Showing 1–27 of 27 results for author: George, A

Optimized near-field optical response via adaptive tip illumination

Authors: Tao Chen, Wei Wang, Ziyang Gan, Daniel Repp, Jinxin Zhan, Antony George, Henrik Schneidewind, Ulf Peschel, Andrey Turchanin, Volker Deckert

Abstract: The performance of tip-enhanced optical microscopy is often limited by inefficient coupling of the excitation field to the plasmonic tip apex, as well as by thermal drift and optical aberrations. Here, we demonstrate that adaptive wavefront shaping based on Zernike mode provides a practical approach to achieving robust near-field optimisation at the tip apex. Using a sequential feedback algorithm,… ▽ More The performance of tip-enhanced optical microscopy is often limited by inefficient coupling of the excitation field to the plasmonic tip apex, as well as by thermal drift and optical aberrations. Here, we demonstrate that adaptive wavefront shaping based on Zernike mode provides a practical approach to achieving robust near-field optimisation at the tip apex. Using a sequential feedback algorithm, initially using the near-field signal, we narrow the illumination point-spread function and suppress sidelobes. This demonstrates that Zernike-mode control can be used for both aberration correction and field engineering. In tip-enhanced Raman measurements of a Janus MoSSe monolayer, conventional near-field optimisation increases the signal intensity by around 1.4 fold. A second optimisation step based directly on the Raman-band intensity yields a further 5 to 15 fold enhancement, depending on the specific tips used. These results establish a systematic, optics-based strategy for optimising tip fields, providing a transferable framework for improving tip-enhanced and related near-field spectroscopies. △ Less

Submitted 15 May, 2026; originally announced May 2026.

Comments: 16 pages, 4 figures

Citizen CATE 2024: Extending Totality During the 8 April 2024 Total Solar Eclipse with a Distributed Network of Community Participants

Authors: Sarah A. Kovac, Amir Caspi, Daniel B. Seaton, Paul Bryans, Joan R. Burkepile, Sarah J. Davis, Craig E. DeForest, David Elmore, Sanjay Gosain, Rebecca Haacker, Marcus Hughes, Jason Jackiewicz, Viliam Klein, Derek Lamb, Valentin Martinez Pillet, Evy McUmber, Ritesh Patel, Kevin Reardon, Willow Reed, Anna Tosolini, Andrei E. Ursache, John K. Williams, Padma A. Yanamandra-Fisher, Daniel W. Zietlow, John Carini , et al. (218 additional authors not shown)

Abstract: The Citizen CATE 2024 next-generation experiment placed 43 identical telescope and camera setups along the path of totality during the total solar eclipse (TSE) on 8 April 2024 to capture a 60-minute movie of the inner and middle solar corona in polarized visible light. The 2024 TSE path covered a large geographic swath of North America and we recruited and trained 36 teams of community participan… ▽ More The Citizen CATE 2024 next-generation experiment placed 43 identical telescope and camera setups along the path of totality during the total solar eclipse (TSE) on 8 April 2024 to capture a 60-minute movie of the inner and middle solar corona in polarized visible light. The 2024 TSE path covered a large geographic swath of North America and we recruited and trained 36 teams of community participants ("citizen scientists") representative of the various communities along the path of totality. Afterwards, these teams retained the equipment in their communities for ongoing education and public engagement activities. Participants ranged from students (K12, undergraduate, and graduate), educators, and adult learners to amateur and professional astronomers. In addition to equipment for their communities, CATE 2024 teams received hands-on telescope training, educational and learning materials, and instruction on data analysis techniques. CATE 2024 used high-cadence, high-dynamic-range (HDR) polarimetric observations of the solar corona to characterize the physical processes that shape its heating, structure, and evolution at scales and sensitivities that cannot be studied outside of a TSE. Conventional eclipse observations do not span sufficient time to capture changing coronal topology, but the extended observation from CATE 2024 does. Analysis of the fully calibrated dataset will provide deeper insight and understanding into these critical physical processes. We present an overview of the CATE 2024 project, including how we engaged local communities along the path of totality, and the first look at CATE 2024 data products from the 2024 TSE. △ Less

Submitted 15 December, 2025; originally announced December 2025.

Journal ref: Solar Physics, Vol. 301, 12 (42pp); 2026 January 22

Valley-dependent emission patterns enabled by plasmonic nanoantennas

Authors: Tobias Bucher, Jingshi Yan, Jan Sperrhake, Zlata Fedorova, Mostafa Abasifard, Rajeshkumar Mupparapu, Haitao Chen, Emad Najafidehaghani, Khosro Zangeneh Kamali, Antony George, Mohsen Rahmani, Thomas Pertsch, Andrey Turchanin, Dragomir N. Neshev, Isabelle Staude

Abstract: Selective control over the emission pattern of valley-polarized excitons in monolayer transition metal dichalcogenides is crucial for developing novel valleytronic, quantum information, and optoelectronic devices. While significant progress has been made in directionally routing photoluminescence from these materials, key challenges remain: notably, how to link routing effects to the degree of val… ▽ More Selective control over the emission pattern of valley-polarized excitons in monolayer transition metal dichalcogenides is crucial for developing novel valleytronic, quantum information, and optoelectronic devices. While significant progress has been made in directionally routing photoluminescence from these materials, key challenges remain: notably, how to link routing effects to the degree of valley polarization, and how to distinguish genuine valley-dependent routing from spin-momentum coupling - an optical phenomenon related to electromagnetic scattering but not the light source itself. In this study, we address these challenges by experimentally and numerically establishing a direct relationship between the intrinsic valley polarization of the emitters and the farfield emission pattern, enabling an accurate assessment of valley-selective emission routing. We report valley-selective manipulation of the angular emission pattern of monolayer tungsten diselenide mediated by gold nanobar dimer antennas at cryogenic temperature. Experimentally, we study changes in the system's emission pattern for different circular polarization states of the excitation, demonstrating a valley-selective circular dichroism in photoluminescence of 6%. These experimental findings are supported by a novel numerical approach based on the principle of reciprocity, which allows modeling valley-selective emission in periodic systems. We further show numerically, that these valley-selective directional effects are a symmetry-protected property of the nanoantenna array owing to its extrinsic chirality for oblique emission angles, and can significantly be enhanced when tailoring the distribution of emitters. This renders our nanoantenna-based system a robust platform for valleytronic processing. △ Less

Submitted 25 September, 2025; originally announced September 2025.

Comments: 38 pages, 6 figures

Role of Coherence in Polarization Response of Hybrid Monolayer MoS$_2$-Gold Nanoparticle Systems

Authors: Laura Valencia Molina, Zlata Fedorova, Angela Barreda, Maximilian Weissflog, Rocio Camacho Morales, Seung Heon Han, Anastasia Romashkina, Antony George, Suprova Das, Ralph Schlegel, Andrey Turchanin, Jeetendra Gour, Falk Eilenberger, Dragomir Neshev, Isabelle Staude

Abstract: Monolayers of transition metal dichacogenides show strong second-order nonlinearity and symmetry-driven selection rules from their three-fold lattice symmetry. This process resembles the valley-contrasting selection rules for photoluminescence in these materials. However, the underlying physical mechanisms fundamentally differ since second harmonic generation is a coherent process, whereas photolu… ▽ More Monolayers of transition metal dichacogenides show strong second-order nonlinearity and symmetry-driven selection rules from their three-fold lattice symmetry. This process resembles the valley-contrasting selection rules for photoluminescence in these materials. However, the underlying physical mechanisms fundamentally differ since second harmonic generation is a coherent process, whereas photoluminescence is incoherent, leading to distinct interactions with photonic nanoresonators. In this study, we investigate the far-field circular polarization properties of second harmonic generation from MoS$_2$ monolayers resonantly interacting with spherical gold nanoparticles. Our results indicate that the coherence of the second harmonic allows its polarization to be mostly preserved, unlike in an incoherent process, where the polarization is scrambled. These findings provide important insights for future applications in valleytronics and quantum nanooptics, where both coherent and incoherent processes can be probed in such hybrid systems without altering sample geometry or operational wavelength. △ Less

Submitted 23 September, 2025; v1 submitted 19 September, 2025; originally announced September 2025.

Phase-Field Modeling of Border Cell Cluster Migration in Drosophila

Authors: Naghmeh Akhavan, Alexander George, Michelle Starz-Gaiano, Bradford E. Peercy

Abstract: Collective cell migration is a fundamental biological process that drives events such as embryonic development, wound healing, and cancer metastasis. In this study, we develop a biophysically informed phase-field model to investigate the collective migration of the border cell cluster in the Drosophila melanogaster egg chamber. Our model captures key aspects of the egg chamber architecture, includ… ▽ More Collective cell migration is a fundamental biological process that drives events such as embryonic development, wound healing, and cancer metastasis. In this study, we develop a biophysically informed phase-field model to investigate the collective migration of the border cell cluster in the Drosophila melanogaster egg chamber. Our model captures key aspects of the egg chamber architecture, including the oocyte, nurse cells, and surrounding epithelium, and incorporates both mechanical forces and biochemical cues that guide cell migration. We introduce the Tangential Interface Migration (TIM) force which captures contact-mediated propulsion generated along interfaces between the border cell cluster and surrounding nurse cells. Our simulations reveal three key features of TIM-driven migration that distinguish it from previous forms of chemotaxis: (1) the necessity of border cell-nurse cell overlap to initiate movement (i.e., border cells cannot move without a nurse cell substrate), (2) motion is tangential to border cell-nurse cell interfaces, and (3) persistent migration even in regions where the spatial slope of chemoattractant is decreasing. Additionally, we demonstrate that with or without geometry-mediated alterations in chemoattractant distribution such as at intercellular junctions we can vary induced migration pauses, independent of mechanical confinement. We capture an experimentally observed transition to dorsal migration at the oocyte with a sustained medio-lateral chemical cue of small amplitude. The results show how spatial constraints and interfacial forces shape collective cell movement and highlight the utility of phase-field models in capturing the interplay between tissue geometry, contact forces, and chemical signaling. △ Less

Submitted 13 August, 2025; originally announced August 2025.

Comments: 19 pages, 9 figures main, 5 figures supplementary

A case study of translating sonifications across musical cultures for an educational application

Authors: Chris M. Harrison, James W. Trayford, Arron George, Leigh Harrison, Rubén García-Benito, Shirin Haque, Rose Shepherd

Abstract: Sonification can be part of educational resources that can be accessible to those who prefer, or require, non-visual learning methods. Furthermore, sonification can contribute to an engaging multi-sensory learning experience, which are known to benefit general learners. Whilst some sonification can be relatively agnostic to musical culture, many sonifications are subject to culturally influenced c… ▽ More Sonification can be part of educational resources that can be accessible to those who prefer, or require, non-visual learning methods. Furthermore, sonification can contribute to an engaging multi-sensory learning experience, which are known to benefit general learners. Whilst some sonification can be relatively agnostic to musical culture, many sonifications are subject to culturally influenced choices, such as the chosen harmonies, rhythmic structures, and instrumentation. This is important when considering how universally inclusive and relatable sonification-based educational resources will be. Here we present a case study of translating a sonification-based educational show about the Solar System, that was originally designed with influences from Euro-American (Western-classical) music, to be more culturally relevant to the Caribbean region. We describe the motivation, approach, some of the challenges, and the initial feedback of the resulting output of the project. Finally, we provide reflections on the importance of further work exploring how educational sonifications can transcend international borders and musical cultures. △ Less

Submitted 9 June, 2025; originally announced June 2025.

Comments: Accepted for publication in Audio Mostly (2025). A html version of this article, with the multi-media embedded is available here: https://www.audiouniverse.org/education/shows/tour-of-the-solar-system/caribbean-autss

Photoluminescence-Based Gas Sensing with MoS2 Monolayers

Authors: Gia Quyet Ngo, Chanaprom Cholsuk, Sebastian Thiele, Ziyang Gan, Antony George, Joerg Pezoldt, Andrey Turchanin, Tobias Vogl, Falk Eilenberger

Abstract: Two-dimensional transition metal dichalcogenides (TMDs) are highly appealing for gas sensors, lab-on-a-chip devices and bio-sensing applications because of their strong light-matter interaction and high surface-to-volume ratio. The ability to grow these van der Waals materials on different substrates and waveguide geometries opens a horizon toward scalable on-chip photonic nanodevices. Here, we re… ▽ More Two-dimensional transition metal dichalcogenides (TMDs) are highly appealing for gas sensors, lab-on-a-chip devices and bio-sensing applications because of their strong light-matter interaction and high surface-to-volume ratio. The ability to grow these van der Waals materials on different substrates and waveguide geometries opens a horizon toward scalable on-chip photonic nanodevices. Here, we report on a versatile technique for real time remote optical gas sensing using two-dimensional TMDs. The adsorption of the gas molecules on the monolayer surface provides a gateway for gas sensing based on charge-transfer-induced photoluminescence variation. For gases that are weakly adsorbed on the surface of monolayer TMDs, purging the monolayers' surface by an inert gas like N2 can desorb gases from the monolayers at room temperature. We demonstrate CO, NO and NO2 detection by monitoring photoluminescence from semiconducting MoS2 monolayers grown on SiO2/Si chips at a level of 10 ppm with fast response time. Observations are supported by our density functional theory calculations, which predict a significant interaction between these gases and MoS2 monolayers. These findings may lead to advances in remote sensing, surface-sensitive bioanalytics and lab-on-a-chip sensors. △ Less

Submitted 2 June, 2025; v1 submitted 9 September, 2024; originally announced September 2024.

Comments: 14 pages, 6 figures

Influence of resonant plasmonic nanoparticles on optically accessing the valley degree of freedom in 2D semiconductors

Authors: Tobias Bucher, Zlata Fedorova, Mostafa Abasifard, Rajeshkumar Mupparapu, Matthias J. Wurdack, Emad Najafidehaghani, Ziyang Gan, Heiko Knopf, Antony George, Falk Eilenberger, Thomas Pertsch, Andrey Turchanin, Isabelle Staude

Abstract: The valley degree of freedom is one of the most intriguing properties of atomically thin transition metal dichalcogenides. Together with the possibility to address this degree of freedom by valley-contrasting optical selection rules, it has the potential to enable a completely new class of future electronic and optoelectronic devices. Resonant optical nanostructures emerge as promising tools for i… ▽ More The valley degree of freedom is one of the most intriguing properties of atomically thin transition metal dichalcogenides. Together with the possibility to address this degree of freedom by valley-contrasting optical selection rules, it has the potential to enable a completely new class of future electronic and optoelectronic devices. Resonant optical nanostructures emerge as promising tools for interacting with and controlling the valley degree of freedom at the nanoscale. However, a critical understanding gap remains in how nanostructures and their nearfields affect the circular polarization properties of valley-selective emission hindering further developments in this field. In order to address this issue, our study delves into the experimental investigation of a hybrid model system where valley-specific emission from a monolayer of molybdenum disulfide is interacting with a resonant plasmonic nanosphere. Contrary to the simple intuition suggesting that a centrosymmetric nanoresonator preserves the degree of circular polarization in the forward scattered farfield by angular momentum conservation, our cryogenic photoluminescence microscopy reveals that the light emitted from the nanoparticle position is largely unpolarized, i.e. we observe depolarization. We rigorously study the nature of this phenomenon numerically considering the monolayer-nanoparticle interaction at different levels including excitation and emission. In doing so, we find that the farfield degree of polarization strongly reduces in the hybrid system when including excitons emitting from outside of the system's symmetry point, which in combination with depolarisation at the excitation level causes the observed effect. Our results highlight the importance of considering spatially distributed emitters for precise predictions of polarization responses in these hybrid systems. △ Less

Submitted 20 June, 2024; v1 submitted 24 January, 2024; originally announced January 2024.

Comments: Tobias Bucher and Zlata Fedorova contributed equally to this work. 29 pages, 6 figures

Kapitza-resistance-like exciton dynamics in atomically flat MoSe$_{2}$-WSe$_{2}$ lateral heterojunction

Authors: Hassan Lamsaadi, Dorian Beret, Ioannis Paradisanos, Pierre Renucci, Delphine Lagarde, Xavier Marie, Bernhard Urbaszek, Ziyang Gan, Antony George, Kenji Watanabe, Takashi Taniguchi, Andrey Turchanin, Laurent Lombez, Nicolas Combe, Vincent Paillard, Jean-Marie Poumirol

Abstract: Being able to control the neutral excitonic flux is a mandatory step for the development of future room-temperature two-dimensional excitonic devices. Semiconducting Monolayer Transition Metal Dichalcogenides (TMD-ML) with extremely robust and mobile excitons are highly attractive in this regard. However, generating an efficient and controlled exciton transport over long distances is a very challe… ▽ More Being able to control the neutral excitonic flux is a mandatory step for the development of future room-temperature two-dimensional excitonic devices. Semiconducting Monolayer Transition Metal Dichalcogenides (TMD-ML) with extremely robust and mobile excitons are highly attractive in this regard. However, generating an efficient and controlled exciton transport over long distances is a very challenging task. Here we demonstrate that an atomically sharp TMD-ML lateral heterostructure (MoSe$_{2}$-WSe$_{2}$) transforms the isotropic exciton diffusion into a unidirectional excitonic flow through the junction. Using tip-enhanced photoluminescence spectroscopy (TEPL) and a modified exciton transfer model, we show a discontinuity of the exciton density distribution on each side of the interface. We introduce the concept of exciton Kapitza resistance, by analogy with the interfacial thermal resistance referred to as Kapitza resistance. By comparing different heterostructures with or without top hexagonal boron nitride (hBN) layer, we deduce that the transport properties can be controlled, over distances far greater than the junction width, by the exciton density through near-field engineering and/or laser power density. This work provides a new approach for controlling the neutral exciton flow, which is key toward the conception of excitonic devices. △ Less

Submitted 23 June, 2023; originally announced June 2023.

Journal ref: Nature Communications volume 14, Article number: 5881 (2023)

Universal abundance fluctuations across microbial communities, tropical forests, and urban populations

Authors: Ashish B. George, James O'Dwyer

Abstract: The growth of complex populations, such as microbial communities, forests, and cities, occurs over vastly different spatial and temporal scales. Although research in different fields has developed detailed, system-specific models to understand each individual system, a unified analysis of different complex populations is lacking; such an analysis could deepen our understanding of each system and f… ▽ More The growth of complex populations, such as microbial communities, forests, and cities, occurs over vastly different spatial and temporal scales. Although research in different fields has developed detailed, system-specific models to understand each individual system, a unified analysis of different complex populations is lacking; such an analysis could deepen our understanding of each system and facilitate cross-pollination of tools and insights across fields. Here, for the first time we use a shared framework to analyze time-series data of the human gut microbiome, tropical forest, and urban employment. We demonstrate that a single, three-parameter model of stochastic population dynamics can reproduce the empirical distributions of population abundances and fluctuations in all three data sets. The three parameters characterizing a species measure its mean abundance, deterministic stability, and stochasticity. Our analysis reveals that, despite the vast differences in scale, all three systems occupy a similar region of parameter space when time is measured in generations. In other words, although the fluctuations observed in these systems may appear different, this difference is primarily due to the different physical timescales associated with each system. Further, we show that the distribution of temporal abundance fluctuations is described by just two parameters and derive a two-parameter functional form for abundance fluctuations to improve risk estimation and forecasting. △ Less

Submitted 12 September, 2023; v1 submitted 15 September, 2022; originally announced September 2022.

Direct Growth of Monolayer MoS$_2$ on Nanostructured Silicon Waveguides

Authors: Athira Kuppadakkath, Emad Najafidehaghani, Ziyang Gan, Alessandro Tuniz, Gia Quyet Ngo, Heiko Knopf, Franz J. F. Löchner, Fatemeh Abtahi, Tobias Bucher, Sai Shradha, Thomas Käsebier, Stefano Palomba, Nadja Felde, Pallabi Paul, Tobias Ullsperger, Sven Schröder, Adriana Szeghalmi, Thomas Pertsch, Isabelle Staude, Uwe Zeitner, Antony George, Andrey Turchanin, Falk Eilenberger

Abstract: We report for the first time the direct growth of Molybdenum disulfide (MoS$_2$) monolayers on nanostructured silicon-on-insulator waveguides. Our results indicate the possibility of utilizing the Chemical Vapour Deposition (CVD) on nanostructured photonic devices in a scalable process. Direct growth of 2D material on nanostructures rectifies many drawbacks of the transfer-based approaches. We sho… ▽ More We report for the first time the direct growth of Molybdenum disulfide (MoS$_2$) monolayers on nanostructured silicon-on-insulator waveguides. Our results indicate the possibility of utilizing the Chemical Vapour Deposition (CVD) on nanostructured photonic devices in a scalable process. Direct growth of 2D material on nanostructures rectifies many drawbacks of the transfer-based approaches. We show that the van der Waals materials grow conformally across the curves, edges, and the silicon-SiO$_2$ interface of the waveguide structure. Here, the waveguide structure used as a growth substrate is complex not just in terms of its geometry but also due to the two materials (Si and SiO$_2$) involved. A transfer-free method like this yields a novel approach for functionalizing nanostructured, integrated optical architectures with an optically active direct semiconductor. △ Less

Submitted 27 May, 2022; originally announced June 2022.

Tuning nanowire lasers via hybridization with two-dimensional materials

Authors: Edwin Eobaldt, Francesco Vitale, Maximilian Zapf, Margarita Lapteva, Tarlan Hamzayev, Ziyang Gan, Emad Najafidehaghani, Christof Neumann, Antony George, Andrey Turchanin, Giancarlo Soavi, Carsten Ronning

Abstract: Mixed dimensional hybrid structures have recently gained increasing attention as promising building blocks for novel electronic and optoelectronic devices. In this context, hybridization of semiconductor nanowires with two-dimensional materials could offer new ways to control and modulate lasing at the nanoscale. In this work, we deterministically fabricate hybrid mixed-dimensional heterostructure… ▽ More Mixed dimensional hybrid structures have recently gained increasing attention as promising building blocks for novel electronic and optoelectronic devices. In this context, hybridization of semiconductor nanowires with two-dimensional materials could offer new ways to control and modulate lasing at the nanoscale. In this work, we deterministically fabricate hybrid mixed-dimensional heterostructures composed of ZnO nanowires and MoS2 monolayers with micrometer control over their relative position. First, we show that our deterministic fabrication method does not degrade the optical properties of the ZnO nanowires. Second, we demonstrate that the lasing wavelength of ZnO nanowires can be tuned by several nanometers by hybridization with CVD-grown MoS2 monolayers. We assign this spectral shift of the lasing modes to an efficient carrier transfer at the heterointerface and the subsequent increase of the optical band gap in ZnO (Moss-Burstein effect). △ Less

Submitted 26 May, 2022; originally announced May 2022.

Journal ref: Nanoscale 2022, 14, 6822-6829

Functional universality in slow-growing microbial communities arises from thermodynamic constraints

Authors: Ashish B. George, Tong Wang, Sergei Maslov

Abstract: The dynamics of microbial communities is incredibly complex, determined by competition for metabolic substrates and cross-feeding of byproducts. Species in the community grow by harvesting energy from chemical reactions that transform substrates to products. In many anoxic environments, these reactions are close to thermodynamic equilibrium and growth is slow. To understand the community structure… ▽ More The dynamics of microbial communities is incredibly complex, determined by competition for metabolic substrates and cross-feeding of byproducts. Species in the community grow by harvesting energy from chemical reactions that transform substrates to products. In many anoxic environments, these reactions are close to thermodynamic equilibrium and growth is slow. To understand the community structure in these energy-limited environments, we developed a microbial community consumer-resource model incorporating energetic and thermodynamic constraints on an interconnected metabolic network. The central ingredient of the model is product inhibition, meaning that microbial growth may be limited not only by depletion of metabolic substrates but also by accumulation of products. We demonstrate that these additional constraints on microbial growth cause a convergence in the structure and function of the community metabolic network -- independent of species composition and biochemical details -- providing a possible explanation for convergence of community function despite taxonomic variation observed in many natural and industrial environments. Furthermore, we discovered that the structure of community metabolic network is governed by the thermodynamic principle of maximum heat dissipation. Overall, the work demonstrates how universal thermodynamic principles may constrain community metabolism and explain observed functional convergence in microbial communities. △ Less

Submitted 18 March, 2022; v1 submitted 11 March, 2022; originally announced March 2022.

Signatures of multi-band effects in high-harmonic generation in monolayer MoS$_2$

Authors: Lun Yue, Richard Hollinger, Can B. Uzundal, Bailey Nebgen, Ziyang Gan, Emad Najafidehaghani, Antony George, Christian Spielmann, Daniil Kartashov, Andrey Turchanin, Diana Y. Qiu, Mette B. Gaarde, Michael Zuerch

Abstract: High-harmonic generation (HHG) in solids has been touted as a way to probe ultrafast dynamics and crystal symmetries in condensed matter systems. Here, we investigate the polarization properties of high-order harmonics generated in monolayer MoS$_2$, as a function of crystal orientation relative to the mid-infrared laser field polarization. At several different laser wavelengths we experimentally… ▽ More High-harmonic generation (HHG) in solids has been touted as a way to probe ultrafast dynamics and crystal symmetries in condensed matter systems. Here, we investigate the polarization properties of high-order harmonics generated in monolayer MoS$_2$, as a function of crystal orientation relative to the mid-infrared laser field polarization. At several different laser wavelengths we experimentally observe a prominent angular shift of the parallel-polarized odd harmonics for energies above approximately 3.5 eV, and our calculations indicate that this shift originates in subtle differences in the recombination dipole strengths involving multiple conduction bands. This observation is material specific and is in addition to the angular dependence imposed by the dynamical symmetry properties of the crystal interacting with the laser field, and may pave the way for probing the vectorial character of multi-band recombination dipoles. △ Less

Submitted 15 February, 2022; v1 submitted 24 December, 2021; originally announced December 2021.

All-optical polarization and amplitude modulation of second-harmonic generation in atomically thin semiconductors

Authors: Sebastian Klimmer, Omid Ghaebi, Ziyang Gan, Antony George, Andrey Turchanin, Giulio Cerullo, Giancarlo Soavi

Abstract: Second-harmonic generation is of paramount importance in several fields of science and technology, including frequency conversion, self-referencing of frequency combs, nonlinear spectroscopy and pulse characterization. Advanced functionalities are enabled by modulation of the harmonic generation efficiency, which can be achieved with electrical or all-optical triggers. Electrical control of the ha… ▽ More Second-harmonic generation is of paramount importance in several fields of science and technology, including frequency conversion, self-referencing of frequency combs, nonlinear spectroscopy and pulse characterization. Advanced functionalities are enabled by modulation of the harmonic generation efficiency, which can be achieved with electrical or all-optical triggers. Electrical control of the harmonic generation efficiency offers large modulation depth at the cost of low switching speed, by contrast to all-optical nonlinear devices, which provide high speed and low modulation depth. Here we demonstrate all-optical modulation of second-harmonic generation in MoS2 with a modulation depth of close to 100% and speed limited only by the fundamental pulse duration. This result arises from a combination of D3h crystal symmetry and the deep subwavelength thickness of the sample, it can therefore be extended to the whole family of transition metal dichalcogenides to provide great flexibility in the design of advanced nonlinear optical devices such as high-speed integrated frequency converters, broadband autocorrelators for ultrashort pulse characterization, and tunable nanoscale holograms. △ Less

Submitted 28 October, 2021; originally announced October 2021.

Journal ref: Nature Photonics 15, 837-842 (2021)

In-fiber second-harmonic generation with embedded two-dimensional materials

Authors: Gia Quyet Ngo, Emad Najafidehaghani, Ziyang Gan, Sara Khazaee, Antony George, Erik P. Schartner, Heike Ebendorff-Heidepriem, Thomas Pertsch, Alessandro Tuniz, Markus A. Schmidt, Ulf Peschel, Andrey Turchanin, Falk Eilenberger

Abstract: Silica-based optical fibers are a workhorse of nonlinear optics. They have been used to demonstrate nonlinear phenomena such as solitons and self-phase modulation. Since the introduction of the photonic crystal fiber, they have found many exciting applications, such as supercontinuum white light sources and third-harmonic generation, among others. They stand out by their low loss, large interactio… ▽ More Silica-based optical fibers are a workhorse of nonlinear optics. They have been used to demonstrate nonlinear phenomena such as solitons and self-phase modulation. Since the introduction of the photonic crystal fiber, they have found many exciting applications, such as supercontinuum white light sources and third-harmonic generation, among others. They stand out by their low loss, large interaction length, and the ability to engineer its dispersive properties, which compensate for the small chi(3) nonlinear coefficient. However, they have one fundamental limitation: due to the amorphous nature of silica, they do not exhibit second-order nonlinearity, except for minor contributions from surfaces. Here, we demonstrate significant second-harmonic generation in functionalized optical fibers with a monolayer of highly nonlinear MoS2 deposited on the fiber guiding core. The demonstration is carried out in a 3.5 mm short piece of exposed core fiber, which was functionalized in a scalable process CVD-based process, without a manual transfer step. This approach is scalable and can be generalized to other transition metal dichalcogenides and other waveguide systems. We achieve an enhancement of more than 1000x over a reference sample of equal length. Our simple proof-of-principle demonstration does not rely on either phase matching to fundamental modes, or ordered growth of monolayer crystals, suggesting that pathways for further improvement are within reach. Our results do not just demonstrate a new path towards efficient in-fiber SHG-sources, instead, they establish a platform with a new route to chi(2)-based nonlinear fiber optics, optoelectronics, and photonics platforms, integrated optical architectures, and active fiber networks. △ Less

Submitted 11 August, 2021; originally announced August 2021.

Comments: 22 pages, 11 figures

Neutron-Induced, Single-Event Effects on Neuromorphic Event-based Vision Sensor: A First Step Towards Space Applications

Authors: Seth Roffe, Himanshu Akolkar, Alan D. George, Bernabé Linares-barranco, Ryad Benosman

Abstract: This paper studies the suitability of neuromorphic event-based vision cameras for spaceflight, and the effects of neutron radiation on their performance. Neuromorphic event-based vision cameras are novel sensors that implement asynchronous, clockless data acquisition, providing information about the change in illuminance greater than 120dB with sub-millisecond temporal precision. These sensors hav… ▽ More This paper studies the suitability of neuromorphic event-based vision cameras for spaceflight, and the effects of neutron radiation on their performance. Neuromorphic event-based vision cameras are novel sensors that implement asynchronous, clockless data acquisition, providing information about the change in illuminance greater than 120dB with sub-millisecond temporal precision. These sensors have huge potential for space applications as they provide an extremely sparse representation of visual dynamics while removing redundant information, thereby conforming to low-resource requirements. An event-based sensor was irradiated under wide-spectrum neutrons at Los Alamos Neutron Science Center and its effects were classified. We found that the sensor had very fast recovery during radiation, showing high correlation of noise event bursts with respect to source macro-pulses. No significant differences were observed between the number of events induced at different angles of incidence but significant differences were found in the spatial structure of noise events at different angles. The results show that event-based cameras are capable of functioning in a space-like, radiative environment with a signal-to-noise ratio of 3.355. They also show that radiation-induced noise does not affect event-level computation. We also introduce the Event-based Radiation-Induced Noise Simulation Environment (Event-RINSE), a simulation environment based on the noise-modelling we conducted and capable of injecting the effects of radiation-induced noise from the collected data to any stream of events in order to ensure that developed code can operate in a radiative environment. To the best of our knowledge, this is the first time such analysis of neutron-induced noise analysis has been performed on a neuromorphic vision sensor, and this study shows the advantage of using such sensors for space applications. △ Less

Submitted 29 January, 2021; originally announced February 2021.

Journal ref: 2021 IEEE ACCESS

The gray body approximation for radiative heat transfer in evacuated tube solar collectors: Effects of envelope infrared transparency

Authors: Mark A. George, Noboru Takamure, David R. McKenzie

Abstract: A theoretical and experimental analysis is carried out of radiative heat transfer in the coaxial geometry of evacuated tube solar collectors. The gray body approximation implicit in the use of an effective emissivity does not strictly apply to evacuated tube solar collectors due to selective absorber coating and partially transmitting outer glass in the thermal infrared, especially when constructe… ▽ More A theoretical and experimental analysis is carried out of radiative heat transfer in the coaxial geometry of evacuated tube solar collectors. The gray body approximation implicit in the use of an effective emissivity does not strictly apply to evacuated tube solar collectors due to selective absorber coating and partially transmitting outer glass in the thermal infrared, especially when constructed from borosilicate. We develop analytic expressions for the heat transfer through the outer envelope and show the equations no longer follow a simple form where an effective emissivity for the system can be defined. To test all approximations in practice, an experiment is performed using an evacuated solar collector manufactured in the 1980s by the Nitto Kohki company in Japan using the effective emissivity approximation to determine the typical heat transfer characteristics using net radiative heat flows in both directions. This method enabled a good fit to temperature-time data for cooling and heating of the inner tube for temperatures between 10 and 85°C. The results confirm that the effective emittance method can be used in situations with typical glass wall thickness and temperatures. At temperatures greater 100°C, the spectral distribution of the emitted radiation falls significantly within the transmitting region of the outer glass and can no longer be neglected. The work has verified the stability of the vacuum in this type of collector as the tube still functions well, maintaining a low emissivity and good vacuum after approximately 40 years in storage conditions. △ Less

Submitted 25 March, 2022; v1 submitted 24 November, 2020; originally announced November 2020.

Comments: 15 pages, 8 figures

Journal ref: Journal of Applied Physics 131, 125001 (2022)

The blending region hybrid framework for the simulation of stochastic reaction-diffusion processes

Authors: Christian A. Yates, Adam George, Armand Jordana, Cameron A. Smith, Andrew B. Duncan, Konstantinos C. Zygalakis

Abstract: The simulation of stochastic reaction-diffusion systems using fine-grained representations can become computationally prohibitive when particle numbers become large. If particle numbers are sufficiently high then it may be possible to ignore stochastic fluctuations and use a more efficient coarse-grained simulation approach. Nevertheless, for multiscale systems which exhibit significant spatial va… ▽ More The simulation of stochastic reaction-diffusion systems using fine-grained representations can become computationally prohibitive when particle numbers become large. If particle numbers are sufficiently high then it may be possible to ignore stochastic fluctuations and use a more efficient coarse-grained simulation approach. Nevertheless, for multiscale systems which exhibit significant spatial variation in concentration, a coarse-grained approach may not be appropriate throughout the simulation domain. Such scenarios suggest a hybrid paradigm in which a computationally cheap, coarse-grained model is coupled to a more expensive, but more detailed fine-grained model enabling the accurate simulation of the fine-scale dynamics at a reasonable computational cost. In this paper, in order to couple two representations of reaction-diffusion at distinct spatial scales, we allow them to overlap in a "blending region". Both modelling paradigms provide a valid representation of the particle density in this region. From one end of the blending region to the other, control of the implementation of diffusion is passed from one modelling paradigm to another through the use of complementary "blending functions" which scale up or down the contribution of each model to the overall diffusion. We establish the reliability of our novel hybrid paradigm by demonstrating its simulation on four exemplar reaction-diffusion scenarios. △ Less

Submitted 30 September, 2020; originally announced October 2020.

Comments: 36 pages, 30 figures

Scalable functionalization of optical fibers using atomically thin semiconductors

Authors: Gia Quyet Ngo, Antony George, Robin Tristan Klaus Schock, Alessandro Tuniz, Emad Najafidehaghani, Ziyang Gan, Nils C. Geib, Tobias Bucher, Heiko Knopf, Christof Neumann, Tilman Lühder, Stephen Warren-Smith, Heike Ebendorff-Heidepriem, Thomas Pertsch, Markus A. Schmidt, Andrey Turchanin, Falk Eilenberger

Abstract: Atomically thin transition metal dichalcogenides are highly promising for integrated optoelectronic and photonic systems due to their exciton-driven linear and nonlinear interaction with light. Integrating them into optical fibers yields novel opportunities in optical communication, remote sensing, and all-fiber optoelectronics. However, scalable and reproducible deposition of high quality monolay… ▽ More Atomically thin transition metal dichalcogenides are highly promising for integrated optoelectronic and photonic systems due to their exciton-driven linear and nonlinear interaction with light. Integrating them into optical fibers yields novel opportunities in optical communication, remote sensing, and all-fiber optoelectronics. However, scalable and reproducible deposition of high quality monolayers on optical fibers is a challenge. Here, we report the chemical vapor deposition of monolayer MoS2 and WS2 crystals on the core of microstructured exposed core optical fibers and their interaction with the fibers' guided modes. We demonstrate two distinct application possibilities of 2D-functionalized waveguides to exemplify their potential. First, we simultaneously excite and collect excitonic 2D material photoluminescence with the fiber modes, opening a novel route to remote sensing. Then we show that third harmonic generation is modified by the highly localized nonlinear polarization of the monolayers, yielding a new avenue to tailor nonlinear optical processes in fibers. We anticipate that our results may lead to significant advances in optical fiber based technologies. △ Less

Submitted 2 September, 2020; v1 submitted 8 May, 2020; originally announced May 2020.

Journal ref: Adv. Mater. 2020, 2003826

Chirality provides a direct fitness advantage and facilitates intermixing in cellular aggregates

Authors: Ashish B. George, Kirill S. Korolev

Abstract: Chirality in shape and motility can evolve rapidly in microbes and cancer cells. To determine how chirality affects cell fitness, we developed a model of chiral growth in compact aggregates such as microbial colonies and solid tumors. Our model recapitulates previous experimental findings and shows that mutant cells can invade by increasing their chirality or switching their handedness. The invasi… ▽ More Chirality in shape and motility can evolve rapidly in microbes and cancer cells. To determine how chirality affects cell fitness, we developed a model of chiral growth in compact aggregates such as microbial colonies and solid tumors. Our model recapitulates previous experimental findings and shows that mutant cells can invade by increasing their chirality or switching their handedness. The invasion results either in a takeover or stable coexistence between the mutant and the ancestor depending on their relative chirality. For large chiralities, the coexistence is accompanied by strong intermixing between the cells, while spatial segregation occurs otherwise. We show that the competition within the aggregate is mediated by bulges in regions where the cells with different chiralities meet. The two-way coupling between aggregate shape and natural selection is described by the chiral Kardar-Parisi-Zhang equation coupled to the Burgers' equation with multiplicative noise. We solve for the key features of this theory to explain the origin of selection on chirality. Overall, our work suggests that chirality could be an important ecological trait that mediates competition, invasion, and spatial structure in cellular populations. △ Less

Submitted 10 July, 2020; v1 submitted 25 October, 2017; originally announced October 2017.

Mapping chemical performance on molecular structures using locally interpretable explanations

Authors: Leanne S. Whitmore, Anthe George, Corey M. Hudson

Abstract: In this work, we present an application of Locally Interpretable Machine-Agnostic Explanations to 2-D chemical structures. Using this framework we are able to provide a structural interpretation for an existing black-box model for classifying biologically produced fuel compounds with regard to Research Octane Number. This method of "painting" locally interpretable explanations onto 2-D chemical st… ▽ More In this work, we present an application of Locally Interpretable Machine-Agnostic Explanations to 2-D chemical structures. Using this framework we are able to provide a structural interpretation for an existing black-box model for classifying biologically produced fuel compounds with regard to Research Octane Number. This method of "painting" locally interpretable explanations onto 2-D chemical structures replicates the chemical intuition of synthetic chemists, allowing researchers in the field to directly accept, reject, inform and evaluate decisions underlying inscrutably complex quantitative structure-activity relationship models. △ Less

Submitted 22 November, 2016; originally announced November 2016.

Comments: Presented at NIPS 2016 Workshop on Interpretable Machine Learning in Complex Systems

Measurement of the Shape Factor for the Beta Decay of $^{14}$O

Authors: E. A. George, P. A. Voytas, G. W. Severin, L. D. Knutson

Abstract: We report results from an experiment designed to test the conserved vector current (CVC) hypothesis by measuring the shape of the $β$-decay spectrum for the allowed $0^+\rightarrow 1^+$ ground state decay of $^{14}$O. Measurements of the spectrum intensity were obtained with a superconducting beta spectrometer and will be reported for positron kinetic energies ranging from 1.9 to 4.0 MeV. After di… ▽ More We report results from an experiment designed to test the conserved vector current (CVC) hypothesis by measuring the shape of the $β$-decay spectrum for the allowed $0^+\rightarrow 1^+$ ground state decay of $^{14}$O. Measurements of the spectrum intensity were obtained with a superconducting beta spectrometer and will be reported for positron kinetic energies ranging from 1.9 to 4.0 MeV. After dividing out phase space, Coulomb, and other correction factors, the resulting shape function has a negative slope of several per cent per MeV. We define a parameter $a'$, which is essentially a measure of the average slope of the shape function over the energy range of the measurements, and determine its value to be $a' = -0.0290 \pm 0.0008$ (stat.) $\pm 0.0006$ (syst.). The measured slope parameter is in good agreement with predictions from shell model calculations that respect CVC. △ Less

Submitted 27 October, 2014; originally announced October 2014.

Design and market considerations for axial flux superconducting electric machine design

Authors: Mark D. Ainslie, Ashley George, Robert Shaw, Lewis Dawson, Andy Winfield, Marina Steketee, Simon Stockley

Abstract: In this paper, the authors investigate a number of design and market considerations for an axial flux superconducting electric machine design that uses high temperature superconductors. This work was carried out as part of the University of Cambridge's Centre for Entrepreneurial Learning ETECH Project programme, designed to accelerate entrepreneurship and diffusion of innovations based on early st… ▽ More In this paper, the authors investigate a number of design and market considerations for an axial flux superconducting electric machine design that uses high temperature superconductors. This work was carried out as part of the University of Cambridge's Centre for Entrepreneurial Learning ETECH Project programme, designed to accelerate entrepreneurship and diffusion of innovations based on early stage and potentially disruptive technologies from the University. The axial flux machine design is assumed to utilise high temperature superconductors in both wire (stator winding) and bulk (rotor field) forms, to operate over a temperature range of 65-77 K, and to have a power output in the range from 10s of kW up to 1 MW (typical for axial flux machines), with approximately 2-3 T as the peak trapped field in the bulk superconductors. The authors firstly investigate the applicability of this type of machine as a generator in small- and medium-sized wind turbines, including the current and forecasted market and pricing for conventional turbines. A study is also carried out on the machine's applicability as an in-wheel hub motor for electric vehicles. Some recommendations for future applications are made based on the outcome of these two studies. Next, the cost of YBCO-based superconducting (2G HTS) wire is analysed with respect to competing wire technologies and compared with current conventional material costs. Current wire costs for both 1G and 2G HTS are still too great to be economically feasible for such superconducting devices, but round wire, so-called 3G HTS, conductors may be a promising new option with a view to the future. Finally, different cooling options are assessed for the machine design and the analysis suggests that waiting for the maturation of pulse-tube cooling technology may be the best option for this particular application. △ Less

Submitted 5 March, 2014; v1 submitted 18 September, 2013; originally announced September 2013.

Comments: This paper was presented at the 11th European Conference on Applied Superconductivity held in Genova, Italy, 15-19 September 2013. A condensed version of this paper was submitted to the Journal of Physics: Conference Series (JPCS)

nuSTORM - Neutrinos from STORed Muons: Proposal to the Fermilab PAC

Authors: D. Adey, S. K. Agarwalla, C. M. Ankenbrandt, R. Asfandiyarov, J. J. Back, G. Barker, E. Baussan, R. Bayes, S. Bhadra, V. Blackmore, A. Blondel, S. A. Bogacz, C. Booth, S. B. Boyd, A. Bravar, S. J. Brice, A. D. Bross, F. Cadoux, H. Cease, A. Cervera, J. Cobb, D. Colling, P. Coloma, L. Coney, A. Dobbs , et al. (88 additional authors not shown)

Abstract: The nuSTORM facility has been designed to deliver beams of electron neutrinos and muon neutrinos (and their anti-particles) from the decay of a stored muon beam with a central momentum of 3.8 GeV/c and a momentum acceptance of 10%. The facility is unique in that it will: 1. Allow searches for sterile neutrinos of exquisite sensitivity to be carried out; 2. Serve future long- and short-baseline neu… ▽ More The nuSTORM facility has been designed to deliver beams of electron neutrinos and muon neutrinos (and their anti-particles) from the decay of a stored muon beam with a central momentum of 3.8 GeV/c and a momentum acceptance of 10%. The facility is unique in that it will: 1. Allow searches for sterile neutrinos of exquisite sensitivity to be carried out; 2. Serve future long- and short-baseline neutrino-oscillation programs by providing definitive measurements of electron neutrino and muon neutrino scattering cross sections off nuclei with percent-level precision; and 3. Constitutes the crucial first step in the development of muon accelerators as a powerful new technique for particle physics. The document describes the facility in detail and demonstrates its physics capabilities. This document was submitted to the Fermilab Physics Advisory Committee in consideration for Stage I approval. △ Less

Submitted 31 July, 2013; originally announced August 2013.

Design, Calibration, and Performance of the MINERvA Detector

Authors: L. Aliaga, L. Bagby, B. Baldin, A. Baumbaugh, A. Bodek, R. Bradford, W. K. Brooks, D. Boehnlein, S. Boyd, H. Budd, A. Butkevich, D. A. Martinez Caicedo, C. M. Castromonte, M. E. Christy, J. Chvojka, H. da Motta, D. S. Damiani, I. Danko, M. Datta, R. DeMaat, J. Devan, E. Draeger, S. A. Dytman, G. A. Diaz, B. Eberly , et al. (80 additional authors not shown)

Abstract: The MINERvA experiment is designed to perform precision studies of neutrino-nucleus scattering using $ν_μ$ and ${\barν}_μ$ neutrinos incident at 1-20 GeV in the NuMI beam at Fermilab. This article presents a detailed description of the \minerva detector and describes the {\em ex situ} and {\em in situ} techniques employed to characterize the detector and monitor its performance. The detector is co… ▽ More The MINERvA experiment is designed to perform precision studies of neutrino-nucleus scattering using $ν_μ$ and ${\barν}_μ$ neutrinos incident at 1-20 GeV in the NuMI beam at Fermilab. This article presents a detailed description of the \minerva detector and describes the {\em ex situ} and {\em in situ} techniques employed to characterize the detector and monitor its performance. The detector is comprised of a finely-segmented scintillator-based inner tracking region surrounded by electromagnetic and hadronic sampling calorimetry. The upstream portion of the detector includes planes of graphite, iron and lead interleaved between tracking planes to facilitate the study of nuclear effects in neutrino interactions. Observations concerning the detector response over sustained periods of running are reported. The detector design and methods of operation have relevance to future neutrino experiments in which segmented scintillator tracking is utilized. △ Less

Submitted 22 May, 2013; originally announced May 2013.

Report number: FERMILAB-PUB-13-111-E

Journal ref: Nucl. Inst. and Meth. A743 (2014) 130

Neutrinos from Stored Muons nuSTORM: Expression of Interest

Authors: D. Adey, S. K. Agarwalla, C. M. Ankenbrandt, R. Asfandiyarov, J. J. Back, G. Barker, E. Baussan, R. Bayes, S. Bhadra, V. Blackmore, A. Blondel, S. A. Bogacz, C. Booth, S. B. Boyd, A. Bravar, S. J. Brice, A. D. Bross, F. Cadoux, H. Cease, A. Cervera, J. Cobb, D. Colling, L. Coney, A. Dobbs, J. Dobson , et al. (84 additional authors not shown)

Abstract: The nuSTORM facility has been designed to deliver beams of electron and muon neutrinos from the decay of a stored muon beam with a central momentum of 3.8 GeV/c and a momentum spread of 10%. The facility is unique in that it will: serve the future long- and short-baseline neutrino-oscillation programmes by providing definitive measurements of electron-neutrino- and muon-neutrino-nucleus cross sect… ▽ More The nuSTORM facility has been designed to deliver beams of electron and muon neutrinos from the decay of a stored muon beam with a central momentum of 3.8 GeV/c and a momentum spread of 10%. The facility is unique in that it will: serve the future long- and short-baseline neutrino-oscillation programmes by providing definitive measurements of electron-neutrino- and muon-neutrino-nucleus cross sections with percent-level precision; allow searches for sterile neutrinos of exquisite sensitivity to be carried out; and constitute the essential first step in the incremental development of muon accelerators as a powerful new technique for particle physics. Of the world's proton-accelerator laboratories, only CERN and FNAL have the infrastructure required to mount nuSTORM. Since no siting decision has yet been taken, the purpose of this Expression of Interest (EoI) is to request the resources required to: investigate in detail how nuSTORM could be implemented at CERN; and develop options for decisive European contributions to the nuSTORM facility and experimental programme wherever the facility is sited. The EoI defines a two-year programme culminating in the delivery of a Technical Design Report. △ Less

Submitted 7 May, 2013; originally announced May 2013.

Comments: 59 pages; 24 figures; 5 tables

Report number: CERN-SPSC-2013-015 / SPSC-EOI-009