R. Oulton

Mode structure of the L3 photonic crystal cavity

The authors investigate the multiple confined modes of GaAs L3 photonic crystal air-bridge cavities, using single layers of InAs quantum dots as active internal light sources. Theoretical results for the energies, quality factors, and emission polarizations of the first five modes are compared to experimental data for cavities with lattice periods ranging from 240to270nm. The authors also present in-plane field distributions for each mode. In addition to the well-known quality factor improvement of the fundamental mode, they show that outward displacement of the end-holes selectively redshifts modes with large end-hole-field overlaps, thus reordering the modes.

Mode structure of coupled L3 photonic crystal cavities

We investigate the energy splitting, quality factor and polarization of the fundamental modes of coupled L3 photonic crystal cavities. Four different geometries are evaluated theoretically, before experimentally investigating coupling in a direction at 30◦ to the line of the cavities. In this geometry, a smooth variation of the energy splitting with the cavity separation is predicted and observed, together with significant differences between the polarizations of the bonding and anti-bonding states. The controlled splitting of the coupled states is potentially useful for applications that require simultaneous resonant enhancement of two transitions.

Tailored quantum dots for entangled photon pair creation

Entangled photon pairs are a key requirement for the implementation of quantum teleportation schemes. [1] Typically, such photon pairs are created by parametric down conversion of a strongly attenuated laser beam in a non-linear optical crystal, with limited efficiency. Recently, the decay of a biexciton complex confined in a quantum dot (QD) has been suggested as an efficient source for polarization entangled photon pairs. [2] This concept was based on the assumption of an idealistic QD structure for which the valence band ground state has pure heavy hole character with angular momentum projections Jh,z = ±3/2 along the heterostructure growth direction. When an electron-hole pair is injected, the momenta of the carriers become coupled by the exchange interaction. If the dot has perfect D2d-symmetry, angular momentum is a good quantum number: the optically active states with momenta M = ±1 are degenerate, and their decay leads to emission of � ± -circularly polarized photons. If the dot ground states are occupied by two electrons and two holes, each with opposite spin orientations, a spin singlet biexciton X2 is formed, for whose decay two channels exist, as shown in Fig. 1 (upper panel left). The first photon is emitted with either � + or � − -polarization, and then the second photon with opposite polarization, as long as no spin flip occurs after the first process. Unless a polarization measurement is performed, the two photon polarization state is therefore described by | 2i = (| +i 1 | −i 2+ | −i 1 | +i 2)/ √ 2, forming an entangled state. A key requirement is that the photons emitted at each stage of the cascade are quasi-degenerate within their homogeneous linewidth, such that they cannot be distinguished by an energy measurement. Experiments have failed up to now to demonstrate such an entanglement, as only classical correlations were observed. [3] While some of the idealizations of the original proposal are well fulfilled, for example, for strongly confined self-assembled (In,Ga)As/GaAs quantum dots (such as the long exciton spin relaxation time as com

Optical properties of single charge tuneable InGaAs quantum dots

We present a magneto-optical investigation of neutral and charged few-exciton complexes in individual In0.5Ga0.5As self-assembled quantum dots. The results reveal direct information regarding Coulomb interaction and correlation effects in dots containing a controlled number of excess electrons (Ne=0,1,2,3,4) or holes (Nh=0,1,2). Either red or blue shifts of the emission energy are observed as the dot is loaded with excess electrons or holes, respectively. These observations are discussed in terms of the expected nature of the few-particle wave functions. In addition complementary magneto-optical measurements have been performed for negatively charged dots. With increasing Ne a weak increase of the g-factor and a strong reduction of the diamagnetic shift is observed. A pronounced minimum of the diamagnetic shift occurs for Ne=3.

Measurement of the Knight field and local nuclear dipole-dipole field in an InGaAs/GaAs quantum dot ensemble

We present a comprehensive investigation of the electron-nuclear system of negatively charged InGaAs/GaAs self-assembled quantum dots (QDs) under the influence of weak external magnetic fields (up to 3 mT). We demonstrate that, in contrast to conventional semiconductor systems, these small fields have a profound influence on the electron spin dynamics, via the hyperfine interaction. QDs, with their comparatively limited number of nuclei, present electron-nuclear behavior that is unique to low-dimensional systems. We show that the conventional Hanle effect used to measure electron-spin relaxation times, for example, cannot be used in these systems when the spin lifetimes are long. An individual nucleus in the QD is subject to milli-Tesla effective fields, arising from the interaction with its nearest neighbors and with the electronic Knight field. The alignment of each nucleus is influenced by application of external fields of the same magnitude. A polarized nuclear system, which may have an effective field strength of several Tesla, may easily be influenced by these milli-Tesla fields. This in turn has a dramatic effect on the electron-spin dynamics and we use this technique to gain a measure of both the dipole-dipole field and the maximum Knight field in our system thus allowing us to estimate the maximum Overhauser field that may be generated at zero external magnetic field. We also show that one may fine tune the angle which the Overhauser field makes with the optical axis.

Coupled resonant modes of dual L3-defect planar photonic crystal cavities

We present the realization of 2-D photonic crystal cavities with a dual L3-defect geometry. The experimental results show consistent and predictable splitting of the fundamental modes and reveal clear evidence for strong cavity-cavity coupling.

Planar waveguide architecture for the implementation of a network of optically controlled quantum dot spin qubits

We propose a device architecture for an in-plane network of optically connected quantum dots. At each node of the network, the dot resides at the intersection of two orthogonal waveguides which transmit the full polarization of an emitted photon to another node. A prototype device is presented.

Whispering Gallery Modes in quantum dot micropillar cavities

High Q-factor (~60,000) whispering gallery modes are observed from GaAs based micropillar cavities with embedded InAs quantum dots. Low threshold lasing is observed in WGMs near to the peak of the QD ensemble.

Modes of the L3 defect cavity in InAs quantum dot photonic crystals

We investigate the longest wavelength modes of an L3 photonic crystal cavity. Reordering of modes due to hole displacement is shown theoretically and experimentally. Cavity optimization is explained in terms of dipolar emission cancellation.

Electron spin dynamics in self-assembled quantum dots

Electron spin coherence in self-assembled (In,Ga)As/GaAs quantum dots has been studied by pump-probe Faraday rotation experiments. Several aspects such as creation of spin coherence, spin dephasing, interaction with lattice etc nuclei will be discussed.