Very many congratulations to the EPSRC CDT in Metamaterials PGR Lauren Barr for submitting her PhD thesis on “Giving Metamaterials a Hand – Electromagnetic interactions in chiral metamaterials”, supervised by Prof. Euan Hendry and Prof. Alastair Hibbins.
(The thesis title is a play-on-words, as the term chiral comes from the Greek word for hand, and describes things that have no mirror symmetry – like hands!)
Lauren previously published various articles on twisted-cross metamaterials (Scientific Reports, 2015) and near-field chiral interactions (Physical Review B, 2018), on direct measurements of topological states (Nature Communications, 2017) and ideal Weyl points in microwave metamaterials (Science, 2018), and presented her results at many conferences, e.g. at Metamorphose 2015 (Ruka, Finland), NanoMeta2017 (Seefeld, Austria), CIMTEC 2016 (Perugia, Italy) and Metamaterials 2017 (Marseille, France).
Lauren moved to Exeter in 2014 to join the CDT after completing a Master’s degree in physics at Queen’s University Belfast. From day one, Lauren engaged fully with the opportunities the CDT offered to develop her portfolio of experiences beyond her research project by joining the student advisory group and the SPIE student chapter (now Exeter University Optics and Photonics Society (EUOPS)). Together with her peers, she successfully applied for an $8,000 grant to host an international OSA (Optical Society) network of students (IONS) conference at Exeter in 2019.
Her passion for sharing knowledge and insipring the wider community is outstanding. Lauren coordinated and presented in two videos, highlighting her work to the general public in one case, and to a scientific audience in another; demonstrated light-based experiments at a Big Bang event (2016), acted as a “Metabuddy” for Honiton College students (2107), and worked with children at the Girls into STEMM / Girls into Physics days (2017 and 2015, resp), was selected as a presenter for “Soapbox Science” (2017) and “Pint of Science” (2018), as well as developing and running several unique outreach events such as the “Lighting up RAMM” event at Exeter’s Royal Albert Memorial Museum (2018).
Apart from the time spent working on my research, I also spent many happy hours outside the office. I have fantastic memories of Erick’s (often late-night) house-parties, Sathya’s (often spicy) lunches and Sam’s (slightly competitive) games nights. The CDT has given me the chance to learn much more about many different aspects of physics and research, and meet very cool people from all over the world. After completing my viva I hope to stay in Exeter a little longer, to work on some new projects and pass on some of my experiences to new students. Long-term, I just plan to keep doing the things I find most interesting. (Lauren Barr, August 2018)
Well done, Lauren! We’re very confident that you’ll pass your viva with excellent results.
PhD thesis abstract “Giving Metamaterials a Hand – Electromagnetic interactions in chiral metamaterials”:
The focus of this thesis is the interaction of electromagnetic fields with chiral structures in the microwave regime. Through this study, which focuses on three regimes of electromagnetic interactions, I aim to develop a deeper understanding of the consequences and manifestations of chiral interactions The structures are on the order of, or smaller than, the wavelength of the probing radiation. As the structures are chiral, they have broken inversion symmetry, and exist in two states where one is the mirror image of the other. The results in this thesis can have impacts on future optical communications technologies and methods of sensing biological molecules.
To begin with, the manipulation of the circular polarisation of a propagating beam by bilayer chiral metasurfaces is investigated. The metasurfaces consist of two layers of stacked crosses with a twist between top and bottom layers, forming chiral metamolecules. A broad frequency region of dispersionless polarisation rotation appears between two resonances, due to alignment between electric and magnetic dipoles. The dependence of this effect on the layer separation is studied for two similar metasurfaces.
Evanescent chiral electromagnetic fields are the focus of the next chapter. An array of chiral antennas produces chiral near-fields at their resonant frequency. Aligned and subwavelength helices placed within this field interact differently depending on the handedness of the field with respect to the handedness of the helices. This difference in interaction strength is measured for the helices and an effective medium model where multipolar interactions are forbidden. Comparison of these two systems leads to the conclusion that the contribution to a chiral interaction from multipolar modes is minimal, in contrast to previous publications.
The third study concentrates on the electromagnetic wave bound to an “infinitely long” metal helix. The helix has infinite-fold screw symmetry, and this leads to interesting features in the energy-dispersion of the waves it supports. The broad frequency range of high, tunable, dispersionless index is interpreted using a geometrical approach, and the factors that limit the bandwidth explained. A modified geometry is suggested for increased bandwidth.
The final part of the thesis is dedicated to future work, based on the results presented thus far. Three suggestions for future study are presented, including chiroptical signals from higher-order chiral arrangements, the effect of reflecting surfaces next to chiral objects and the possible use of orbital angular momentum for chiroptical measurements.