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Register a new userSemiconductor quantum dots with light-hole exciton ground state: fabrication and fine structure
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Quantum dots (QDs) can act as convenient hosts of two-level quantum szstems, such as single electron spins, hole spins or excitons (bound electron-hole pairs). Due to quantum confinement, the ground state of a single hole confined in a QD usually has dominant heavy-hole (HH) character. For this reason light-hole (LH) states have been largely neglected, despite the fact that may enable the realilzation of coherent photon-to-spin converters or allow for faster spin manipulation compared to HH states. In this work, we use tensile strains larger than 0.3% to switch the ground state of excitons confined in high quality GaAs/AlGaAs QDs from the conventional HH- to LH-type. The LH-exciton fine structure is characterized by two in-plane-polarized lines and, ~400 micro-eV above them, by an additional line with pronounced out-of-plane oscillator strength, consistent with theoretical predictions based on atomistic empirical pseudopotential calculations and a simple mesoscopic model.
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We use temporally resolved intensity cross-correlation measurements to identify the biexciton-exciton radiative cascades in a negatively charged QD. The polarization sensitive correlation measurements show unambiguously that the excited two electron triplet states relax non-radiatively to their singlet ground state via a spin non conserving flip-flop with the ground state heavy hole. We explain this mechanism in terms of resonant coupling between the confined electron states and an LO phonon. This resonant interaction together with the electron-hole exchange interaction provides an efficient mechanism for this, otherwise spin-blockaded, electronic relaxation.
The fine structure of the neutral exciton in a single self assembled InGaAs quantum dot is investigated under the effect of a lateral electric field. Stark shifts up to 1.5 meV, an increase in linewidth, and a decrease in photoluminescence intensity were observed due to the electric field. We show that the lateral electric field strongly affects the exciton fine structure splitting due to active manipulation of the single particle wave-functions. Remarkably, the splitting can be tuned over large values and through zero.
We investigate theoretically the spectral and dynamical effects of the short-range exchange interaction between a single manganese (Mn) atom hosted by cylindrical CdTe quantum dots and its light-hole excitons or biexcitons. Our approach is based on the Kohn-Luttinger KP theory and configuration interaction method, the dynamics of the system in the presence of intraband relaxation being derived from the von Neumann-Lindblad equation. The complex structure of the light-hole exciton absorption spectrum reveals the exchange-induced exciton mixing and depends strongly on the Mn position. In particular, if the Mn atom is closer to the edges of the cylinder the bright and dark light-hole excitons are mixed by the hole-Mn exchange alone. Consequently, their populations exhibit exchange-induced Rabi oscillations which can be viewed as optical signatures of light-hole spin reversal. Similar results are obtained for mixed biexcitons, in this case the exchange-induced Rabi oscillations being damped by the intraband hole relaxation processes. The effect of light-hole heavy-hole mixing is also discussed.
Preparation of a specific quantum state is a required step for a variety of proposed practical uses of quantum dynamics. We report an experimental demonstration of optical quantum state preparation in a semiconductor quantum dot with electrical readout, which contrasts with earlier work based on Rabi flopping in that the method is robust with respect to variation in the optical coupling. We use adiabatic rapid passage, which is capable of inverting single dots to a specified upper level. We demonstrate that when the pulse power exceeds a threshold for inversion, the final state is independent of power. This provides a new tool for preparing quantum states in semiconductor dots and has a wide range of potential uses.
In a charge tunable device, we investigate the fine structure splitting of neutral excitons in single long-wavelength (1.1mu m < lambda < 1.3 mu m) InGaAs quantum dots as a function of external uniaxial strain. Nominal fine structure splittings between 16 and 136 mu eV are measured and manipulated. We observe varied response of the splitting to the external strain, including positive and negative tuning slopes, different tuning ranges, and linear and parabolic dependencies, indicating that these physical parameters depend strongly on the unique microscopic structure of the individual quantum dot. To better understand the experimental results, we apply a phenomenological model describing the exciton polarization and fine-structure splitting under uniaxial strain. The model predicts that, with an increased experimental strain tuning range, the fine-structure can be effectively canceled for select telecom wavelength dots using uniaxial strain. These results are promising for the generation of on-demand entangled photon pairs at telecom wavelengths.
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