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Ford Wagner
D.Phil. Candidate
Clarendon Laboratory Room 245
Phone (office): +44 (0) 1865 272339
Phone (lab): +44 (0) 1865 282649
Email:
ford.wagner@physics.ox.ac.uk
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Research interests
Terahertz spectroscopy
Publications
- The influence of subwavelength geometry on extracting the electrical properties of semiconductors by terahertz spectroscopy,
FM Wagner, T Haggren, JC Bürger, TJ Keat, K Peng, C Uswachoke, DA Damry, H Kraus, HJ Joyce, HH Tan, C Jagadish, T Siday, MB Johnston
APL Photonics, 10:076123 (2025)
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pdf | doi:10.1063/5.0273919 ]
Terahertz (THz) spectroscopy is a non-contact technique well-suited for probing the ultrafast electrical conductivity of semiconductor nanostructures, where conventional methods are often impractical. However, geometric resonances in these nanostructures can distort the measured THz response, complicating the extraction of the intrinsic material properties. Here, we use THz spectroscopy to study GaAs nanowires of varying lengths and observe that the resonant frequency increases as the nanowire length decreases. Further measurements on a model system of well-defined GaAs microsquares (ranging in size from 500 x 500 to 7 x 7 mu m(2)) confirmed the relationship between structure size and resonant frequency. We verify that a plasmon-based model successfully separates the geometric response from the intrinsic charge-carrier response. Thus, the model allows for the accurate evaluation of critical material properties in nanostructured semiconductors, such as electron mobility. Fluence-dependent measurements and finite-difference time-domain simulations confirm the plasmonic nature of the resonances. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license - Direct and integrating sampling in terahertz receivers from wafer-scalable inas nanowires,
K Peng, NP Morgan, FM Wagner, T Siday, CQ Xia, D Dede, V Boureau, V Piazza, AFI Morral, MB Johnston
Nat. Commun., 15:103 (2024)
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pdf | doi:10.1038/s41467-023-44345-1 ]
Terahertz (THz) radiation will play a pivotal role in wireless communications, sensing, spectroscopy and imaging technologies in the decades to come. THz emitters and receivers should thus be simplified in their design and miniaturized to become a commodity. In this work we demonstrate scalable photoconductive THz receivers based on horizontally-grown InAs nanowires (NWs) embedded in a bow-tie antenna that work at room temperature. The NWs provide a short photoconductivity lifetime while conserving high electron mobility. The large surface-to-volume ratio also ensures low dark current and thus low thermal noise, compared to narrow-bandgap bulk devices. By engineering the NW morphology, the NWs exhibit greatly different photoconductivity lifetimes, enabling the receivers to detect THz photons via both direct and integrating sampling modes. The broadband NW receivers are compatible with gating lasers across the entire range of telecom wavelengths (1.2-1.6 mu m) and thus are ideal for inexpensive all-optical fibre-based THz time-domain spectroscopy and imaging systems. The devices are deterministically positioned by lithography and thus scalable to the wafer scale, opening the path for a new generation of commercial THz receivers. Authors report on nanofacet engineering of wafer-scalable InAs nanowires enabling the operation of THz photodetectors in direct or integrating sampling mode, with performance comparable to commercial InP technology. - Optimised spintronic emitters of terahertz radiation for time-domain spectroscopy,
FM Wagner, S Melnikas, J Cramer, DA Damry, CQ Xia, K Peng, G Jakob, M Klaui, S Kicas, MB Johnston
J. Infrared Millim. Terahertz Waves, 44:52–65 (2023)
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pdf | doi:10.1007/s10762-022-00897-9 ]
Spintronic metal thin films excited by femtosecond laser pulses have recently emerged as excellent broadband sources of terahertz (THz) radiation. Unfortunately, these emitters transmit a significant proportion of the incident excitation laser, which causes two issues: first, the transmitted light can interfere with measurements and so must be attenuated; second, the transmitted light is effectively wasted as it does not drive further THz generation. Here, we address both issues with the inclusion of a high-reflectivity (HR) coating made from alternating layers of SiO2 and Ta2O5. Emitters with the HR coating transmit less than 0.1% of the incident excitation pulse. Additionally, we find that the HR coating increases the peak THz signal by roughly 35%, whereas alternative attenuating elements, such as cellulose nitrate films, reduce the THz signal. To further improve the emission, we study the inclusion of an anti-reflective coating to the HR-coated emitters and find the peak THz signal is enhanced by a further 4%.
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