M. Zielinski

Multiexciton complexes in InAs self-assembled quantum dots

We review our recent work on multiexciton complexes in InAs self-assembled quantum dots using a combination of effective mass, k⋅p, and atomistic sp3s∗d5 tight-binding approaches. The single-particle levels from effective mass, k⋅p, and atomistic tight-binding models are used as input into configuration-interaction calculation of multiexciton spectra. We describe the principles of the atomistic approach and apply all these computational tools to illustrate the concept of hidden symmetry as underlying principle in energy levels of multiexciton complexes, optical detection of electron spin polarization, tunneling of holes in quantum dot molecules, and tuning of multiexciton spectra with lateral electric fields for entangled photon pair generation.

Controlling a nanowire quantum dot band gap using a straining dielectric envelope.

We tune the emission wavelength of an InAsP quantum dot in an InP nanowire over 200 meV by depositing a SiO(2) envelope using plasma-enhanced chemical vapor deposition without deterioration of the optical quality. This SiO(2) envelope generates a controlled static strain field. Both red and blue shift can be easily achieved by controlling the deposition conditions of the SiO(2). Using atomistic empirical tight-binding calculations, we investigate the effect of strain on a quantum dot band structure for different compositions, shape, and crystal orientations. From the calculations, we estimate the applied strain in our experiment. This enables engineering of the band gap in nanowires with unprecedented possibilities to extend the application range of nanowire devices.

Deterministic Writing and Control of the Dark Exciton Spin Using Single Short Optical Pulses

We experimentally demonstrate deterministic optical writing of a quantum dot-confined dark exciton, in a pure quantum state using one optical pulse. We then control the spin state of this long-lived exciton using picosecond optical pulses.

The emergence of classical behaviour in magnetic adatoms

A wide class of nanomagnets shows striking quantum behaviour, known as quantum spin tunnelling (QST): instead of two degenerate ground states with opposite magnetizations, a bonding-antibonding pair forms, resulting in a splitting of the ground-state doublet with wave functions linear combination of two classically opposite magnetic states, leading to the quenching of their magnetic moment. Here we study how QST is destroyed and classical behaviour emerges in the case of magnetic adatoms, where, contrary to larger nanomagnets, the QST splitting is in some instances bigger than temperature and broadening. We analyze two different mechanisms for the renormalization of the QST splitting: Heisenberg exchange between different atoms, and Kondo exchange interaction with the substrate electrons. Sufficiently strong spin-substrate and spin-spin coupling renormalize the QST splitting to zero allowing the environmental decoherence to eliminate superpositions between classical states, leading to the emergence of spontaneous magnetization. Importantly, we extract the strength of the Kondo exchange for various experiments on individual adatoms and construct a phase diagram for the classical to quantum transition.

Photon Cascade from a Single Crystal Phase Nanowire Quantum Dot.

We report the first comprehensive experimental and theoretical study of the optical properties of single crystal phase quantum dots in InP nanowires. Crystal phase quantum dots are defined by a transition in the crystallographic lattice between zinc blende and wurtzite segments and therefore offer unprecedented potential to be controlled with atomic layer accuracy without random alloying. We show for the first time that crystal phase quantum dots are a source of pure single-photons and cascaded photon-pairs from type II transitions with excellent optical properties in terms of intensity and line width. We notice that the emission spectra consist often of two peaks close in energy, which we explain with a comprehensive theory showing that the symmetry of the system plays a crucial role for the hole levels forming hybridized orbitals. Our results state that crystal phase quantum dots have promising quantum optical properties for single photon application and quantum optics.

Strain-induced energy gap variation in ZnTe/ZnMgTe core/shell nanowires

Strain-induced changes of ZnTe energy gap in ZnTe/ZnMgTe core/shell nanowires arising from lattice mismatch between the core and the shell semiconductor are studied by means of optical methods. It is shown that the increase of the Mg content in the shell, as well as the increase of the shell thickness result in an effective redshift of the near band edge photoluminescence from ZnTe nanowire cores, which reflects directly the decrease of energy gap under tensile strain conditions. The conclusions are supported by theoretical calculations in terms of the valence force field model. The observed change of ZnTe energy gap can be as large as 120 meV with respect to the unstrained conditions and can be tuned in a continuous manner by adjusting shell parameters, which open a path towards an effective band gap engineering in these structures.

Ab initio calculation of band edges modified by (001) biaxial strain in group IIIA–VA and group IIB–VIA semiconductors: Application to quasiparticle energy levels of strained InAs/InP quantum dot

Results of first-principles full potential calculations of absolute position of valence and conduction energy bands as a function of (001) biaxial strain are reported for group IIIA–VA (InAs, GaAs, InP) and group IIB–VIA (CdTe, ZnTe) semiconductors. Our computational procedure is based on the Kohn–Sham form of density functional theory (KS DFT), local spin density approximation (LSDA), variational treatment of spin-orbital coupling, and augmented plane wave plus local orbitals method (APW+lo). The band energies are evaluated at lattice constants obtained from KS DFT total energy as well as from elastic free energy. The conduction band energies are corrected with a rigid shift to account for the LSDA band gap error. The dependence of band energies on strain is fitted to polynomial of third degree and results are available for parameterization of biaxial strain coupling in empirical tight-binding models of IIIA–VA and IIB–VIA self-assembled quantum dots (SAQDs). The strain effects on the quasiparticle energ...

Building semiconductor nanostructures atom by atom

We present an atomistic tight-binding approach to calculating the electronic structure of semiconductor nanostructures. We start by deriving the strain distribution in the structure using the valence force field model. The strain field is incorporated into the tight-binding electronic structure calculation carried out in the frame of the effective bond orbital model and the fully atomistic sp3d5s* approach. We apply the method to a vertically coupled self-assembled double-dot molecule. Using the effective mass approach, we establish the existence of electronic bonding and antibonding molecular orbitals for electrons and holes, whose probability density is shared equally between the dots. In the atomistic calculation we recover the molecular character of electron orbitals, but find that structural and atomistic details of the sample modify the hole orbitals, leading to a strongly asymmetric distribution of the probability density between the dots.

Linear scaling approach for atomistic calculation of excitonic properties of 10-million-atom nanostructures

Numerical calculations of excitonic properties of novel nanostructures, such as nanowire and crystal phase quantum dots, must combine atomistic accuracy with an approachable computational complexity. The key difficulty comes from the fact that excitonic spectra details arise from atomicscale contributions that must be integrated over a large spatial domain containing a million and more of atoms. In this work we present a step-by-step solution to this problem: combined empirical tight-binding and configuration interaction scheme that unites linearly scaling computational time with the essentials of the atomistic modeling. We benchmark our method on the example of wellstudied self-assembled InAs/GaAs quantum dot. Next, we apply our atomistic approach to crystal phase quantum dots containing more than 10 million atoms.

Tunnel-Coupled Quantum Dots: Atomistic Theory of Quantum Dot Molecules and Arrays

The electronic structure and optical properties of quantum dot molecules and arrays are determined by use of atomistic tight-binding theory. Electron states follow the artificial molecule analogy. Hole levels are much more complex.