Ewch i’r prif gynnwys

Seminars

Mae'r cynnwys hwn ar gael yn Saesneg yn unig.

We run a regular programme of seminars for staff and students.

All seminars will take place in Queen's Buildings North, Room N/3.28 unless otherwise stated.

2020 seminars

It seems likely that during the earliest stages of star formation, protostellar discs may be massive enough to be susceptible to a gravitational instability. This will manifest as spiral density waves, which can act to transport angular momentum outwards, potentially playing a key role in the early growth of the central protostar. Additionally, if these discs are very unstable, they may fragment to form bound objects. However, these will tend to be relatively massive objects on wide orbits. In this talk, I will present our current understanding of self-gravitating protostellar discs, the role they may play in star formation, the chances of observing this phase in a protostellar system, and the possibility that direct fragmentation may explain some of the directly imaged, wide-orbit, planetary-mass objects.

I will discuss the origin of an unexpected thermo-refractive noise contribution that we recently identified in nanophotonic waveguides made of amorphous materials [1]. This noise that corresponds to dynamic fluctuations in the picosecond time range sets a fundamental detection limit in photonic integrated circuits, in particular in integrated Raman sensor. Beyond the implications for sensing applications, the presence of this noise is a direct experimental signature of irreversible thermodynamic effects where inertia phenomena cannot be ignored. It also highlights the limits of current theories of thermo-refractive noise at high frequencies. Moreover, the possibility of experimentally accessing the optical spectrum of this noise contribution could be an opportunity to improve our understanding of optical non-linear effects in the presence of dissipation and go beyond models that ignore memory effects.

[1] N. Le Thomas, A. Dhakal, A. Raza, F. Peyskens, R. Baets, “Impact of fundamental thermodynamic fluctuations on light propagating in photonic waveguides made of amorphous materials”, Optica 5, 328-326 (2018).

In this talk I will describe our efforts to combine Low Energy Electron Microscopy (LEEM) and Molecular Beam Epitaxy (MBE) of III-As, and I will demonstrate how LEEM-MBE can provide new information about kinetic mechanisms of (In,Ga)As epitaxy. LEEM enable us to observe the surface of the sample in real time with 5 nm resolution in x/y plane and atomic resolution in z-axis. LEEM contrast provides information about local changes in diffraction conditions and dynamics of atomic steps. Therefore, LEEM enable us to obtain real-time imaging of kinetics of surface phases, changes in strain fields or changes of surface chemical potential of the different elements throughout the sample’s surface [1].

In our recent experiments we have imaged the formation of new terraces on the GaAs(001) surface and develop a technique that combines droplet epitaxy and LEEM to provide a full image of the surface phase diagram of GaAs (001). Our results shed light on the stability of the controversial 6x6 phase on GaAs (001) surfaces which is shown to be metastable during Langmuir evaporation, but can be stable over a narrow range of chemical potential under As flux [2,3]. We show that real-time imaging at growth conditions can be used as feedback to translate formation energy from T-0K to growth temperatures in density- functional theory calculations (Figure 1). Our recent results demonstrate that LEEM-MBE can help provide the missing pieces in the understanding fully epitaxial processes.

[1] E. Bauer, Rep. Prog. Phys. 57, 895 (1994) [2] K. Hannikainen et al. 123, 186102 (2019) [3] C.X. Zheng et al. 3, 124603 (2019) [4] A. Ohtake, Surf. Sci. Rep. 63, 295 (2008).

A new exploration of the Universe has recently started through gravitational-wave observations. On August 17, 2017, the first observation of gravitational waves from the inspiral and merger of a binary neutron-star system by the Advanced LIGO and Virgo network, followed 1.7 s later by a weak short gamma-ray burst detected by the Fermi and INTEGRAL satellites initiated the most extensive world-wide observing campaign which led to the detection of multi-wavelength electromagnetic counterparts. Multi-messenger discoveries are revealing the enigmas of the most energetic transients in the sky, probing neutron-stars physics, relativistic astrophysics, nuclear physics, nucleosynthesis, and cosmology. The talk will give an overview of the astrophysical implications of the gravitational-wave and multi-messenger observations, the prospects and challenges of the current and future gravitational-wave detectors.

Modeling, which includes developing, testing, and refining models, is a central activity in physics. Modeling is most fully represented in the laboratory where measurements of real phenomena intersect with theoretical models, leading to refinement of models and experimental apparatus. However, experimental physicists use models in complex ways and the process is often not made explicit in physics laboratory courses. We have developed a framework to describe the modeling process in physics laboratory activities. The framework has guided our course transformations, research into student leaning, and our assessment of student outcomes. I will present the framework, how we use it to transform our lab courses, and a new scalable assessment used to measure students’ modeling ability.

The dynamics of quasi-particles in non-equilibrium states of matter reveal the underlying microscopic coupling between electronic, spin and vibrational degrees of freedom. We aim for a quantum-state-resolved picture of coupling on the level of quasi-particle self-energies, which goes beyond established ensemble-average descriptions, and which requires ultrafast momentum-resolving techniques. The dynamics of electrons and excitons is measured with four-dimensional time- and angle-resolved photoelectron spectroscopy (trARPES), featuring a high-repetition-rate XUV laser source [1] and momentum microscope detector [2]. I will exemplify this experimental approach by discussing electron and exciton dynamics in the semiconducting transition metal dichalcogenide WSe2 [3,4]. Our approach provides access to the transient distribution of hot carriers in the entire Brillouin zone of photo-excited semiconductors and allows the quantification of energy relaxation dynamics. I will sketch the capability of multidimensional photoemission spectroscopy of providing orbital information [5], of visualizing the change of the electronic structure during phase transitions [6,7], and of revealing interfacial energy transfer processes in nanoscale heterostructures. The complementary view of ultrafast phonon dynamics is obtained through femtosecond electron diffraction. The elastic and inelastic scattering signal reveals the temporal evolution of vibrational excitation of the lattice and momentum-resolved information of transient phonon populations [8].

[1] M. Puppin et al., Rev. Sci. Inst. 90, 23104 (2019). [2] J. Maklar et al., arXiv:2008.05829 (2020). [3] R. Bertoni et al., Phys. Rev. Lett. 117, 277201 (2016). [4] D. Christiansen et al., Phys. Rev. B 100, 205401 (2019). [5] S. Beaulieu et al., Phys. Rev. Lett., accepted; arXiv:2006.01657 (2020). [6] C.W. Nicholson et al., Science 362, 821 (2018). [7] S. Beaulieu et al., arXiv:2003.04059 (2020). [8] L. Waldecker et al., Phys. Rev. Lett. 119, 036803 (2017).

Van der Waals Semiconductors such as transition metal dichalcogenides (TMDs) mark a new frontier for condense matter physics and the optoelectronics. The two-dimensionality of the monolayer TMDs and weak dielectric screening yield a significant enhancement of the Coulomb interaction. As a result, the optical properties of TMDs are widely dominated by excitons, Coulomb-bound electron–hole pairs. With high exciton binding energy, large exciton oscillator strength, and unprecedented integration flexibility with optical architectures, TMDs provide a new platform to study exciton polaritons, a new quasi-particle formed by strong coupling between an exciton and a photon. In this talk, I will begin with the excitons polaritons in TMDs monolayers coupled with a one-dimensional photonic crystal [1]. Then I will introduce two types of TMDs heterobilayers and talk about how the properties of excitons are controlled by heterostructures [2,3]. Lastly, these two types of heterobilayers are integrated with optical cavities, which give rise to exciton-photon interactions in weak and strong coupling regimes respectively [4].

References:
1. Zhang, L., Gogna, R., Burg, W., Tutuc, E. & Deng, H. Photonic-crystal exciton-polaritons in monolayer semiconductors. Nature Communications 9, 1–8 (2018). 2. Zhang, L. et al. Highly valley-polarized singlet and triplet interlayer excitons in van der Waals heterostructure. Phys. Rev. B 100, 041402 (2019). 3. Zhang, L. et al. Twist-angle dependence of moiré excitons in WS 2 /MoSe 2 heterobilayers. Nature Communications 11, 5888 (2020). 4. Paik, E. Y*. Zhang, L*. et al. Interlayer exciton laser of extended spatial coherence in atomically thin heterostructures. Nature 576, 80–84 (2019).

Super star clusters were an important ingredient of star formation in the early universe, where they played an crucial role also in self-regulating star formation through their feedback. They most likely were the progenitors of globular clusters. Therefore, it is important to understand how they came into being. In today's universe, they still are formed copiously in starburst galaxies, but are exceedingly rare in our own Galaxy. There is one candidate star-forming region in the Galactic center region, SgrB2, which has the potential to form one or even two super star clusters today. I will present multi-scale and multi-wavelength studies of this region which shed some light on the formation process.

Light has the remarkable capacity to reveal quantum features under ambient conditions, making exploration of the quantum world feasible in the laboratory and field. Further, the availability of high-quality integrated optical components makes it possible to conceive of large-scale quantum states by bringing together many different quantum light sources and manipulating them in a coherent manner and detecting them efficiently. By this route, we can envisage a scalable photonic quantum network that will facilitate the preparation of distributed quantum correlations among many light beams. This will enable a new regime of state complexity to be accessed - one for which it is impossible using classical computers to determine the structure and dynamics of the system. This is a new regime not only for scientific discovery, but also practical purpose: the same complexity of big quantum systems may be harnessed to perform tasks that are impossible using known future information processing technologies. For instance, ideal universal quantum computers may be exponentially more efficiently than classical machines for certain classes of problems, and communications may be completely secure. Photonic quantum machines will open new frontiers in quantum science and technology.

Biography: Professor Ian Walmsley FRS is the second Provost of Imperial College London since September 2018 and is also Chair in Experimental Physics at the College. Before joining Imperial, Professor Walmsley served as Pro-Vice-Chancellor (Research and Innovation) and Hooke Professor of Experimental Physics at the University of Oxford. Professor Walmsley graduated from Imperial with first class honours in physics in 1980, and completed his PhD at the University of Rochester before working as a postdoc at Cornell University. He became Assistant Professor of Optics at the University of Rochester in 1988, and held a number of roles there before joining the University of Oxford in 2001 as Professor of Experimental Physics. He was also a Senior Visiting Fellow at Princeton University. He was appointed Pro-Vice-Chancellor (Research) of the University of Oxford in 2009, becoming Pro-Vice-Chancellor (Research and Innovation) in 2015. At Oxford, he led the Networked Quantum Information Technologies Hub and headed up the creation of the Rosalind Franklin Institute. He was a member of the EPSRC Physics Strategic Advisory Team and was on the Max Planck Institute for Quantum Optics’ Science Advisory Board.In recognition of his contributions to quantum optics and ultrafast optics, Professor Walmsley was elected a Fellow of the Royal Society in 2012. He is also a Fellow of the Institute of Physics, the American Physical Society and the Optical Society of America.

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2019 seminars

This seminar will focus on three absorbing lines of research into nanophotonics, representing three consecutive time scales of the physics involved. It will begin with the first experimental observation of a chiroptical effect that was predicted 40 years ago [1,2]. Next, it will consider the smallest backjets (‘nanojets’) ever created and will discuss how they can serve to assemble novel metamaterials [3,4]. Finally, drawing inspiration from steampunk science fiction, it will illustrate how a vapour stabilization technique can greatly enhance quantum sensors [5]. When light shines on metal nanoparticles (NPs), it is initially (fs timescale) absorbed by the electrons. These electrons give rise to “instantaneous” nonlinear optical processes, such as second harmonic (SH) generation, whereby two photons at the fundamental frequency are converted into a single photon at twice that frequency \Omega. This SH generation is promising for applications, based on frequency conversion (for laser manufacturing), on nonlinear optical characterization[6,7] (e.g. microscopy) and on metasurfaces (for ultrathin telecom components). Our team recently demonstrated that in chiral metal NPs (those that lack mirror symmetry) the intensity of light, scattered at the SH frequency, is proportional to the chirality, see Fig. 1 [in the poster]. This effect was predicted 40 years ago, it is >10,000 more sensitive than corresponding linear optical effects and it could enable safer pharmaceuticals.

Taking little other than common cuticle, loaded with a small amount of melanin, butterflies have evolved some stunning metasurfaces. Often only microns thick, these act as selective reflectors and polarizers as well as being sometimes very strong scatterers (white) or very strong absorbers (black) of electromagnetic radiation. Similarly surface-structured metals, metasurfaces, can lead to unexpected effects: for example selective absorption, even at long wavelengths where metals are expected to behave as almost perfect mirrors, or even negative refraction.

The membranes of cells are made for a large part of lipids assembled in bilayers. Membranes behave as 2D-liquid, but they also have peculiar mechanical properties since they can be bent in the perpendicular direction, as well as stretched. The Brownian motion of inclusions embedded into membranes, such as trans-membrane proteins is not trivial and has been well described by the long-time accepted Saffman-Delbrück model. However, rich behavior arises for inclusions with non-symmetric shapes, that locally bend membranes. Coupling between inclusions' distribution and density can be observed as well as deviation to inclusion mobility described by Saffman-Delbrück. I will illustrate with some examples these peculiar properties specific to fluid membranes, and discuss some consequences for living cells.

Supernovae are the incredibly luminous deaths of stars that play vital roles in chemical enrichment, galaxy feedback mechanisms, and stellar evolution. In particular, Type Ia supernovae, the explosions of white dwarf stars in binary systems, were instrumental in the discovery of dark energy. However, what are their progenitor systems, and how they explode, remains a mystery. There is increasing observational evidence that there are multiple ways in which white dwarfs can explode. I will review the status of what we know about the stellar systems that produce Type Ia supernovae, as well as discuss the recently discovered zoo of peculiar transients that are also predicted to result from the explosions of white dwarfs, such as He-shell mergers, tidal disruption events, violent mergers. Distinguishing between these explosion scenarios and understanding their diversity is vital for producing the best samples for future precision measurements of the cosmological parameters.

Despite the fact that most of the radiation emitted in the universe since the Big Bang is in the THz range, readily available THz sources did not emerge until the end of the 20th century. Also, the detection of THz waves was proven to be very challenging. Altogether slowed down the adoption of THz waves (and technology), making this spectral window sandwiched between the optical and microwave regimes (ca. 0.3 – 3 THz; wavelength range between 1 mm and 0.1 mm) relatively unexplored. A great deal of effort is now carried out to develop such spectrum as it holds promise for next generation of wireless communication, medical diagnosis, security applications (chemical fingerprinting and standoff screening) and industrial control processes. The potential of THz in these realms arises from the ability of THz radiation to provide more bandwidth than microwaves/millimetre-waves and to pass through many optically opaque materials (e.g., clothing, paper, etc.), as well as the fact that specific rotations, vibrations or librations of molecules and molecular aggregates occur in this frequency range. In addition, THz radiation is non-ionizing and safe, unlike X-rays. Terahertz time-domain spectroscopy has emerged as a main spectroscopic modality to fill this so-called THz-gap and this seminar will showcase the use of the technique for two applications: (1) development of flexible low-loss low-dispersion waveguides ('cables'); (2) understanding of the extraordinary transmission phenomenon.

To be confirmed.

To be confirmed.

Our basic picture of condensed matter involves drawing a distinction between 'order' and 'disorder', as evidenced, respectively, by Bragg peaks and blobs or rings of scattering. However, modern, high resolution X-ray and neutron scattering sources can resolve a third type of scattering: 'pinch points' - near singularities in the structure factor that indicate special type of highly correlated state that is neither entirely ordered, nor entirely disordered. In this talk I will explain how pinch points are in part an illusion and in part an important diagnostic of a very special state. To explain this, I will refer to several types of material and meta-material, including water ice, spin ice, artificial spin ice, ionic solids and models of dipolar liquids, electrolytes and superfluids. I will also point out analogies with general physics.

Astrometry from space has unique advantages over ground-based observations: the all-sky coverage, relatively stable and temperature- and gravity-invariant operating environment delivers precision, accuracy and sample volume several orders of magnitude greater than ground-based results. Even more importantly, absolute astrometry is possible. The European Space Agency Cornerstone mission Gaia is delivering that promise. Gaia provides 5-D phase space measurements, 3 spatial coordinates and two space motions in the plane of the sky, for a representative sample of the Milky Way’s stellar populations (over 1 billion stars, being ~1% of the stars over 50% of the volume). Full 6-D phase space data is delivered from line-of-sight (radial) velocities for the 300million brightest stars. These data make substantial contributions to astrophysics and fundamental physics on scales from the Solar System to cosmology, from asteroids to gravitational waves. A few example results illustrating the rapidly changing understanding of the history of our Milky Way will be given.

To be confirmed.

To be confirmed.

To be confirmed.

2018 seminars

Microelectromechanical (MEMS) and nanoelectromechanical systems (NEMS) are ideal candidates for exploring quantum fluids, since they can be manufactured reproducibly, cover the frequency range from hundreds of kilohertz up to gigahertz and usually have very low power dissipation.

Their small size offers the possibility of probing the condensate on scales comparable to, and below, the coherence length. That said, there have been hitherto no successful measurements of NEMS res- onators in the liquid phases of helium.

Here we report the operation of doubly-clamped aluminum nanocantilevers in superfluid 4He at temperatures spanning the superfluid transition. The devices are shown to be very sensitive detectors of the superfluid density and the normal fluid damping. We use nanomechanical resonators with extremely high quality factor to probe superfluid 4He at millikelvin temperatures, as well.

The high sensitivity of these devices to thermal excitations in the environment makes it possible to drive them using the momentum transfer from phonons generated by a nearby heater. This so-called phonon wind is a reverse thermomechanical effect that until now has never been demonstrated.

Analysing, constructing, and translating between graphical, pictorial, and mathematical representations of physics ideas and reasoning flexibly through them (representational competence) is a key characteristic of expertise. It is challenging for learners to develop, but little instruction is explicitly designed with this purpose in mind.

This talk will focus on the role of interactive computer simulations with appropriate scaffolding in supporting representational learning. We have been developing combined simulation-tutorials for the learning of quantum mechanics, whereby students first work on problems independently, constructing representations they will later see in the simulation, followed by further problems with simulation support.

This talk will describe the structure and sequencing of the simulation-tutorials and present results from pre-, mid- and post-tests to assess student learning.