Department of Physics John Anderson Research Colloquia

Colloquia Schedule 2015-2016

Semester I

Semester II


Semiconductor Nanowires: A platform for future electronics

Prof Anders Gustafsson (Solid State Physics and NanoLund, University of Lund), 30th September 2015, 3pm JA3.14

As the size of many electronic devices shrink, new and alternative routes are needed to further the performance of electronics in general. One approach is to use semiconductor nanowires to base the devices on. These wires typically have a length of several microns with diameters of around a tenth of a micron. The wires grow with a small footprint and it is therefore possible to combine materials that cannot be combined in bulk material without generating dislocations due to lattice mismatch. The wires are either seeded by metal particles, typically gold, or grown on patterned substrates. The crystal structure can be zincblende, wurtzite or a mixture of the two. The latter does not occur in bulk growth of III-V materials, except for III-nitrides. Nanowires give the opportunity to study properties of non-nitride wurtzite material. The composition of the nanowires can be controlled and grown in different geometries, including homogeneous core only, core-shell, radial quantum well(s) and axial heterostructures. It is also possible to design the structures so that there are secondary wires growing out of the first wires to form networks of connected wires.

In this presentation, I will concentrate on the optical properties of nanowires. The diameter and length of nanowires make them very suitable for studies using cathodoluminescence (CL). A conventional scanning electron microscope was used to study the spatial and spectral variation along the nanowires. All these structures are intended for use as light emitters, varying from near infrared to deep ultraviolet. I will demonstrate thickness variations in radial quantum wells of InAs/InP and GaAs/AlGaAs, where thickness variations were observed on a scale of one monolayer. The AlGaAs barriers can show some interesting features, as the corners where the side facets meet have different compositions. In some cases, these corners can form either quantum dots or quantum wires, revealed by CL imaging. I will also present investigations of nanowire-based III-nitride structures. GaN nanowires have the ability to split along the length when the substrate is cleaved, giving access to the interior of the layers in a radial structure. In radial device structures, it is possible to distinguish between the radial quantum-well and the p- and n-type GaN layers by their emission energies. Finally, I will present data on flat micro substrates of micron-sized hexagons seeded by nanowires, for red (InGaN) to ultraviolet (AlGaN) devices.

Chiral interaction of light and matter in confined geometries

Prof Arno Rauschenbeutel (Vienna University of Technology), 14th October 2015, 3pm, JA 3.14

When light is strongly transversally confined, significant local polarization components that point in the direction of propagation arise. In contrast to paraxial light fields, the corresponding intrinsic angular momentum of the light field is position-dependent - an effect referred to as spin-orbit interaction of light. Remarkably, the light's spin can even be perpendicular to the propagation direction. The interaction of emitters with such light fields leads to new and surprising effects. For example, when coupling gold nanoparticles or atoms to the evanescent field surrounding a silica nanophotonic waveguide, the intrinsic mirror symmetry of the particles’ emission is broken. This allowed us to realize chiral nanophotonic interfaces in which the emission direction of light into the waveguide is controlled by the polarization of the excitation light [1] or by the internal state of the atoms [2], respectively. Moreover, we employed this chiral interaction to demonstrate nonreciprocal transmission of light at the single-photon level through a silica nanofiber [3]. The resulting optical diode is the first example of a new class of nonreciprocal nanophotonic devices which exploit the chiral interaction between quantum emitters and transversally confined photons.

Spin qubits and radio-frequency optomechanics with carbon nanotubes

Dr Edward Laird (Department of Materials, University of Oxford), 28th October 2015, 3pm JA3.14

Carbon nanotubes are versatile materials in which many aspects of condensed matter physics come together. The chemical vapour deposition process through which nanotubes are synthesized leaves very low disorder; by incorporating these pristine nanotubes we can fabricate clean electrical devices in which delicate effects can be studied. Recently, we developed the technology of stamping, which allows single nanotubes to be positioned in quantum dot devices, and is compatible with many other fabrication techniques. I will present two sets of experiments that make use of the unique properties of clean nanotube devices.

I will begin by describing the first qubit in this material, the spin-valley qubit. This makes use of both the electron spin and the valley magnetic moment, which are coupled by the spin-orbit interaction.  To realize this qubit, we first used gate potentials to define a double quantum dot. Then, we made use of the Pauli exclusion principle to configure the device as an electrical spin filter.  Finally, we exploited a bend in the nanotube to manipulate the qubit electrically, reading it out and characterizing its coherence properties via the current through the device.

In the second part of the talk, I will present electromechanical measurements of vibrating nanotubes. Nanotubes combine light mass (leading to large zero-point motion) and high stiffness (leading to large mode spacing), making them potentially interesting for studying the quantum limit of mechanical motion. We have measured nanotube mechanical modes with frequencies as high as 39 GHz, implying efficient cooling to the ground state in a standard laboratory cryostat. We have also shown how to measure nanotube mechanics without passing a current, by coupling optomechanically to a radio-frequency tank circuit. I will discuss how we may build on these experiments to study quantum effects in a vibrating nanotube.


  • "A valley-spin qubit in a carbon nanotube”, EA Laird, F Pei, LP Kouwenhoven, Nature Nanotechnology 8, 565-568 (2013)
  • “A high quality factor carbon nanotube mechanical resonator at 39 GHz”, EA Laird, F Pei, W Tang, GA Steele, LP Kouwenhoven, Nano Letters 12 193-197 (2011)
  • “Quantum transport in carbon nanotubes”, E.A. Laird, F. Kuemmeth, G. Steele, K. Grove-Rasmussen, J. Nygård, K. Flensberg, L.P. Kouwenhoven, Reviews of Modern Physics 87, 703 (2015)

Laser Dynamics and Dynamic Lasers

Dr Antonio Hurtado (Institute of Photonics, Physics Department, University of Strathclyde), 11th November 2015, 3pm JA3.14

Semiconductor lasers can undergo a rich variety of nonlinear dynamics, (e.g. periodic oscillations, chaotic regimes, etc.) when subject to external optical injection or feedback.

In this seminar I will review our recent analyses on the nonlinear dynamics of different laser structures (e.g. Vertical-Cavity Surface Emitting Lasers, nanostructure lasers, etc.) outlining their potentials for application on a wide range of technologies. In particular, I will discuss the use of laser dynamics for ultra-high frequency generation, all-optical storage and information processing using stable temporal patterns and the ultrafast optical emulation of neuronal dynamics for non-traditional computing paradigms.

I will finish by briefly introducing our ongoing research at the Institute of Photonics on Dynamic Lasers. This research focuses on the manipulation, transfer and printing of nanoscale lasers for their integration onto bespoke nanophotonic systems.

Twisted laser pulses and twisted plasma waves at very high intensities

Prof Jose Tito Mendonça (IPFN/IST, Universidade de Lisboa, Portugal), 25th November 2015, 3pm JA3.14

We give an overview of our recent work on the physics of intense laser pulses carrying orbital angular momentum (OAM), and on the resulting laser-matter interactions. Twisted laser pulses are electromagnetic vortices, which can be represented by Laguerre-Gauss modes. They have been explored in optics during the last twenty years, but recently the problem emerged in plasma physics, where different kinds of new processes have been identified. Intense laser-matter interactions lead to the formation of dense plasmas, therefore plasma effects are naturally linked with intense laser pulses with OAM. First of all, qualitative changes occur in a plasma because, not only electromagnetic waves but also longitudinal oscillations (such as plasmons and phonons) with OAM can be excited. In particular, these electrostatic waves with OAM display qualitatively new kinetic properties, such as a modified Landau resonance.

Other nonlinear effects associated with laser OAM propagation in plasmas were also considered. This includes Raman and Brillouin stimulated scattering, the inverse Faraday effect with linear polarization, radiation pressure effects and high harmonic generation with OAM. In particular, it has been shown that twisted laser wakefields can be excited by short laser pulses with OAM, leading to the formation of toroidal (or donut shaped) density perturbations behind the pulse.

This could possibly lead to efficient positron acceleration in the nonlinear regime, and to the formation of energetic electron hollow beams. New regimes of intense magnetic field generation have also been identified. Finally, we discuss perspectives for future theoretical and experimental work related with OAM laser beams and other twisted modes in a plasma.

New Frontiers in Photonics: Is there more to the photon than meets the eye?

Prof. David L. Andrews (University of east Anglia), 9th December 2015, 3pm JA3.14

How much do we really know about the nature of a photon? At a time when whole industries are being built on ‘photonics’, recent research makes the paradoxical nature of the photon more than ever evident. Numerous developments in the field of optical vortices, plasmonics and nonlinear optics hinge on exotic properties of the wave-front and phase structures of light, down to the level of the photon itself. As a result we are discovering new fundamental principles, and finding new applications ranging including nanomanipulation, quantum information and all-optical switching. This colloquium highlights some of the recent discoveries, and puzzles that now need to be solved.

Nano-Optics with Fast Electrons

Dr. Mathieu Kociak (Laboratoire de Physique des Solides, Université Paris-Sud XI, Orsay, France), 27th January 2016, 3pm JA3.14

How light behaves and interacts with matter at the nanometer scale is a fascinating subject. Indeed, at this scale, both the electromagnetic field and the electron wave functions may be subject to confinement. This is why the optical properties of nano-objects will in general depend drastically on their shape, size and local environment. This is the case for surface plasmons on metallic nanoparticles, which can be viewed as classical electromagnetic standing waves, or for the excitons in quantum emitters (such as Quantum Dots), where the confinement now affects the excitons wavefunction.

The typical sizes at which confinement becomes crucial range from few angströms (for excitons) to tens or hundred of nanometers (for plasmons). It is thus important to have tools able to probe optical and structural properties at these scales. Of course, regular optical microscopies and spectroscopies are not able to deliver such spatial resolution. Recently, electron spectroscopies such as Electron Energy Loss Spectroscopy (EELS) and Cathodoluminescence (CL) used in a Scanning Electron Microscope (STEM) have shown to address this issue.

In this presentation, I will thus present how recent technical and conceptual developments in EELS and CL have allowed to explore various aspects of nano-optics (plasmonics, photonics, quantum optics) at the scale relevant for plasmons and quantum emitters: few nanometers.

Note: With  A Losquin, S Meuret, Z Mahfoud, M. Tencé, L Tizei, L F Zagonel*, K. March, O Stéphan

(*) Also at Brazilian Nanotechnology National Laboratory, Brazilian Center for Research in Energy and Materials, 13083-970, Campinas, Brazil.


Water, honey and electrons - evidence for electronic hydrodynamics in naturally occurring materials.

Prof. Andrew Mackenzie, Max Planck Institute fir Chemical Physics of Solids, Dresden & School of Physics & Astronomy, University of St. Andrews , 10th February 2016, 3pm JA3.14

Electrical transport in solids is almost always analysed using an approximation in which all scattering is assumed to relax the momentum of the electrons. Although this can be justified in the vast majority of cases, because the electrons are moving in a lattice to which momentum is efficiently transferred, recent measurements by several groups give evidence that it is not always true. In ultra-pure systems with extremely long mean free paths, the momentum-conserving collisions that are ignored in standard theory can become more rapid than the momentum-relaxing ones. In this limit, the electronic flow moves into a hydrodynamic regime in which the electron fluid’s viscosity dominates the resistance measured in flow through constrained channels. Although not very well known by people working on bulk materials, the study of such effects goes back over fifty years in the theoretical literature and over twenty years in experiments on high purity two-dimensional electron gases. I will try to review the history of the field, then describe the new experiments, and finally make some comments about extending the investigation to other systems.

Matters of Gravity: 3

Dr. Nicholas Lockerbie (University of Strathclyde), 24th of February, 2016, 3pm JA3.25

On 14 September, 2015, at 09:50 and 45 seconds, UTC (Coordinated Universal Time—a similar time to that of the GMT time zone) the two detectors of the Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States observed simultaneously a transient gravitational-wave signal. It was the middle of the night at both LIGO Hanford WA (01:50 and 45 seconds), and LIGO Livingston LA (03:50 and 45 seconds), but low-latency, automatic, search algorithms, designed for the detection of generic gravitational-wave transients, reported the detection within 3 minutes of data acquisition.  The LIGO detectors, each measuring 4 km ´ 4 km, are arguably the most sensitive detectors on Earth.  They are capable of detecting differential changes in the dimensions of space itself on the order of 10-19 m/ÖHz, at frequencies ~200 Hz, and they recorded these transient signals with a signal-to-noise ratio of 24.  Astrophysical models subsequently showed the signals to have come from a binary Black-Hole merger, approximately 1.3 billion light-years away.  I shall discuss the nature of gravitational waves (GWs), the design of the LIGO instruments and how the GWs were detected, and the part that the University of Strathclyde played in this extraordinary discovery.  I shall conclude with some comments on the nature of this particular source of gravitational waves, and what this might augur for the future.

Hybrid quantum information processing with atoms, photons and superconducting circuits

Dr. Jonathan Pritchard (University of Strathclyde), 9th of March, 2016, 3pm JA3.14

Hybrid quantum computation exploits the unique strengths of disparate quantum technologies, enabling realization of a scalable quantum device capable of both fast gates and long coherence times. We propose a quantum interface for creating hybrid entanglement between neutral atoms, superconducting circuits and optical photons. The interface is mediated by coupling superconducting circuits to Rydberg excited single atoms using chip-based coplanar waveguide microwave cavities. We have developed a simple gate scheme to enable entanglement of an atomic qubit with a microwave photon, with fidelity calculations based on realistic parameters giving Bell-state preparation fidelity exceeding 0.999 on μs timescales. Experimental progress towards the coherent excitation of a single atom above a coplanar waveguide in a 4 K cryostat at the University of Wisconsin-Madison will be presented, along with an overview of a newly established experiment at the University of Strathclyde focused on quantum networking of hybrid systems.

White Dwarfs: Why all astronomers should love them

Prof Martin Barstow (Physics and Astronomy, Leicester), 23rd of March, 2016, 3pm JA3.14

White dwarfs may just be the dying embers of the vast majority of stars in the galaxy, but they are of enormous importance for astrophysics. They are implicated in the mechanisms for the supernovae type Ia explosions used to determine the age and scale of the Universe; they can be used as background sources to probe the structure of the galaxy; their atmospheres bear the imprint of debris from rocky extra-solar planets and finally, they can be used to probe variations of fundamental constants in strong gravity. This talk will survey scientific results from the study of white dwarfs and show how they can contribute to understanding of several fundamental problems facing astronomers.

Quantum metrology with Bose Einstein Condensates

Prof Markus Oberthaler (University of Heidelberg), 20th of April, 2016, 3pm JA3.14

One aspect of metrology, the science of measurement, is the exploration of the ultimate precision limit. It is known for quite some time that the new possibilities in quantum mechanics allow the surpassing of the ultimate classical precision limit given by counting statistics. Quantum metrology is about the exploration of these new limits. The goal is the generation and characterization of useful quantum mechanical resources for going beyond the classical precision limits. Since the gain in precision is intimately connected to quantum entanglement in many particle systems these investigations are also interesting from the fundamental point of view. In this colloquium I will discuss in detail how Bose Einstein condensates can be used to generate entangled many particle states which push atom interferometry beyond the classical limits. I will use the system of two component atomic condensates as a model system for explaining how quantum correlations arise and how they can be used for improved estimation of a phase shift in an atom interferometer. The simplest form of useful many particle quantum states are spin squeezed states which can be classified as Gaussian states. With the experimental platform of ultracold gases even non-gaussian states can be generated. Their proper characterization involves the quantum Fisher information which will be explained in detail.