Bell Burnell PhD Studentship

Many microalgae swim and bias their motion in response to useful environmental stimuli. For example, they can actively track the light they need to grow (phototaxis). Passive mechanisms also orient cells: gravity causes bottom-heavy microalgae to swim upwards (gravitaxis). In fluid flows, e.g. flow in a pipe, microalgae swim at an angle dependent on the balance between gravitational and viscous torques on a cell (gyrotaxis). Thus, beautiful ‘plumes’ of cells emerge in downwelling flows because of the bias on individual swimmers. This biophysics is well-known in the field of biologically active fluids, but has largely been ignored in the bioengineering of industrially-relevant swimming algae, such as Dunaliella salina [1]. These algae are grown in, and harvested from, photobioreactors, open ponds or transparent pipes in which algal suspensions are flowed and grow photosynthetically. Recent work by our group has developed and tested the theory of dispersion for swimming algae in infinite pipes [2], and shown that light and porous media can be used to concentrate swimmers [3], but has not tackled the scale of photobioreactors directly. In this project, aspects of the theoretical biophysics of swimming suspensions of algae will be explored and applied to finite-length pipe flows, with the goal of predicting suspension behaviour in photobioreactors. For example, a model that predicts phototactic accumulation of algae in a photobioreactor to light will be developed. This could be used to explore the coupling between suspension biofluid dynamics and the photosynthetic growth of microalgae in the bioreactor. There will be the opportunity to carry out experiments to test the model predictions using Dr Croze’s pilot-scale photobioreactors hosted in the labs of Dr Gary Caldwell (School of Natural and Environmental Sciences).

[1] M. A. Bees & O. A. Croze, Mathematics for streamlined biofuel production from unicellular algae. Biofuels 5, 53 (2014) https://doi.org/10.4155/bfs.13.66

[2] O. A. Croze, R. N. Bearon & M. A. Bees, Gyrotactic swimmer dispersion in pipe flow: testing the theory, J. Fluid Mech. 816, 481-506 (2017) https://doi.org/10.1017/jfm.2017.90

[3] P. Prakash & O. A. Croze, Photogyrotactic concentration of a population of swimming microalgae across a porous layer, Front. Phys. (2021) https://www.frontiersin.org/articles/10.3389/fphy.2021.744428

Supervisor: Dr Otti Croze

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Atomically-thin single-photon emitters at room temperature

The Quantum Photonics Group at Newcastle University (led by Dr Jonathan Mar) performs experimental research at the interface of solid-state quantum optics and nanophotonics for applications in quantum computing and quantum communication and cryptography. In particular, we are interested in the optical properties of excitons and carrier spins in semiconductor quantum dots and defect colour centres in 2D materials and diamond and their optical coupling to nanophotonic and nanoplasmonic devices towards the realization of single-photon emitters and spin-based quantum bits.

Since the Nobel-prize winning discovery of graphene in 2004, a whole host of novel atomically-thin 2D materials beyond graphene has been discovered, exhibiting a wide variety of fascinating and technologically useful properties. One such 2D material is hexagonal boron nitride (h-BN). Unlike transition metal dichalcogenides which need to be cooled to cryogenic temperatures, h-BN has been shown to host bright single-photon emitters even up to room temperature due to atomic defect states located deep within its large bandgap. Practical single-photon emitters such as these defect colour centers in h-BN are therefore essential building blocks for the realistic implementation of quantum computing, quantum communication and cryptography, as well as a wide range of other transformative quantum technologies.

In this PhD project, the student will investigate the optical properties of room-temperature single-photon emitters due to defect colour centres in 2D h-BN for applications in quantum information processing and other practical quantum technologies. The student will examine 2D h-BN samples obtained via chemical vapour deposition, mechanical exfoliation (i.e. the “Scotch tape” method), and liquid-phase exfoliation.

The experimental work of this project will be carried out in Newcastle University’s world-class research facilities and using state-of-the-art experimental tools. The student will receive in-depth training and become proficient in using advanced techniques such as Raman spectroscopy and microscopy, high-resolution photoluminescence spectroscopy and microscopy, photon correlation measurements to confirm single-photon emission, nanoscale device fabrication, state-of-the-art electron microscopy, and theoretical device and materials modelling. Additionally, the student will have the opportunity to interact closely with our collaborators in the Cambridge Graphene Centre at Cambridge University and the National Institute for Materials Science (Japan), as well as with leading industry partners and academic members of the Newcastle-Durham Joint Quantum Centre.

Supervisor: Dr Jonathan Mar

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Ultralow-voltage operating atomically thin quantum light emitting diodes

Quantum dot light emitting diodes (LEDs) are considered as key elements in the next generation emerging display and lighting technologies due to their excellent colour purity and high brightness. LEDs turn on upon radiative recombination of electrons and holes injected under external voltage. The minimum voltage required for it corresponds to the bandgap of emissive semiconductors. However, using a mechanism called upconversion, it is possible to lower this voltage. As a result, charge carriers whose energy was originally too low to overcome the material’s bandgap can now easily cross this potential barrier, recombine and emit light.

In this project, we will use novel atomically thin (2D) graphene-like materials. We will combine them like LEGO game blocks to create devices emitting quantum light at ultra-low voltages. The project will be based on the recently observed effect of upconversion in 2D materials [1]. We will work towards further lowering the minimal operating voltage on the route towards energy-efficient light sources. We will also produce quantum dot-like emitters of single photons (quantum light) in these devices and tune them to emit light in the telecommunication wavelength range necessary for applications in secure quantum communications. We will also explore potential fundamental applications in exciton condensation and superfluidity.

The project will involve nanofabrication, optical and electron transport measurements, scanning probe microscopy and instrumentation development.

[1] J. Binder et al. Nature Communications, 10, 2335 (2019)

Supervisor: Dr Aleksey Kozikov

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Metamaterials, plasmonics and light-matter interaction

Metamaterials (artificial electromagnetic media) and metasurfaces (their 2D version) can provide full control of light-matter interactions by arbitrarily tailoring the electromagnetic response of matter.

They can be applied at different frequency ranges such as: acoustics, microwaves, Terahertz and optics.

They offer great opportunities in applications such as:

Levitation and invisibility cloaking
Plasmonic nano structures
Quasi-optical devices
Spatiotemporal modulation of fields and waves

Our group is focused on the theoretical, numerical, and experimental study of metamaterials and metasurfaces from their basic principles and their applications to spatial, temporal and spatiotemporal manipulation of wave propagation.

Supervisor: Dr Victor Pacheco-Peña

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Ultracold Quantum Sensors & Atomtronics

Quantum technologies with ultracold atoms is an exciting area of intense experimental and theoretical research. One of the promising avenues in this direction is the use of (neutral) ultracold atomic gases in closed circuits in what has become known as the emerging field of atomtronics [1], a key strength of which derives from the experimental flexibility of potential landscapes, which allows for new quantum device architectures and functionalities which have no analog in conventional electronics. Within this broader context, which facilitates a variety of different research avenues, the specific aim of this PhD project is to devise and numerically simulate strategies for ultracold quantum sensors – similar to the recently-observed atomic analogue of the superconducting quantum interference device [2] – utilizing atom neutrality and sensitivity to external fields to produce next-generation rotation and acceleration sensors. This project focusses on numerically modelling ultracold atom dynamics across single and multiple connected closed (ring-trap like) geometries, with particular attention paid on quantum transport across Josephson junctions, stability and transport of persistent currents, and interaction across coupled superfluids, by means of advanced stochastic and kinetic models [3] which fully account for the interaction of the superfluid gas with the co-existent incoherent thermal cloud. As such, the successful student will gain experience across diverse thematic areas, including non-equilibrium quantum modelling under realistic experimental conditions; ultracold quantum sensors; dynamics of persistent currents; superflow transport across Josephson junctions and coupling across different superfluid components, while simultaneously learning high-end numerical computing and being exposed to collaborations with leading experimental groups in the UK and beyond.

[1] L. Amico et al., Roadmap on Atomtronics: State of the art and perspective, AVS Quantum Sci. 3, 039201 (2021) ( https://doi.org/10.1116/5.0026178 )

[2] C. Ryu et al., Quantum interference of currents in an atomtronic SQUID, Nat. Communications 11, 3338 (2020) ( https://doi.org/10.1038/s41467-020-17185-6 )

[3] N. Proukakis et al., Finite Temperature and Non-Equilibrium Dynamics, World Scientific (2013) ( https://doi.org/10.1142/p817 )

Supervisor: Prof Nick Proukakis

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The cosmic web as a laboratory for fundamental physics

The 21st century has transformed cosmology from a speculative field into a precision science driven by theoretical methods, numerical simulations and galaxy survey data. Measurements of galaxy clustering and weak gravitational lensing will map the 3d matter distribution over the past 10 billion years and answer fundamental questions in physics: What are the properties of the early universe? What is the nature of dark energy? What are the characteristics of dark matter?

A slice through the Euclid Flagship mock catalog of 2.6 billion galaxies displaying the growth of structure over 10 billion years from early (green/right) to late times (red/left).

Unravelling these mysteries is difficult because the information is hidden in the galaxy distribution that has been shaped by nonlinear clustering and is characterised by complex non-Gaussian statistics. As a PhD student working on this project, you will develop and apply state-of-the art statistical, analytical and computational techniques to extract the maximal information on fundamental physics from the late-time matter distribution. You will be part of a local research team in Newcastle and have the option to join the Euclid Consortium to contribute to the ESA space mission Euclid which will launch a dedicated satellite in 2022 to map the dark universe across one third of the sky.

Supervisor: Dr Cora Uhlemann

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Cosmic Shear Cosmology

Weak gravitational lensing is now established as one of the most compelling probes of cosmology, allowing to map out the dark matter distribution on the sky while providing some of the best measurements of its clumpiness and abundance. Existing cosmic shear data such as that from the Kilo Degree Survey (KiDS) are currently being analysed and novel methods are being deployed in order to maximize their scientific outcome.

Among these, simulation-based approaches are receiving an increasing level of attention for their potential at better capturing the information contained in the data. In this project, the PhD student will specialize in some of these novel analysis techniques, validate the methods on simulations at first, then apply the findings on the KiDS legacy data, and subsequently on the first data release of the LSST and Euclid surveys.

Supervisor: Dr Joachim Harnois-Deraps

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From Galaxies to Cosmology

Cosmological galaxy surveys have in recent years begun to place field-leading constraints on the parameters of our model of the Universe. The statistical power of these data sets will only continue to grow in the coming years, with the Rubin Observatory Legacy Survey of Space and Time (LSST), slated to come online in 2023, set to increase the number of galaxies to the tens of billions. At the same time, exciting indications of potential cracks in the standard cosmological model (ΛCDM) have emerged, notably a discrepancy in the values of key cosmological parameters as measured in the early and late Universe. If confirmed, this could indicate the breakdown of ΛCDM as a universal standard model of cosmology.

Will LSST confirm ΛCDM, or will it point us towards new physics? To ensure a robust answer to this question, we need to significantly improve our understanding and treatment of the ways in which, on cosmological scales, galaxies behave not just as ways to trace the cosmic dark matter but as complex astrophysical objects.

In this PhD project, you will develop new modelling tools for and make novel measurements of two key astrophysical effects which impact cosmological measurements of gravitational lensing and cosmic structure: the intrinsic alignment between orientations and shapes of nearby galaxies due to tidal physics and environment effects, and the so-called ‘galaxy bias’ – the link between the galaxy distribution and the underlying dark matter field. In doing so, you will both ensure rigorous answers to burning cosmological questions and improve upon our understanding of the astrophysical behaviour of galaxies at a population level.

Supervisor: Dr Danielle Leonard

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Dust in the Wind: new insights into the impact of supermassive black holes with the James Webb Space Telescope (JWST)

The James Webb Space Telescope (JWST) is the most anticipated space observatory of this generation. With unprecedented capabilities for infra-red astronomy, it will revolutionise our studies of galaxies and supermassive black holes. In this project, you will work with some of the first data that will be taken with JWST to explore the central regions of nearby active galactic nuclei (AGNs), galaxies in which supermassive black holes are growing. Collaborating with the Galaxy Activity, Torus and Outflow Survey (GATOS), you will use a wide range of multi-wavelength datasets from other modern telescopes to uncover dusty winds emanating from these AGNs, and quantitatively constrain their mass and energy outflow rates. You will compare the JWST images to advanced simulations of the AGN environment, and add in information from JWST spectroscopy to understand the interplay between black holes and star-formation in these galaxy centres.

As a motivated student taking on this project, you will be expected to work with a large international team on some of the most cutting-edge observations available. A strong background in data analysis and programming will be beneficial, particularly Python in order to use available JWST analysis tools and develop new ones. Familiarity with the topics of AGN science and infra-red astronomy is beneficial, but not required.

Links:

Newcastle University Astronomy research group

JWST programme summary

GATOS

Supervisor: Dr David Rosario

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X-ray polarization: a new window to understand black holes

Black holes represent the most catastrophic end point of stellar evolution and posses the most extreme gravitational field possible. The vast majority of black holes are invisible to us. However, we can detect those that are accreting material, since the accretion disc that forms around the black hole becomes hot enough to glow brightly in X-rays. This reveals two populations of black holes: X-ray binaries – whereby a stellar-mass black hole accretes from a stellar companion – and active galactic nuclei – whereby a supermassive black hole accretes from its host galaxy. However, the vicinity of the black hole is far too small to directly image, and so indirect mapping techniques are required if we are to observe how matter behaves just before it falls forever beyond the event horizon. Until now, we have only been able to measure the brightness of the X-rays and how this depends on wavelength and time. This will change in December 2021 when NASA's Imaging X-ray Polarimetry Explorer (IXPE) launches. IXPE will be the first satellite for more than 40 years capable of measuring the polarisation of X-rays. Since its sensitivity is more than 100 times that of its predecessors, it will be able to make the first firm detections of polarisation for X-ray binaries and active galactic nuclei. The successful candidate for this PhD project will become an associate member of the IXPE team in order to analyse and develop models for new IXPE data from X-ray binaries and active galactic nuclei. In particular, we will analyse how the polarization degree and angle depend on X-ray wavelength and we will apply state-of-the-art techniques to test whether or not the polarization angle is swinging back and forth with time; which is predicted to happen if the inner accretion flow is being caused to wobble around the black hole spin axis by a relativistic effect called frame dragging. We have theoretical expectations, but we do not really know what the polarization of these objects will be until the data start to come down from IXPE. The successful candidate will be at the forefront of this journey of discovery, looking at black holes through the entirely new window of X-ray polarization.

Supervisor: Dr Adam Ingram

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