Polarization Anisotropies in Strain-Free, Asymmetric and Symmetric Quantum Dots Grown by Droplet Epitaxy

We provide an extensive and systematic investigation of exciton dynamics in droplet epitaxial quantum dots comparing the cases of (311)A, (001) and (111)A surfaces. In spite of a similar s-shell exciton structure common to the three cases, the absence of a wetting layer for (311)A and (111)A samples leads to a larger carrier confinement with respect to (001), where a wetting layer is present. Moreover, this leads to a more pronounced dependence of the binding energies of s-shell excitons on the quantum dot size and to a strong anti-binding character of the positive charged exciton for smaller quantum dots. In-plane geometrical anisotropies of (311)A and (001) quantum dots lead to a large electron-hole fine interaction (fine structure splitting, FSS ~ 100 ueV) whereas for the three-fold symmetric (111)A counterpart this figure of merit is reduced of about one order of magnitude. In all these cases we do not observe any size dependence of the fine interactions. Heavy-hole/light-hole mixing is present in all the studied cases leading to a broad spread of linear polarization anisotropy (from 0 up to about 50%) irrespective of surface orientation (symmetry of the confinement), fine interactions and nanostructure size. These results are important for the further development of ideal single and entangled photon sources based on semiconductor quantum dots.

Спиновая динамика отрицательно заряженных экситонов в квантовых точках InP/(In,Ga)P в магнитном поле

The dynamics of the photoluminescence negative circular polarization of the InP/(In,Ga)P quantum dots ensemble was studied. We find that in the time-resolved dependences of the polarization there are no oscillations in Voigt magnetic field. Also, with increasing field the polarization declines to zero. Such behavior is attributed to the peculiarities of the negatively charged exciton spin dynamics, particularly, to the fact that in the negatively charged exciton ground state the spin dynamics is governed by the heavy hole. We show that magnetic field depolarization of the photoluminescence occurs once the field of dynamically polarized nuclear spins acting on electron spins is surpassed.

Quantum plasmonic control of trions in a picocavity with monolayer WS2

Monitoring and controlling the neutral and charged excitons (trions) in two-dimensional (2D) materials are essential for the development of high-performance devices. However, nanoscale control is challenging because of diffraction-limited spatial resolution of conventional far-field techniques. Here, we extend the classical tip-enhanced photoluminescence based on tip-substrate nanocavity to quantum regime and demonstrate controlled nano-optical imaging, namely, tip-enhanced quantum plasmonics. In addition to improving the spatial resolution, we use the scanning probe to control the optoelectronic response of monolayer WS2 by varying the neutral/charged exciton ratio via charge tunneling in Au-Ag picocavity. We observe trion “hot spots” generated by varying the picometer-scale probe-sample distance and show the effects of weak and strong coupling, which depend on the spatial location. Our experimental results are in agreement with simulations and open an unprecedented view of a new range of quantum plasmonic phenomena with 2D materials that will help to design new quantum optoelectronic devices.