We study a type-II GaAs/AlAs quantum dot system that enables intervalley coupling between electronic states in the F and Xz valleys, probed optically at the single-dot level. This structure intrinsically supports both direct and indirect excitons, with their spatial separation tunable via an external electric field. Stark shift spectroscopy reveals a clear double anticrossing in the neutral exciton line as a direct signature of intervalley coupling. Experimental observations, including measurements of static dipole moments and electron g factors, confirm electron confinement within the AlAs barrier and highlight the involvement of barrier-localized states. We further propose that these states may contribute to the formation of multicharged excitonic complexes. A simplified model allows us to estimate the interfacial coupling strength from the measured anticrossing amplitudes.

We demonstrate efficient in-plane optical fiber collection of single photon emission from quantum dots embedded in photonic crystal cavities. This was achieved via adiabatic coupling between a tapered optical fiber and a tapered on-chip photonic waveguide coupled to the photonic crystal cavity. The collection efficiency of a dot in a photonic crystal cavity was measured to be 5 times greater via the tapered optical fiber compared to collection by a microscope objective lens above the cavity. The single photon source was also characterized by second order photon correlations measurements giving g(2)(0)=0.17 under non-resonant excitation. Numerical calculations demonstrate that the collection efficiency could be further increased by improving the dot-cavity coupling and by increasing the overlap length of the tapered fiber with the on-chip waveguide. An adiabatic coupling of near unity is predicted for an overlap length of 5 microns.

Semiconductor excitons are commonly seen as hydrogen atom. This analogy requires a unique hole mass. In reality, this is not so due to the complexity of the semiconductor band structure. The precise consequences on the exciton physics of the Coulomb coupling between heavy and light holes remain a tricky open problem. Through an "optimized perturbative" approach that uses excitons with a flexible hole mass as a basis, we show that for zero exciton wave vector, the heavy-light hole mass difference does not split the (2x4) exciton degeneracy in zinc-blende-like semiconductors, the hole mass for binding energy being close to the average mass inverse. By contrast, for nonzero exciton wave vector that physically breaks the crystal symmetry, the exciton degeneracy splits into two branches quantized along the exciton wave vector, with nontrivial center-of-mass dependence not only on the heavy and light hole masses, but also on the electron mass.

We use two-photon excitation to create biexcitonic complexes with two electron-hole pairs in different orbital levels of a GaAs/AlAs quantum dot. In addition to p-shell emission of the biexcitonic triplet states generated by two-photon excitation, we observe additional higher-energy resonances which are a signature of the radiative cascade of two-photon excited singlet states. The detection of these signals obtained in a high excitation regime is made possible by the use of a waveguiding structure in which the quantum dots are inserted, allowing an orthogonal excitation and detection geometry with an excellent laser rejection.

We report measurements of the radiative lifetimes and coherence times of the dark and bright excitons in an asymmetric GaAs/AlGaAs quantum dot. The dots, fabricated by partial infilling of asymmetric in situ etched nanoholes, have low symmetry, which leads to significant dark-bright mixing as demonstrated by dark-bright anticrossing in magnetophotoluminescence spectra. Using an orthogonal excitation-detection waveguiding geometry and quasiresonant excitation, we compare the coherence properties, measured by Michelson interferometry, of the dark and bright exciton from the same dot in the absence of an external magnetic field.

Photon absorption in a semiconductor produces bright excitons that recombine very fast into photons. Here, we show that in a quantum dot set close to a p-doped reservoir, this absorption can produce a dark duo, i.e., an electron-hole pair that does not emit light. This unexpected effect relies on the fact that the wave function for a hole leaks out of a finite-barrier dot less than for an electron. This difference can render the positively charged trio unstable in the dot by tuning the applied bias voltage in a field-effect device. The unstable trio that would result from photon absorption in a positively charged dot has to eject one of its two holes. The remaining duo can be made dark with a probability close to 100% after a few pumping cycles with linearly polarized photons, in this way engineering long-lived initial states for quantum information processing.

We demonstrate here electrical control of the sign of the circularly polarized emission from the negatively charged trion, going from co- to contrapolarized with respect to the circular polarization of the laser, using a GaAs/AlAs quantum dot (QD) embedded in a field effect structure. The voltage range over which the trion is negatively (contra) circularly polarized is shown to be dependent on the laser excitation energy within the $P$-shell resonance. The negative polarization never exceeds $\ensuremath{\sim}\ensuremath{-}15%$, in stark contrast to measurements on InAs/GaAs QDs reported by M. E. Ware et al. [Phys. Rev. Lett. 95, 177403 (2005).] in which a negative polarization reaching $\ensuremath{-}95%$ was observed. This result is shown to be a consequence of the low-symmetry confinement potential of these GaAs/AlAs QD, which are fabricated by partial infilling of asymmetric droplet-etched nanoholes. This low QD symmetry also leads to optical activity of the dark spin configuration of the triplet state, which we measure experimentally by photoluminescence excitation spectroscopy. A simple, semiquantitative model explaining both the optical activity of the dark spin configuration and the maximum degree of negative polarization is presented.

Symmetry breaking of the confinement in In(Ga)As quantum dots leads to mixing between the bright and dark exciton states and an increase in the oscillator strength of the dark exciton. We demonstrate here that the change in degree of linear polarization of the bright and dark excitons as a function of applied magnetic field, where both parallel and perpendicular magnetic field components are applied, can be used to quantify the dark-bright coupling. Experimental results for a dot where the coupling is weak and the dark exciton is undetectable in zero applied magnetic field are in good agreement with theoretical simulations based on atomistic calculations reported by M. Zieli\ifmmode \acute{n}\else \'{n}\fi{}ski et al. [Phys. Rev. B 91, 085403 (2015)].

We have investigated the optical properties of the nitrogen vacancy (NV)(-) center in 3C-SiC to determine the photoluminscence zero phonon line (ZPL) associated with the E-3 -> (3)A(2) intracenter transition. Combining electron paramagnetic resonance and photoluminescence spectroscopy, we show that the NV- center in 3C-SiC has a ZPL line at 1.468 mu m in excellent agreement with theoretical predictions. The ZPL line can be observed up to T = 100 K. The negatively charged NV center in 3C-SiC is the structural isomorphe of the NV center in diamond and has equally a spin S = 1 ground state and a spin S = 1 excited state, long spin lattice relaxation times and presents optically induced groudstate spin polarization. These properties make it already a strong competitor to the NV center in diamond, but as its optical domain is shifted in the near infrared at 1.5 mu m, the NV center in 3C-SiC is compatible with quantum photonic networks and silicon based microelectronics.

We show that electron paramagnetic resonance (EPR) tagged high resolution photoexcitation spectroscopy is a powerful method for the correlation of zero phonon photoluminescence spectra with atomic point defects. Applied to the case of NV centers in 4H-SiC it allows to associate the photoluminescence zero phonon lines (ZPL) at 1243, 1223, 1180, and 1176 nm with the (hk, kk, hh, kh) configurations of the NV- centers in this material. These results lead to a revision of a previous tentative assignment. Contrary to theoretical predictions, we find that the NV centers in 4H-SiC show a negligible Franck-Condon shift as their ZPL absorption lines are resonant with the ZPL emission lines. The high subnanometer energy resolution of this technique allows us further to resolve additional fine-structure of the ZPL lines of the axial NV centers which show a doublet structure with a splitting of 0.8 nm. Our results confirm that NV centers in 4H-SiC provide strong competitors for sensing and qubit application due to the shift of their optical transitions into the technology compatible near-infrared region and the superior material properties of SiC. Given that single center spin readout will be realized, they are suitable for scalable nanophotonic devices compatible with optical communication network.

We have studied the coherent spin dynamics of excitons bound to acceptors, A0X, immersed in a CdTe quantum well by using time resolved photo-induced Faraday rotation. We have also measured the time-resolved differential transmission in order to determine a A0X lifetime of 220 ps, which is independent of the applied magnetic field. We show that at low magnetic field, the spin of A0X is completely frozen during a time, ≅ 4.5 ns, at least twenty times longer than its lifetime. We compare the spin properties of A0X with the spin properties of other charged excitons systems, and we conclude that the hyperfine interaction of the photo-created electron spin with nuclear spins is very likely to be at the origin of the observed spin dephasing times.

We present evidence of near-infrared photoluminescence (PL) signature of nitrogen vacancy centers (NCVSi)-in silicon carbide (SiC). This center exhibits an S = 1 ground state spin similar to the NV- center in diamond. We have performed photoluminescence excitation measurements at cryogenic temperature and demonstrated efficient photoexcitation of distinct photoluminescence from (NCVSi)-in 4H-SiC. Furthermore, by correlating the energies of measured zero phonon lines (ZPLs) with theoretical values derived from hybrid density functional theory each of the ZPLs has been associated to the respective occupation of hexagonal (h) and quasicubic (k) lattice sites in close analogy to neutral divacancy centers (VCVSi)(0) in the same material. Finally, with the appropriate choice of excitation energy we demonstrated the selective excitation of (NCVSi)(-) PL with no contamination by (VCVSi)(0) PL, thereby opening the way towards the optical detection of (NCVSi)(-) electron spin resonance.

Publisher’s Note, 1 December 2016 : This paper, originally published on 11/9/2016, was withdrawn at request of the authors.

We demonstrate here the phase control of the neutral exciton quantum beats in InGaAs/GaAs quantum dots. A longitudinal magnetic field is used as a tuning parameter to change the phase of the oscillations in a deterministic way. This effect arises from the competition between the Zeeman splitting and the electron/hole exchange interaction on the exciton dipole symmetry. To explore this mechanism, we have developed a pump-probe setup based on the optical heterodyne detection of the quantum dots reflectivity allowing one to measure the exciton dynamics from a small quantum dots ensemble (similar to 300). Particular attention is paid to give a detailed theoretical analysis of the measurements. The experimental results are in excellent agreement with the model.

The effect of the lattice-mismatch strain and of the charge carrier confinement profile, on the optical properties of thermally annealed self-assembled In x Ga1−x As/GaAs quantum dots (QDs), is theoretically analyzed by using a recently developed 40-band k.p model. First, to evaluate the composition and size of QDs as a function of thermal annealing conditions, we model the In/Ga interdiffusion by a Fickian diffusion. Second, we investigate the decrease of the strain effects on the carrier confinement potentials with annealing by solving the Schrödinger equation separately for electrons and holes. It is clearly found that the strain strongly modifies the QD potential profile, leading to a different electron and hole energy distribution. Finally, we carry on a comparison between theoretical calculations and photoluminescence (PL) experimental results performed in thermal annealed samples. A good agreement is obtained for the energy blueshift and the linewidth narrowing of the PL spectra measured on annealed QD ensemble. These results prove the relevance of the present approach to describe the optoelectronic properties of the nanostructures through the post-growth thermal annealing treatment.

The initialization of a resident hole spin by the absorption of a circularly polarized light at resonance involves the formation of an excited state called a trion state. For a pure heavy hole, this optical initialization is mediated by the hyperfine electron-nuclear coupling in the trion state. We show here that for a mixed-hole spin an additional mechanism for the optical initialization appears, associated to 'crossed transitions'; it becomes dominant and keeps a high level of hole spin polarization when the magnetic field screens the electron-nuclear interaction. Finally, using a simple model, we obtain a good theoretical agreement with pulsed pump-probe experiments.

This chapter is devoted to the description of the interaction of polarized light with carrier spins and nuclear spins in semiconductor quantum dots. A historical starting point of these original experiments is the close analogy between quantum dot physics and atomic physics. In 1952, Brossel Kastler and Winter investigated mercury atoms in a weak magnetic field which splits the electron Zeeman levels. By irradiation of the atoms with circularly polarized light the authors could selectively populate one of the electron Zeeman levels [9]. This process has since been referred to as optical pumping. Soon afterwards the first optical pumping of carrier spins in a semiconductor was reported [31]. The initial pumping of spin-orientated conduction electrons in silicon induced by polarized light led to polarization of the nuclear spins of the atoms of the silicon lattice via the hyperfine interaction. This dependence of the nuclear magnetization on the polarization of the absorbed light is at the heart of the experiments described in this chapter. A review of the nuclear spin effects in bulk semiconductors can be found in [37]. The hyperfine interaction between carrier and nuclear spins gives even more spectacular results in quantum dots as shown in pioneering work on optically detected nuclear magnetic resonance ODNMR [23] and orientation of one spin species will have a strong influence on the other [25, 7]. Below we detail a selection of the most remarkable consequences of nuclear spin physics on the optical properties of quantum dots.

We have obtained the optical pumping of hole spins, in p-doped InAs/GaAs quantum dots, via the generation of an intermediate trion state by a train of circularly polarized pulses. We show that we can optically control the orientation of the initialized hole spin, independently of the orientation of the intermediate trion state, by choosing the excitation energy of the circularly polarized light. This brings a supplementary degree of freedom for hole-spin manipulations in quantum dots.

In this study, we evidence hole spin mode locking in a largely inhomogeneous p-doped InAs quantum dot (QD) ensemble, Delta g(x)(h)/g(x)(h) approximate to 34%, which allows us to reveal a long spin coherence time of T-2(h) approximate to 0.8 mu s. In addition, with a two-pump experiment, we demonstrate, in a low magnetic range 60-120 mT, the possibility to synchronize and tune a single subset of QDs through the distribution. Experiments are supported by an analysis within the density matrix approach.