Clare Foden

High-frequency acousto-electric single-photon source

We propose a single optical photon source for quantum cryptography based on the acousto-electric effect. Surface acoustic waves (SAWs) propagating through a quasi-one-dimensional channel have been shown to produce packets of electrons which reside in the SAW minima and travel at the velocity of sound. In our scheme these electron packets are injected into a p-type region, resulting in photon emission. Since the number of electrons in each packet can be controlled down to a single electron, a stream of single (or N) photon states, with a creation time strongly correlated with the driving acoustic field, should be generated.

Optically induced bistability in the mobility of a two-dimensional electron gas coupled to a layer of quantum dots

We report a bistability in the resistance of a GaAs/AlGaAs modulation doped field effect transistor in which a layer of InAs self-organized quantum dots has been grown near the electron channel. Brief optical illumination causes a large, persistent drop in the two-dimensional electron gas (2DEG) resistance which can be recovered by allowing a current to flow through the Schottky gate. We demonstrate that illumination reduces the number of electrons trapped in the quantum dots, lowering their potential and thereby enhancing the 2DEG mobility. This bistability could be the basis of an optical memory or sensitive phototransistor.

Improving the light extraction efficiency of red-emitting conjugated polymer light emitting diodes

We demonstrate a significant improvement in the external electroluminescence efficiency of red-emitting polymer light emitting diodes (LEDs) by modifying the optical structure of the device. By using a cathode composed of a thin (5nm) film of calcium backed with an optically thick film of silver, we measured improvements in the external efficiency of polymer LEDs by a factor of 1.6 times compared to a device using a cathode composed of calcium backed with lower reflectivity aluminum. By incorporating the LED into a microcavity structure (to form a resonant cavity LED) it is possible to obtain additional (but rather smaller) improvements in external efficiency of the order of 1.15 times, compared to a standard LED utilizing the same cathode. By combining high reflectivity cathode∕mirror materials with a low finesse cavity structure, we show that the external efficiency of a LED can be improved by as much as 1.8 times compared to a standard (noncavity) LED. Our results are in good agreement with those of th...

Quantum magnetic confinement in a curved two-dimensional electron gas

The ability to produce deliberately shaped or curved two-dimensional electron gases in semiconductors using recent developments in technology, for example regrowth of III-V semiconductors on patterned or etched substrates, opens the possibility of investigating not only the behaviour of electrons in a curved quasi-two-dimensional space, and the effects of varying that curvature, but also presents a novel way of investigating electron transport properties in a non-uniform transverse high magnetic field. It is shown that a semi-infinite two-dimensional electron gas subjected to a non-uniform magnetic field has, in addition to current-carrying edge states, one-dimensional states which lie within the interior of the gas, which also have a finite dispersion, an effect which may be used to create quantum wires or other structures. It is also shown that, in the absence of a magnetic field, curvature of the two-dimensional electron gas gives rise to a potential variation which is inversely proportional to the square of the radius of curvature, an effect which may also be used to confine the electronic motion to one dimension.

Charge recombination in distributed heterostructures of semiconductor discotic and polymeric materials.

Control of microstructure and energetics at heterojunctions in organic semiconductors is central to achieve high light-emitting or photovoltaic device efficiency. We report the observation of an emissive exciplex formed between an electron-accepting discotic material (hexaazatrinaphthylene or HATNA-SC12) and a hole accepting conjugated polymer {poly[9,9- dioctylfluorene-co-N-(4-butylphenyl)diphenylamine] or TFB}. In contrast to polymer-polymer systems, we find here that the exciplex is strongly localized at the interface, acting as an energy bottleneck with inefficient transfer to bulk exciton states and with low yield of charge separation.

Probing the Fermi surfaces of coupled double quantum wells in the presence of an in-plane magnetic field

The anticrossing of the dispersion relations of two two-dimensional electron gases (2DEGs) in an in-plane magnetic field results in distorted Fermi surfaces, leading to density of states and effective mass changes. We have used two different techniques to probe such changes with both the in-plane field and carrier density being systematically altered. The first technique uses a fixed perpendicular field to form Landau levels. By following the evolution of these Landau levels with in-plane field, carrier density and temperature, we are able to determine changes in the subband populations and effective masses. The second technique uses a third conducting layer as a detector of chemical potential changes in the coupled electron gases. This method also allows the determination of effective mass changes in an in-plane field. Calculations of the dispersion curves have been made, illustrating the effect of the parallel field upon subband occupancy and effective mass. These calculations are compared with the experimental data from both methods and are found to be in good agreement.

Improving the light extraction efficiency of polymer LEDs using microcavities and photonic crystals

A significant fraction of light generated within an organic light emitting diode (OLED) is often trapped within the structure within waveguide modes and is unable to escape usefully from the device. Addressing this issue is of significant importance, as it potentially offers a route to improve the external efficiency of OLEDs. Here, we discuss a number of methods to improve light extraction efficiency from conjugated-polymer LEDs. Firstly we explore the use of low finesse optical microcavities to redistribute trapped-light into externally propagating modes. The improvements obtained by simply adopting a microcavity structure on its own are rather small, however we then show that they can be improved significantly by improving the reflectivity of the cathode. Finally, we show that by engineering a photonic crystal beneath the anode of a polymer LED, a significant improvement in external efficiency (by a factor of 2) can be achieved. Such an approach is anticipated to be readily scalable to a manufacturing environment.

Cyclotron resonance studies of strongly coupled double quantum wells in tilted magnetic fields near the quantum and semi-classical limits

Abstract Far-infrared cyclotron resonance (CR) spectroscopy has been used to study a pair of strongly coupled two-dimensional electron gases (2DEGs) formed in two GaAs quantum wells separated by a thin AlGaAs barrier. The degree of wave function hybridisation, along with the effect of a magnetic field parallel to the plane of the electron gas, have been investigated near both the quantum and semi-classical limits, corresponding to low and high filling factors, respectively. Near the quantum regime, the CR transitions in a small parallel field reveal anticrossing between the Landau levels associated with different hybridised subbands. The energies and intensities of these transitions change with front gate bias, yielding information on the bias dependence of the wave function hybridisation and the subband energy splitting. Near the semi-classical limit and with strong parallel magnetic fields, two CR peaks are observed. The corresponding cyclotron masses are compared to those expected for non-circular Fermi contours created by anticrossing of the parabolic dispersion curves associated with the coupled 2DEGs. Experimental results in both limits are compared with predictions from self-consistent solutions of Poissons and Schrodingers equations.

Electron transport in a non-planar 2DEG

We investigate electron transport in a spatially varying magnetic field generated by applying a uniform field to a non-planar 2DEG grown by MBE on a substrate patterned with etched facets. When a magnetic field is applied in the plane of the substrate, the resistance measured across a facet is quantised. Magneto-oscillations with the same periodicity are measured between probes on the planar region adjacent to the facet, even though there is no perpendicular component of magnetic field in that region. By rotating the direction of the magnetic field, we produce a normal component of field on the facet which is of opposite sign to that on the planar regions. We observe plateaux in the resistance across the facet at values determined by the sum of the inverse filling factors.

Anomalous integer quantum Hall states in coupled double quantum wells and the effect of Landau level broadening

In this paper, we discuss the effect of a strong perpendicular magnetic field component upon a range of coupled double quantum well (CDQW) devices. With increasing inter-layer tunnelling, we observe a transition from double- to single-layer characteristics in the magneto-transport data. The effect of the perpendicular field upon a CDQW has also been considered through a self-consistent Poisson/single-particle Schrodinger model. The implications of the experimental and theoretical results for the current analysis of the transport properties of CDQWs are discussed.