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Dr Terng Junn Keat
Postdoctoral Research Fellow
Clarendon Laboratory Room 247
Phone (office): +44 (0) 1865 282096
Phone (lab): +44 (0) 1865 272087 / +44 (0) 1865 282649
Email:
junn.keat@physics.ox.ac.uk
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Research interests
Terahertz spectroscopy, terahertz imaging and microscopy
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 - The 3237 cm-1 diamond defect: ultrafast vibrational dynamics, concentration calibration, and relationship to the n3vh0 defect,
TJ Keat, DJL Coxon, RJ Cruddace, VG Stavros, ME Newton, J Lloyd-hughes
Diam. Relat. Mat., 141:110661 (2024)
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pdf | doi:10.1016/j.diamond.2023.110661 ]
The dynamics of the 3237 cm- 1 local vibrational mode in diamond, associated with an unknown defect, was investigated using ultrafast infrared pump-probe spectroscopy. When pumped at 3237 cm -1, a degenerate probe was used to study the ground state's recovery, while a non-degenerate probe tracked excited state absorption at 3029 cm-1, corresponding to the 1 -> 2 vibrational state transition. The similar population lifetimes for the ground state recovery and excited state absorption suggests a single population decay pathway, with a lifetime of T1 = 2.2 +/- 0.1 ps. Perturbed free induction decay signals observed in negative time delays gave the dephasing time of the coherent state between the 0 and 1 vibrational states, and further predicted the 3029 cm- 1 transition. Images from FTIR microscopy show that the 3237 cm- 1 feature and the 3107 cm- 1 absorption line from the N3VH0 defect are not correlated, and our pump-probe study shows the 3237 cm- 1 feature does not share a common ground state with the N3VH0 defect, both of which suggest that this local vibrational mode does not originate from the N3VH0 defect. A calibration factor was obtained via a Morse potential model constrained by the observed transition energies, which relates the concentration of the defect producing the 3237 cm- 1 feature to its absorption coefficient measured by FTIR spectroscopy. Based on FTIR absorption spectroscopy under uniaxial stress, we further assign a trigonal symmetry character to the defect that gives the 3237 cm- 1 feature. The results presented are consistent with the theory that the 3237 cm- 1 feature originates from the N4VH defect, the quantification of which allows better tracking of the nitrogen content in diamond. - Dephasing dynamics across different local vibrational modes and crystalline environments,
TJ Keat, DJL Coxon, M Staniforth, MW Dale, VG Stavros, ME Newton, J Lloyd-hughes
Phys. Rev. Lett., 129:237401 (2022)
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pdf | doi:10.1103/PhysRevLett.129.237401 ]
The perturbed free induction decay (PFID) observed in ultrafast infrared spectroscopy was used to unveil the rates at which different vibrational modes of the same atomic-scale defect can interact with their environment. The N3VH0 defect in diamond provided a model system, allowing a comparison of stretch and bend vibrational modes within different crystal lattice environments. The observed bend mode (first overtone) exhibited dephasing times T-2 = 2.8(1)ps, while the fundamental stretch mode had surprisingly faster dynamics T-2 < 1.7 ps driven by its more direct perturbation of the crystal lattice, with increased phonon coupling. Further, at high defect concentrations the stretch mode's dephasing rate was enhanced. The ability to reliably measure T-2 via PFID provides vital insights into how vibrational systems interact with their local environment.
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