discuss the influence of the cascaded emission occurring under increased excitation power on the observed decay time of the excitonic transition.

Frequency cavity pulling induced by a single semiconductor quantum dot

We investigate the emission properties of a single semiconductor quantum dot deterministically coupled to a confined optical mode in the weak coupling regime. A strong pulling, broadening and narrowing of the cavity mode emission is evidenced when changing the spectral detuning between the emitter and the cavity. These features are theoretically accounted for by considering the phonon assisted emission of the quantum dot transition. These observations highlight a new situation for cavity quantum electrodynamics involving spectrally broad emitters.

MBE growth and characterization of a II–VI distributed Bragg reflector and microcavity lattice-matched to MgTe

Abstract We present the realization and characterization of a 20-fold, fully lattice-matched epitaxial distributed Bragg reflector based on (Cd,Zn)Te and (Cd,Zn,Mg)Te layers. We also present a microcavity based on (Cd,Zn,Mg)Te containing a (Cd,Zn)Te quantum well. Reflectivity spectra, photoluminescence imaging in real space and in far field are presented.

Far field emission of micropillar and planar microcavities lattice-matched to ZnTe

We investigate light emission from ZnTe-based microcavities containing CdTe quantum dots (QDs), with 2D (planar cavity) and 0D (pillar cavities) photonic confinement. The angular distribution from the planar cavity is presented as well as 2D cross-sections of the far field distribution of radiation from the micropillars. The efficient and desirable modification of the isotropic radiation of the QDs is shown for such structures. The diffraction observed is found to be inherent for such experiments with large numerical aperture of the lens and small diameters of the investigated pillars. This diffraction is successfully modeled.

Impact of phonons on dephasing of individual excitons in deterministic quantum dot microlenses

Optimized light–matter coupling in semiconductor nanostructures is a key to understand their optical properties and can be enabled by advanced fabrication techniques. Using in situ electron beam lithography combined with a low-temperature cathodoluminescence imaging, we deterministically fabricate microlenses above selected InAs quantum dots (QDs), achieving their efficient coupling to the external light field. This enables performing four-wave mixing microspectroscopy of single QD excitons, revealing the exciton population and coherence dynamics. We infer the temperature dependence of the dephasing in order to address the impact of phonons on the decoherence of confined excitons. The loss of the coherence over the first picoseconds is associated with the emission of a phonon wave packet, also governing the phonon background in photoluminescence (PL) spectra. Using theory based on the independent boson model, we consistently explain the initial coherence decay, the zero-phonon line fraction, and the line shape of the phonon-assisted PL using realistic quantum dot geometries.