Edmund Harbord

Oblique Hanle measurements of InAs/GaAs quantum dot spin-light emitting diodes

We report on studies of electrical spin injection from ferromagnetic Fe contacts into semiconductor light emitting diodes containing single layers of InAs∕GaAs self-assembled quantum dots (QDs). An oblique magnetic field is used to manipulate the spin of the injected electrons in the semiconductor. This approach allows us to measure the injected steady-state spin polarization in the QDs, Pspin as well as estimate the spin losses in the QD spin detector. After subtraction of magneto-optical effects not related to spin injection, we measured a Pspin of 7.5% at 15 K and estimated an injected spin polarization before QD recombination of around 20%.

Persistent template effect in InAs/GaAs quantum dot bilayers

The dependence of the optical properties of InAs/GaAs quantum dot (QD) bilayers on seed layer growth temperature and second layer InAs coverage is investigated. As the seed layer growth temperature is increased, a low density of large QDs is obtained. This results in a concomitant increase in dot size in the second layer, which extends their emission wavelength, reaching a saturation value of around 1400 nm at room temperature for GaAs-capped bilayers. Capping the second dot layer with InGaAs results in a further extension of the emission wavelength, to 1515 nm at room temperature with a narrow linewidth of 22 meV. Addition of more InAs to high density bilayers does not result in a significant extension of emission wavelength as most additional material migrates to coalesced InAs islands but, in contrast to single layers, a substantial population of regular QDs remains.

Growth, optical properties and device characterisation of InAs/GaAs quantum dot bilayers

The growth and optical properties of InAs/GaAs quantum dot (QD) bilayers are investigated, where the strain interactions between closely spaced QD layers are exploited to tailor the optical properties of the system. The underlying (seed) layer acts as a template for subsequent growth of the upper layer, whose properties can then be modified due to the greater freedom in the choice of growth conditions. Extension of the emission wavelength of the QDs is observed, to 1400 nm at room temperature for GaAs-capped bilayers and extending to 1515 nm for InGaAs-capped bilayers. The QDs in the second layer are highly uniform, resulting in an inhomogeneous broadening of <20 meV and, for small separations between QD layers, efficient carrier tunnelling results in suppression of emission from the seed layer. Edge-emitting lasers incorporating QD bilayers operating either in the ground state or first excited state at 1340 nm at room temperature are demonstrated, showing comparable behaviour to QD lasers containing independent layers. Ground state lasing at 1425 nm at 250 K is also observed.

Ultrafast absorption recovery dynamics of 1300 nm quantum dot saturable absorber mirrors

We compare the performance of two quantum dot saturable absorber mirrors with one device operating at the quantum dot ground state transition whereas the other operates at the first excited state transition. Time-resolved photoluminescence and heterodyne four-wave mixing experiments demonstrate faster recovery of the excited-state device compared to the ground-state device. Femtosecond pulses were achieved with both devices, with the ground-state device producing 91 fs pulses and the excited-state device producing 86 fs pulses in a Cr:forsterite laser. The fast absorption recovery dynamics indicates the potential of devices exploiting excited-state transitions for use in high repetition rate lasers.

The influence of size distribution on the luminescence decay from excited states of InAs/GaAs self-assembled quantum dots

We compare the time integrated and time resolved spectra of two samples having coincident ground state emission peaks: one consisting of highly uniform quantum dots, the other grown under conditions which produce a broad distribution of quantum dot sizes. The photoluminescence decay of the ground states in both samples is monoexponential from which we deduce a lifetime of ∼1100 ps independent of excitation power. The excited state decays for the two samples are biexponential with fast and slow components of ∼300 and ∼1100 ps, respectively. These are also independent of excitation power but their contribution to the decay curve changes with power. The data allow us to unequivocally associate the fast component with the excited state decay of larger dots and the slow component with the ground state decay of smaller dots which emit at the same energy. Furthermore, taking into account the degeneracy of the ground state and the optical selection rules for exciton recombination in a confined system we show that...

Charged quantum dot micropillar system for deterministic light-matter interactions

This work was funded by the Future Emerging Technologies (FET) programme within the Seventh Framework Programme for Research of the European Commission, FET-Open, FP7-284743 [project Spin Photon Angular Momentum Transfer for Quantum Enabled Technologies (SPANGL4Q)] and the German Ministry of Education and research (BMBF) and Engineering and Physical Sciences Research Council (EPSRC) [project Solid State Quantum Networks (SSQN)]. J.G.R. is sponsored by the EPSRC fellowship EP/M024458/1.

Optical spin-filtering effect in charged InAs/GaAs quantum dots

We present time resolved photoluminescence results using nonresonant polarized light which show that the electron spin-flip time is much longer than the recombination time for an ensemble of p-doped InAs/GaAs quantum dots. Under continuous wave excitation the degree of optical polarization of the ground state is found to be around 10%. However, the excited state polarization is twice this value. We attribute this effect to Pauli blocking of the injected spin population captured into the dots and show that the effect persists up to room temperature. For resonant excitation, values are nearly doubled in accordance with increased spin injection efficiency.

Resolving Zeeman splitting in quantum dot ensembles

This letter presents a technique for the investigation of the fine structure and spin properties of quantum dot (QD) ensembles, allowing measurement of QD parameters previously accessible only from studies of individual QDs. We show how ∼μeV splittings can be deduced from information contained in the shape of the ensemble polarization spectra and demonstrate the effectiveness of this technique by measuring Zeeman splittings, g-factors, and sensitivity to QD fine structure effects.

Carrier spin dynamics in self-assembled quantum dots

Exploitation of spin in the solid state has attracted considerable recent interest for spintronics and quantum computing applications. Semiconductor nanostructures offer improvement in spin relaxation and spin coherence times due to localization of carriers, which leads to the suppression of traditional relaxation and dephasing mechanisms. Quantum dots (QDs), with 3D localization of carriers, are particularly strong candidates for solid state qubits. In this chapter, we investigate how the properties of self-assembled QDs influence the spin of confined carriers, and how these properties can be tailored by choice of appropriate growth conditions. Spin lifetimes of single carriers and excitons are enhanced in QDs but significant spin relaxation and dephasing mechanisms remain, including exchange interaction with other carriers, hyperfine interaction with nuclei and multiphonon processes. These are reviewed and strategies to minimize or eliminate them are discussed. The exploitation of spin coherence in single QDs and coupled QD structures is introduced, with reference to the requirements for implementation of quantum computing schemes.