Seminars

Speaker: Vani Lanka (Dr Sam Ladak's group)
Date: Thursday 9 February 2023
Time: 14:30
Venue: Queen's buildings North N3.23 with zoom live streaming

Artificial spin-ices (ASI) consist of periodically/aperiodically arranged nanomagnets on lattice sites moulded to various geometries. Extensive research upon frustration has shown limitations in 2D, therefore extending these systems into 3D gives rise for unconventional states of matter to be studied. In particular emergent magnetic monopoles, spin textures and dynamics. 3DASI follows local ordering principles, known as the ice rules, which aim to minimise magnetic charge. This research aims to develop energetically favourable 3DASI systems with extensively degenerate ground states, allowing artificial monopoles to move in a divergence-free vacuum.

Two-photon lithography has allowed bulk spin ice geometries to be fabricated with high spatial resolution, potentially overcoming the limitations seen in 2D. By performing magnetic force microscopy (MFM) studies with the aid of micromagnetic simulations, magnetic monopole defects and transport within the lattice have been realised, revealing glimpses of charge ordered ground states. Monte Carlo simulations have demonstrated a rich phase diagram of ground states including various charge ordered states.

Thus far our work has suggested that charge ordered ground states are achievable with regions of highly correlated magnetic charge. Our recent research aims to adjust lattice parameters, wire thickness/length and surface energetics to directly visualise the ground state. We model an annealing process using various demagnetising protocols. We also present future advances within exploration of the phase space via thermally active ASI aims to isolate magnetic monopoles.

Dynamically, these systems open up a collection of technological applications through advancing memory, data storage and encryption. Overall, the field of artificial spin-ice has significant avenues to explore with exciting outcomes for the future.

References:

1. Williams, G., Hunt, M., Boehm, B. et al. Two-photon lithography for 3D magnetic nanostructure fabrication. Nano Res. 11, 845-854 (2018).

2. May, A., Hunt, M., Van Den Berg, A. et al. Realisation of a frustrated 3D magnetic nanowire lattice. Commun Phys 2, 13 (2019).

3. May, A., Saccone, M., van den Berg, A. et al. Magnetic charge propagation upon a 3D artificial spin-ice. Nat Commun 12, 3217 (2021).

Speaker: Tristan Burman (Prof Peter M Smowton's group)
Date: Thursday 23 February 2023
Time: 14:30
Venue: Translational Research Hub Seminar Room 0.01

Abstract:

Optoelectronics are currently going through a period of development similar to that seen in electronics over the last fifty years, devices are being miniaturised and integrated circuits are being developed with an increasing number of functions on chip. Laser devices are considered to be one of the more difficult components to integrate, and further research is required to produce an efficient manufacturing process that can be applied to large substrates for dense integration of lasers. Typically, the laser facets are manufactured via cleaving, this process breaks the wafer into pieces and is therefore inappropriate for monolithic integrated circuits.

Using etching to manufacture the laser facets overcomes the integration issue however each come with their own draw backs. Wet etching is unable to reliably produce the vertical profile required for laser facets. Dry etching can achieve the vertical profile using methods preferred by large scale manufacturing however, it does tend to introduce a sidewall roughness that the manufacture process must be optimised to reduce. The impact that the sidewall roughness has on facet reflectivity and therefore device performance is yet to be fully understood. There are commonly held beliefs that an RMS roughness below the emission wavelength divided by an arbitrary value will produce a tolerable loss in efficiency [1]. There are also models that that attempt to better relate an RMS roughness of a surface to its reflectivity [2]. These models are often used to determine a facets reflectivity from a roughness measurement or vice versa. However, they are developed for a single material system and rarely verified on real world devices.

This work independently determines the facet reflectivity and roughness of InP lasers operating at telecom wavelengths manufactured on 100mm wafers, and comparing results to the theoretical models.

Reference:

1. Francis, D. A., et al. Effect of facet roughness on etched facet semiconductor laser diodes. Applied Physics Letters 68, 1598-1600 (1996).

2. D. A. Stocker, et al. Facet roughness analysis for InGaN/GaN lasers with cleaved facets, Applied Physics Letters 73, 25-1927 (1998).

Speaker: James Meiklejohn (Dr Samuel Shutts's group)
Date: Thursday 16 March 2023
Time: 14:30
Venue: Translational Research Hub Seminar Room 0.01

We investigate the suitability of different device designs for Vertical Cavity Surface Emitting Lasers (VCSELs) intended for use in miniaturised magnetometers required in applications such as geophysical mapping, underwater ordinance detection and monitoring electrical pulses generated within the human body. Novel geometries are motivated by the requirement to increase the output power while maintaining beam quality and optical coherence, and we explore the use of multiple closely-spaced emitters that can be coupled together in-plane to create a single output.

Variations in the design and spacing of the devices geometry are examined, and the results discussed with reference to the physical mechanisms involved in the coupling, including the in-plane propagation of the optical field and the effect of the mesa shape on reflections in the lateral direction. The threshold current, maximum output power and side-mode suppression ratio are measured at a range of temperatures up to the operating temperature of 75 deg.C, and we compare the key operating characteristics of conventional and novel device designs. With reference to the requirements of the application, such as single-mode emission at the caesium D2 transition at 852.2 nm and with output-powers over 1 mW, we identify the best-performing devices and briefly discuss the suitability of these devices as optical sources in miniature magnetometers.

Speaker: Freya Turley (Prof Wolfgang Langbein's group)
Date: Thursday 23 March 2023
Time: 14:30
Venue: Queen's buildings North N3.23 with zoom live streaming

Abstract:

The interaction between proteins and lipid membranes is a fundamental process underpinning key functions in cell biology. Despite the importance of such system, many questions are still unanswered concerning the mechanism of protein organisation and function within the cell membrane, and how this is modulated by the lipid environment, due to the lack of suitable measurement techniques.

We are developing a novel optical microscopy technique called Interferometric Gated Off-axis reflectometry (iGOR) which promises to be a step-change, enabling us to monitor the motion of single membrane proteins with high sensitivity (~10kDa estimated smallest size limit) rapidly (~1ms) in 3D label-free with ~10nm localisation precision, simultaneously with local membrane properties at the protein site, such as membrane curvature and thickness changes.

As lipid membrane model systems, we are using giant unilamellar vesicles (GUVs) of about 30-50um diameter. This is a versatile system where we can control the lipid composition and the osmolarity of the internal and external solutions.

We have optimised GUV stability using sucrose concentration differences, characterised via a quantitative differential interference contrast microscopy (qDIC) method developed in house. We found that using a concentration difference between the internal and external sucrose solution >0.2mM establishes an internal pressure too great for a unilamellar vesicle to withstand. We developed a numerical simulation model fitting the experimental qDIC data to accurately determine the lamellarity. An overview of these qDIC results and our progress on characterising lipid membrane dynamics and the insertion of pore forming proteins using iGOR will be presented.

Plasmons are excited in metallic nanostructures and have the ability to enhance light intensity by several orders of magnitudes. Due to this property, they have been extensively used for sensing applications. Recently, there has been a renewed interest to couple and dress quantum emitters (such as quantum dots, dye molecules etc) with plasmons. The aim is to generate new photonic platforms for exploring light-matter interactions. The highlight so far has been the demonstration of single molecule strong coupling with plasmons at room temperature [1]. In my talk, I will briefly introduce plasmons and give a quick overview of the most recent work in the field of Quantum Plasmonics. I will then explain the plasmon properties that allow strong coupling at room temperature and demonstrate the unique properties of nano-cavities [2,3]. I will present my work that focuses on understanding the photonic modes in plasmonic nanocavities [4], the Rabi oscillations of emitters with plasmonic nano-cavities, and their complex coupling with multiple photonic modes of different character [5]. If time permits, I will discuss how one can access specific chemical bonds within a single molecule using plasmons [6].

[1] Chikkaraddy, R., de Nijs B., et al. Nature, 535, 127 (2016)
[2] Kongsuwan, N., Demetriadou A., et al., ACS Photonics, 5, 186 (2018)
[3] Mertens J., Demetriadou A., et al., Nano Letters, 16, 5605 (2016)
[4] Kongsuwan N. , Demetriadou A., et al., ACS Photonics, 7, 463 (2020)
[5] Demetriadou A., Hamm J. et al., ACS Photonics, 4, 2410 (2017)
[6] Benz, F., Schmidt M., et al., Science, 354, 726 (2016)

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The advent of high resolution hydrodynamical cosmological simulations allows us to now study the dynamics of barred galaxies, such as our own Milky Way, within the full ΛCDM cosmological context. I will present what we have learned about the formation history of our galaxy and its inner structures -- such as the bar and the boxy/peanut bulge -- by comparing the chemo-dynamical properties of stellar populations of these inner regions to the Auriga cosmological simulations. In particular, I will present evidence of the almost entirely in-situ formation of the Galaxy's bulge, and of its unusually quiescent merger history. I will also show how studying the dynamics of barred galaxies in cosmological simulations -- in particular the interaction through dynamical friction of the bar and the dark matter halo -- can help us shed light on the amount of dark matter in massive spiral galaxies. I will discuss these findings within the context of the recently reported 'failed feedback problem' as well as within the context of galaxy formation and evolution in general.

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The symmetry of a material plays a fundamental role in determining its physical properties. Symmetry breaking can modify the physics of a system and produce new and unusual behavior. Superconductivity is one of the best examples of a symmetry-breaking phenomenon. In conventional superconductors, gauge symmetry is broken, while in unconventional superconductors other symmetries may also be broken. In this talk, I present some recent results on Time-Reversal Symmetry Breaking in Unconventional Superconductor.

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Meeting ID: 829 2884 3353
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Meeting ID: 922 8539 2843
Password: 168617

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Meeting ID: 829 2884 3353
Password: 780047

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Meeting ID: 922 8539 2843
Password: 168617

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Meeting ID: 922 8539 2843
Password: 168617

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Meeting ID: 829 2884 3353
Password: 780047

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Meeting ID: 922 8539 2843
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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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