P. W. Fry

Electric-field-dependent carrier capture and escape in self-assembled InAs/GaAs quantum dots

Photoluminescence and complementary photocurrent spectroscopy, both as a function of electric field, are used to probe carrier capture and escape mechanisms in InAs/GaAs quantum dots. Carrier capture from the GaAs matrix is found to be highly field sensitive, being fully quenched in fields of only 15 kV/cm. For fields less than 20 kV/cm, carriers excited in the wetting layer are shown to be captured by the dots very effectively, whereas for fields in excess of 50 kV/cm tunnel escape from the wetting layer into the GaAs continuum is dominant. For excitation directly into the dots, radiative recombination dominates up to 100 kV/cm.

OBSERVATION OF ULTRAHIGH QUALITY FACTOR IN A SEMICONDUCTOR MICROCAVITY

Observation of a very high-quality factor (Q) of ∼30,000 is reported for a planar semiconductor microcavity grown by molecular-beam epitaxy using in situ optical monitoring. The very high Qs are measured in pillars of 5–10μm diameter, and are approximately a factor of 3 higher than measured in planar structures before etching. The higher values in the pillars are ascribed to the elimination of the effects of in-plane dispersion, diffraction, and lateral inhomogeneities, thus allowing the intrinsic Q of the planar structure to be observed. Spectrally resolved mode mapping is reported, accounting qualitatively for the decrease of Q with increasing mode number in the pillars.

Pinning induced by inter-domain wall interactions in planar magnetic nanowires

Pinning Induced by Inter-Domain Wall Interactions in Planar Magnetic Nanowires T.J. Hayward 1 , M.T. Bryan 1 , P.W. Fry 2 , P.M. Fundi 1 , M.R.J. Gibbs 1 , D.A. Allwood 1 , M.-Y. Im 3 and P. Fischer 3 Department of Engineering Materials, University of Sheffield, Sheffield, UK Nanoscience and Technology Centre, University of Sheffield, Sheffield UK Center for X-ray Optics, Lawrence Berkeley Natl Lab, Berkeley, CA, USA PACS: 07.85.Tt, 75.60.Ch, 75.75.+a, 85.70.Kh We have investigated pinning potentials created by inter-domain wall magnetostatic interactions in planar magnetic nanowires. We show that these potentials can take the form of an energy barrier or an energy well depending on the walls’ relative monopole moments, and that the applied magnetic fields required to overcome these potentials are significant. Both transverse and vortex wall pairs are investigated and it is found that transverse walls interact more strongly due to dipolar coupling between their magnetization structures. Simple analytical models which allow the effects of inter- domain wall interactions to be estimated are also presented. There is great interest in developing memory [1] and logic [2] devices based upon the controlled motion and interaction of domain walls (DWs) in ferromagnetic planar nanowires. Such domain walls have particle-like properties which allow them to be propagated around complex circuits using rotating magnetic fields [3,4] or short electric current pulses [5], and hence they may be used to represent binary data in a similar way to electric charge in conventional microelectronics. DWs in planar magnetic nanowires have head-to-head (H2H) or tail-to-tail (T2T) character (Fig 1(a)), and consequently they carry a net monopole moment (i.e. a localised excess of north (H2H) or south (T2T) magnetic poles). Therefore, to a first approximation DWs in adjacent nanowires will interact via a Coulomb-like potential: if the DWs have like monopole moments there will be a repulsive interaction, whereas if they have opposite monopole moments their interaction will be attractive. Understanding these effects and how they affect DW propagation is likely to be important to the development of DW based devices, where large nanowire densities will be desirable. So far there have been relatively few investigations into these effects, with studies characterizing attractive coupling between walls with opposite monopole moments for a limited range of nanowire geometries and DW structures [6,7]. We have also previously demonstrated that DW interaction energies are dependent to some degree on the DWs magnetization structure and chirality [8].

Control of polarized single quantum dot emission in high-quality-factor microcavity pillars

Small size microcavity pillars with elliptical cross section and high quality factors Q are reported and are shown to provide nearly 100% linearly polarized single photon sources. It is shown that the polarization of the emission of quantum dots embedded within the pillars can be controlled by using the coupling of the dot emission with the photonic modes. A notable dependence of the Q value is found on the polarization of the mode even though calculations of the mode profiles show that the electric field distribution is very similar.

Switchable Cell Trapping Using Superparamagnetic Beads

Ni81Fe19 microwires are investigated as the basis of a switchable template for positioning magnetically labeled neural Schwann cells. Magnetic transmission X-ray microscopy and micromagnetic modeling show that magnetic domain walls can be created or removed in zigzagged structures by an applied magnetic field. Schwann cells containing superparamagnetic beads are trapped by the field emanating from the domain walls. The design allows Schwann cells to be organized on a surface to form a connected network and then released from the surface if required. As aligned Schwann cells can guide nerve regeneration, this technique is of value for developing glial-neuronal coculture models in the future treatment of peripheral nerve injuries.

SINGLE PHOTON SOURCES BASED UPON SINGLE QUANTUM DOTS IN SEMICONDUCTOR MICROCAVITY PILLARS

Semiconductor microcavity pillars with both circular and elliptical cross-section containing semiconductor quantum dots are shown to be good candidates for efficient single photon sources. Pillars with small diameters are shown to have exceptionally high quality factors and the reduction in the measured quality factor as the pillar diameter is reduced is shown to agree well with finite difference time domain simulation. These pillars exhibit a Purcell enhancement of the quantum dot emission when the dots are on-resonance with the cavity mode and strong photon antibunching. The use of the polarized modes of an elliptical micropillar allows the polarization of the emitted single photons to be selected.

Electronic properties of InAs/GaAs self-assembled quantum dots studied by photocurrent spectroscopy

Abstract The power of photocurrent spectroscopy to study the electronic properties of InAs/GaAs self-assembled quantum dots is described. From comparison of results from different samples it is shown that photocurrent provides a direct means to measure absorption spectra of quantum dots. Studies in high-electric field enable the electron–hole vertical alignment to be determined. Most surprisingly this is found to be opposite to that predicted by all recent predictions. Comparison with theory shows that this can only be explained if the dots contain significant amounts of gallium, and have a severely truncated shape. The nature of the ground and excited state transitions, carrier escape mechanisms from dots and in-plane anisotropies are also determined.

Modal gain and lasing states in InAs/GaAs self-organized quantum dot lasers

The modal gain of an InAs/GaAs self-organized quantum dot laser is determined from a measurement of the normal incidence, interband photocurrent. The maximum modal gain of the ground state transition is shown to have a value of (7±3) cm−1, considerably smaller than typical values for comparable quantum well lasers. The photocurrent technique is demonstrated to be a convenient and simple method for determining the spectral form of the gain and for comparing the modal gain of different devices. The consequences of the small modal gain for the laser characteristics are discussed.

Realization of the Manipulation of Ultracold Atoms with a Reconfigurable Nanomagnetic System of Domain Walls

Planar magnetic nanowires have been vital to the development of spintronic technology. They provide an unparalleled combination of magnetic reconfigurability, controllability, and scalability, which has helped to realize such applications as racetrack memory and novel logic gates. Microfabricated atom optics benefit from all of these properties, and we present the first demonstration of the amalgamation of spintronic technology with ultracold atoms. A magnetic interaction is exhibited through the reflection of a cloud of (87)Rb atoms at a temperature of 10 μK, from a 2 mm × 2 mm array of nanomagnetic domain walls. In turn, the incident atoms approach the array at heights of the order of 100 nm and are thus used to probe magnetic fields at this distance.

An Optical Nanocavity Incorporating a Fluorescent Organic Dye Having a High Quality Factor

We have fabricated an L3 optical nanocavity operating at visible wavelengths that is coated with a thin-film of a fluorescent molecular-dye. The cavity was directly fabricated into a pre-etched, free-standing silicon-nitride (SiN) membrane and had a quality factor of Q = 2650. This relatively high Q-factor approaches the theoretical limit that can be expected from an L3 nanocavity using silicon nitride as a dielectric material and is achieved as a result of the solvent-free cavity-fabrication protocol that we have developed. We show that the fluorescence from a red-emitting fluorescent dye coated onto the cavity surface undergoes strong emission intensity enhancement at a series of discrete wavelengths corresponding to the cavity modes. Three dimensional finite difference time domain (FDTD) calculations are used to predict the mode structure of the cavities with excellent agreement demonstrated between theory and experiment.