Colloquia Schedule 2014-2015

Semester I

Semester II

* Note: Outside of regular schedule.

Self-Organizing Structures in Relativistic Laser Plasma

Prof Juergen Meyer-ter-Vehn (Munich), 1st October 2014

Recent progress in high power laser technology allows to generate laser intensities up to 1021 W/cm2. Such lasers are now available in a number of laboratories worldwide. Typically the pulses have femtosecond duration and carry electric fields in the order of Teravolts/m. They ionize dense target materials and form laser plasma of highly relativistic electrons. Light pressure tends to separate electrons from ions creating TV/m electrostatic fields, and huge currents build up carrying up to Gigagauss magnetic fields. The tight coupling between electromagnetic and plasma fields is highly non-linear and leads to self-organisation of density structures that are amazingly stable.

Beyond the intrinsic beauty of these structures, they are of remarkable practical importance. We discuss bubble structures that represent micro-accelerator cavities trapping, focusing, and accelerating electron beams. At near-critical plasma densities, we find self-focusing, highly magnetized, laser plasma channels in which the laser light drives electron beams directly by an inverse FEL mechanism. Recent results show that, driven by circular polarized light, these beams may form helical structures emitting intense synchrotron radiation of MeV photons. A major part of the results was found from particle-in-cell simulation already 10 - 20 years ago. The great challenge today is experimental demonstration.

Simulations and theory for ultrafast x-ray scattering experiments

Dr Adam Kirrander (Edinburgh), 15th October 2014

New x-ray free electron lasers, such as the LCLS at Stanford and the XFEL in Hamburg, provide very short (~30 fs) and very intense pulses of coherent x-rays making it possible to attempt completely new types of experiments. In particular, the large numbers of hard x-ray photons per pulse open up for the study of non-crystalline samples, including comparatively dilute gases.

For the chemical physics community, the prospect of so-called ‘molecular movies’ based on ultrafast x-ray scattering is attractive as it suggests that complex photochemical processes can be studied in a direct fashion. In this talk, we will outline recent and ongoing experimental efforts, and present simulations of the electron dynamics in Ar atoms and of the ring-opening reaction in the molecule 1,3-cyclohexadiene. Direct calculation of elastic scattering from ab initio wave functions will be discussed, as well as our current efforts to account for inelastic scattering effects.

Cathodoluminescence hyperspectral imaging of light-emitting nanostructures

Dr Paul Edwards (Strathclyde), 29th October 2014

CL_Nanorods Characterising the emission from such small structures is beyond the spatial resolution of diffraction-limited optical techniques. This can be circumvented by the use of cathodoluminescence (CL) spectroscopy, in which we analyse the emission excited from the material under a focused beam of energetic electrons. This technique is extended to CL hyperspectral imaging in which a multidimensional dataset is acquired containing both spatially- and spectrally-resolved information from a sample. I will describe the instrumentation and software I have developed for these measurements, and show how they have been crucial in the advancement of novel semiconductor nanostructures (such as the InGaN/GaN core/shell nanorod array pictured here).

The development of blue light-emitting diodes based on the group III nitrides has been of enormous technological importance, as acknowledged by the recent award of the Nobel Prize for Physics to the pioneers of this field. Unlike comparable materials, the luminescence from (Al,Ga,In)N alloys is strongly influenced by nanometre-scale inhomogeneity, and the role of this in limiting the detrimental effects of the abundant crystal defects in this material system is still contentious. Nano-scale features are also increasingly being introduced deliberately using lithographic patterning, providing a number of benefits: reduced elastic strain; improved light extraction; access to alternative lattice planes; and directional emission through photonic crystal effects.

How Gallium Nitride can save energy, purify water and improve your health!

Prof Sir Colin Humphreys (Cambridge), 26th November 2014

t4_920314933461285974 The 2014 Nobel Prize in Physics went to three Japanese scientists for inventing GaN LEDs. This reflects the GaN is a truly remarkable new man-made material! The huge increase in Wi-Fi demand means that the RF (radio frequency) capacity will soon be exceeded. We can use light as a carrier of data instead of RF, a technique which is being pioneered by the Institute of Photonics in Strathclyde, led by Martin Dawson, with whom we have a collaboration. Finally, there is increasing evidence that lighting is important for our health. We can tailor the quality of LED lighting to improve our health, our productivity at work and our exam performance!

Nanoscale Imaging of Neurotoxic Proteins

Prof Clemens F. Kaminski (Cambridge), 10th December 2014

Misfolding and aggregation of peptides and proteins is a characteristic of many neurodegenerative disorders, including Parkinson’s Disease (PD) and Alzheimer’s (AD). Their common feature is that normally unstructured and soluble proteins, misfold and aggregate into insoluble amyloid fibrils, which make up the deposits in the brains of patients suffering from these devastating illnesses. A key requirement to gain insight into molecular mechanisms of disease and to progress in the search for therapeutic intervention is a capability to image the aggregation process and structure of ensuing aggregates in situ.

In this talk I will give an overview of research to gain insight on the aggregation state of alpha synuclein (relevant to PD) beta-amyloid and Tau (relevant to AD) in vitro [1], in cells [2, 3] and in live model organisms [4]. In particular we wish to understand how these and similar proteins nucleate to form toxic structures and to correlate such information with phenotypes of disease [3]. I will show how direct stochastic optical reconstruction microscopy, dSTORM, and multiparametric imaging techniques, such as spectral and lifetime imaging, are capable of tracking amyloidogenesis in vitro, see figure 1, and in vivo, and how we can correlate the appearance of certain aggregate species with toxic phenotypes [5].

Using multiparametric imaging methods we follow the trafficking of aggregates between cells and see how the misfolded state propagates from cell to cell. I will show how such information at the molecular level guides our understanding of disease pathology in humans.

Kaminski_s

Fig. 1: α-synuclein elongation assay performed with dSTORM super-resolution microscopy. Small fibril seed species, shown in green, are incubated in solution containing monomeric α-synuclein, shown in red. α-synuclein seeds were covalently labelled with Alexa Fluor® 568 and monomer with Alexa Fluor® 647 dyes, respectively. The images show the time-sequenced growth of individual α-synuclein fibrils. Clearly, growth takes place from both ends of the seed fibril, extending to several micrometers in length after 24 hours. The last image shows a conventional fluorescence microscopy image, blurred by optical diffraction. Adapted with permission from27, copyright 2014 American Chemical Society.

[1] Pinotsi D, Büll AK, Galvagnion C, Dobson CM, Kaminski-Schierle GS, Kaminski CF, "Direct Observation of Heterogeneous Amyloid Fibril Growth Kinetics via Two-Color SuperResolution Microscopy," Nano Letters (2013), 14 (1), 339–345
[2] Kaminski Schierle GS, van de Linde S, Erdelyi M, Esbjörner EK, Klein T, Rees E, Bertoncini CW, Dobson CM, Sauer M, and Kaminski CF, "In Situ Measurements of the Formation and Morphology of Intracellular ß-Amyloid Fibrils by Super-Resolution Fluorescence Imaging", J. Am. Chem. Soc., 133 (33), pp 12902–12905 (2011)
[3] Esbjörner, E.K., Chan, F., Rees, E., Erdelyi, M., Luheshi, L.M., Bertoncini, C.W., Kaminski, C.F., Dobson, C.M., and Kaminski Schierle, G.S., “Direct Observations of Amyloid β Self-Assembly in Live Cells Provide Insights into Differences in the Kinetics of A β(1 –40) and A β(1 –42) Aggregation,” Chemistry & Biology (2014).
[4] Kaminski Schierle GS, Bertoncini CW, Chan FTS, van der Goot AT, Schwedler S, Skepper J, Schlachter S, van Ham T, Esposito A, Kumita JR, Nollen EAA, Dobson CM, Kaminski CF, "A FRET sensor for non-invasive imaging of amyloid formation in vivo", ChemPhysChem, 12(3), 673–680, (2011)
[5] Michel CH, Kumar S, Pinotsi D, Tunnacliffe A, St George-Hyslop P, Mandelkow E, Mandelkow E-M, Kaminski CF, Kaminski Schierle GS, "Extracellular Monomeric Tau is Sufficient to Initiate the Spread of Tau Pathology", J. Biol. Chem. (2014), 289: 956-967.

Towards efficient solid state lighting using ion beam techniques

Prof Katharina Lorenz (IST Lisbon, Strathclyde Visiting Professor), 21st January 2015

Current commercial “white LEDs” are based on blue-emitting InGaN LEDs coupled to a yellow-emitting phosphor. The efficiency of these phosphor conversion LEDs is limited by energy losses inherent to the absorption and emission processes. The alternative approach of combining three (blue, green and red emitting) LEDs promises to be more efficient and more versatile. However, the internal quantum efficiency (IQE) for nitride-based LEDs decreases dramatically in the green and red spectral region. Theoretical work suggests that InGaN/GaN quantum wells with graded compositional profiles may increase IQE and mitigate Auger losses when compared to conventional, abrupt QWs. In this work we investigate the possibility of quantum well intermixing to achieve such graded layers. A combination of ion irradiation and thermal annealing can promote the interdiffusion of InGaN QWs and GaN barrier layers. Another approach to achieve long wavelength emission from III-nitride devices is the doping with optically active rare earth ions. A review on recent advances of rare earth implantation in GaN layers and nanostructures will be presented.

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Figure: 3 MV Tandem accelerator at the Laboratory of Accelerators and Radiation Technologies (LATR), of the Nuclear and Technological Campus in Lisbon

Order, Disorder, Symmetry and Complexity

Prof Daniel Stein (NYU), 22nd January 2015

One of the deepest scientific questions we can ask is, How might complexity arise? That is, starting from simple, undirected processes subject to physical and chemical laws, how could structures with complex shapes and patterns arise, and even more perplexing, what processes could give rise to living cells, and how might they then organize themselves into complex organisms, leading ultimately to such things as brains, consciousness, and societies?

We are far from answering these questions at almost any level, but they have attracted increasing attention in the scientific community, and some initial headway has been made. The basic problem can be reframed as one involving the self-organization of microscopic constituents into larger assemblies, in such a way that the process leads to an increase of information, the creation of new patterns, and eventually increasing hierarchical levels of complex structure. The key to understanding these processes cannot be found in any single (natural or social) scientific field but rather in collaborations that cross many disciplinary boundaries.

Although we are still at the initial stages of inquiry, new and interesting approaches and points of view have arisen. In this talk I present one that arises from the point of view of physics. We start by describing the (well-understood) phenomenon of matter organizing itself into simple ordered structures, like crystals and magnets, and then explore how our ideas are affected when we consider the effects of randomness and disorder, pervasive in the physical world, and focusing in particular on glasses and spin glasses. We will see that randomness and disorder are, paradoxically, essential for more ordered, complex structures to arise. Using these ideas, we provide some hints (but only hints) as to how we can gain a handle on issues related to the increase of complexity. Underlying all of our considerations is the notion of symmetry in physics: where it comes from and how matter "breaks" its inherent symmetry to create new information and ever-increasing complexity.

This is a non-technical talk and is designed for natural scientists at all levels.

Relativistic surface high-harmonic generation

Prof Gerhard Paulus (Jena), 28th January 2015

Coherent radiation in the XUV spectral regime with attosecond pulse duration can not only be generated by high-harmonic generation in gases, but also by interaction of terawatt laser pulses with surfaces (surface high-harmonic generation, SHHG). Under suitable conditions, an overdense plasma oscillating with relativistic velocities is generated. Therefore, the laser which has generated this plasma, is reflected and modulated (relativistically oscillating mirror, ROM). The Doppler effect in conjunction with relativistic retardation results in the creation of high-order harmonics. More elaborate models predict the spectral shape of the harmonic emission as well as the efficiency. We have tested the present theoretical understanding of SHHG in a series of experiments at the JETI terawatt laser facility. For the first time, the efficiency of surface harmonic generation was measured using an XUV spectrometer calibrated at a synchrotron. The efficiency at the 21st harmonic was found to be 10-5, falling short of expectations. Extremely short plasma scale lengths lead to low efficiencies, also contrary to expectations. The reason is that the strong restoring forces in the plasma inhibit large oscillation amplitudes and thus large velocities [1]. Methods and results to overcome this problem will be presented.

We also found a new effect that can be directly related to the attosecond time structure of surface harmonics: At plasma scale lengths of the order λ/5, the harmonics exhibit a distinct fine structure. Analytical and numerical modeling have revealed that the fine structure in harmonic emission can be attributed to denting of the plasma by the radiation pressure of the incident laser pulse. The attosecond pulses emitted by the plasma therefore have a time-dependent temporal separation. In frequency domain, this leads to the observed spectral features [2]. The effect can largely be cancelled by an appropriately pre-chirped laser pulse. High-harmonic spectra are regularly characterized by a cutoff beyond a certain harmonic order. The cutoff energy scales with a parameter characteristic for the process responsible for high harmonic generation. In case of gas harmonics this is the ponderomotive energy, in the present case of surface harmonics the relativistic γ-factor. The intriguing observation we have made is a strong enhancement of a particular harmonic beyond the SHHG roll-off. We show that the effect is due to frequency mixing mediated by a relativistic nonlinearity [3].

[1] C. Roedel et al., Phys. Rev. Lett. 109, 125002 (2012).
[2] M. Behmke et al., Phys. Rev. Lett. 106, 185002 (2011).
[3] C. Roedel et al., (submitted for publication).

Scalable and efficient photon generation for integrated optical systems

Dr Michael Strain (Institute of Photonics, Strathclyde), 11th February 2015

The generation of correlated or entangled photon pairs using bulk non-linear optical crystals is a mature technology and widely employed as the starting point for many optical experiments. Scaling such experiments to operate on multiple photon pairs is a non-trivial task and takes large amounts of lab-space to accomplish. Recent advances in on-chip photonic systems, originally designed for telecommunications applications, provide an alternative to bulk optical systems for the generation and manipulation of light. These devices are extremely compact, allowing for dense optical device integration in mm2 areas and opening up the prospect for scalable optical experimental setups on a single chip.

Recent work demonstrating correlated and entangled photon generation from ultra-compact silicon resonator devices will be presented. Tuning of the non-linear efficiency is achieved using simple electronic control of the chip. The issue of pump light filtering is also addressed, as this presents a major obstacle to producing fully on-chip photon generation and manipulation circuits.

MICE as a step towards the Neutrino Factory

Prof Ken Long (Imperial College, London, and STFC), 25th February 2015

Muon beams of low emittance provide the basis for the intense, well-characterised neutrino beams necessary to elucidate the physics of flavour at the Neutrino Factory and to provide lepton-anti-lepton collisions at energies of up to several TeV at the Muon Collider. The International Muon Ionization Cooling Experiment (MICE) will demonstrate ionization cooling; the technique by which it is proposed to reduce the phase-space volume occupied by the muon beam at such facilities.

The status of the MICE construction project will be described together with a summary of the performance of the principal components. Plans for the commissioning and operation of the experiment will be outlined and the measurement programme that will begin next spring will be described. The crucial role of MICE in the development of the long- and short-baseline neutrino programmes will be outlined and the opportunity to develop neutrino beams based on stored muons as a new technique for particle physics will be described.

Materials and surfaces for optical and particle beams

Prof Amin Abdolvand (Dundee), 11th March 2015

Advanced materials are a key requirement for leading-edge manufacturing, and have been identified as one of the UK Government’s Eight Great Technologies. They constitute a classic, general-purpose technology due to the range of their potential applications. Advancement in the engineering of materials is of paramount importance for a myriad of industrial initiatives and for the technological progression of society in general.

I head the Materials and Photonics Systems (MAPS) Group at the University of Dundee. In MAPS, we combine the strengths of materials with advanced photonics, two key strands of technology at extreme scales that enable us to develop and deploy materials and processes at the cutting edge of technology. In this talk, I will briefly introduce a few strands from our group’s research towards the manufacture and processing of functional materials with light, for a broad range of applications. The focus will be on some of the emerging and most exciting applications.

The emission, propagation and scattering of microwaves by fusion plasmas

Dr Roddy Vann (Dept. of Physics, University of York, York Plasma Institute), 25th March 2015

Nuclear fusion offers the possibility of carbon-free energy with an almost limitless fuel supply. Tokamak experiments have made great progress in confinement of plasmas with temperatures in the keV range, necessary to make fusion power a reality. Diagnosing these plasmas is becoming increasingly challenging as we move towards a reactor due to the large number of high energy neutrons that are produced by the fusion reactions. Microwave diagnostics can be constructed from the same radiation-resilient materials as the vacuum vessel and are therefore unusually well-suited to installation on a reactor. It is therefore prudent to maximise the diagnostic possibilities based on emitted or reflected microwave radiation.

One such diagnostic is the Synthetic Aperture Microwave Imaging (SAMI) system that was installed on MAST at the UK’s Culham Centre for Fusion Energy and is currently being transferred to the newly upgraded NSTX tokamak at the Princeton Plasma Physics Laboratory. This diagnostic uses an array of independently-phased antennas to create, for the first time, a 2-D microwave image of the tokamak plasma. Imaging the spontaneous emission has allowed us to infer some of the physics behind edge-localised modes (ELMs), which limit the tokamak performance. By imaging the back-scattered signal, we have demonstrated a novel technique for directly measuring the magnetic field in the plasma edge, which is important for understanding the plasma’s stability.

The tokamak plasma edge is strongly turbulent and we have developed simulation tools to calculate how this turbulent layer scatters the incident microwave signal: these results will also be discussed; YouTube has this example.

A single charge in a Bose-Einstein condensate: from two to few to many-body physics

Prof Tilman Pfau (Stuttgart), 25th April 2015

Electrons attract polarizable atoms via a 1/r4 potential. For slow electrons the scattering from that potential is purely s-wave and can be described by a Fermi pseudopotential. To study this interaction Rydberg electrons are well suited as they are slow and trapped by the charged nucleus. In the environment of a high pressure discharge Amaldi and Segre, already in 1934 observed a lineshift proportional to the scattering length [1].

At ultracold temperatures and Rydberg states with medium size principle quantum numbers n, one or two ground state atoms can be trapped in the meanfield potential created by the Rydberg electron, leading to so called ultra-long range Rydberg molecules [2].

At higher Rydberg states the spatial extent of the Rydberg electron orbit is increasing. For principal quantum numbers n in the range of 100-200 and typical BEC densities, up to several ten thousand ground state atoms are located inside one Rydberg atom, We excite a single Rydberg electron in the BEC, the orbital size of which becomes comparable to the size of the BEC. We study the coupling between the electron and phonons in the BEC [3].

We also observe evidence for ultracold collisions involving a single ion which is shielded by a Rydberg electron. Reactive processes due to few-body Langevin dynamics are mostly l-changing and lead to molecule formation.

As an outlook, the trapping of a full condensate inside a Rydberg atom of high principal quantum number, the imaging of the Rydberg electron's wave function by its impact onto the surrounding ultracold cloud as well as the observation of polaron formation seem to be within reach [4].

[1] E. Amaldi and E. Segre, Nature 133, 141 (1934)
[2] C. H. Greene, et al., PRL 85, 2458 (2000); V. Bendkowsky et al., Nature 458, 1005 (2009)
[3] J . B. Balewski, et al., Nature 502, 664 (2013)
[4] T. Karpiuk, et al., arXiv:1402.6875

Validation of quantum devices

Prof Matthias Troyer (ETH), 13th May 2015

About a century after the development of quantum mechanics we have now reached an exciting time where non-trivial devices that make use of quantum effects can be built. While a universal quantum computer of non-trivial size is still out of reach there are a number commercial and experimental devices: quantum random number generators, quantum encryption systems, and analogue quantum simulators. In this colloquium I will present some of these devices and validation tests we performed on them. Quantum random number generators use the inherent randomness in quantum measurements to produce true random numbers, unlike classical pseudorandom number generators which are inherently deterministic. Optical lattice emulators use ultracold atomic gases in optical lattices to mimic typical models of condensed matter physics. Finally, I will discuss the devices built by Canadian company D-Wave systems, which are special purpose quantum simulators for solving hard classical optimization problems.

Superfluids of light

Prof David Snoke (Pittsburgh, SUPA Distinguished Visitor), 1st July 2015

It is possible to engineer the properties of photons in an optical medium to have an effective mass and repulsive interactions, so that they act like a gas of atoms. These "renormalized photons" are called polaritons. In the past ten years, several experiments have demonstrated many of the canonical effects of Bose-Einstein condensation and superfluidity of polaritons. In this talk I will review some of this past work and present recent results which show quantized circulation of the polariton condensate in a ring trap.