School of Physics - Research Publications
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ItemNo Preview AvailableA 3D printed flow sensor for microfluidic applications(ELSEVIER SCIENCE SA, 2023-11-01)
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ItemSensing Coherent Nuclear Spin Dynamics with an Ensemble of Paramagnetic Nitrogen Spins(American Physical Society, 2024)The unpolarized spin environment surrounding a central spin qubit is typically considered as an incoherent source of dephasing, however, precise characterization and control of the spin bath can yield a resource for storing and sensing with quantum states. In this work, we use nitrogen-vacancy (NV) centers in diamond to measure the coherence of optically dark paramagnetic nitrogen defects (P1 centers) and detect coherent interactions between the P1 centers and a local bath of 13C nuclear spins. The dipolar coupling between the P1 centers and 13C nuclear spins is identified by signature periodic collapses and revivals in the P1 spin coherence signal. We then demonstrate, using a range of dynamical decoupling protocols, that the probing NV centers and the P1 spins are coupled to independent ensembles of 13C nuclear spins. Our work illustrates how the optically dark P1 spins can be used to extract information from their local environment and offers new insight into the interactions within a many-body system.
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ItemVacuum-Free Liquid-Metal-Printed 2D Semiconducting Tin Dioxide: The Effect of Annealing(American Chemical Society, 2024)Thin film transistors (TFTs) offer unparalleled opportunities for the fabrication of multifunctional electronic and optoelectronic devices. In this work, we report a vacuum-free liquid metal exfoliation technique for rapidly printing ∼2 nm-thick layer of oxide from molten tin. We explore the effect of rapid thermal annealing at 450 °C on the stoichiometry, morphology, and crystal structure of the resulting tin oxide nanosheets. The annealed samples exhibit a dominant SnO2 phase and a high degree of transparency (>99%) in the visible spectra. Field-effect transistors based on the two-dimensional (2D) SnO2 films show typical n-channel conduction with a field-effect mobility of ∼7.5 cm2 V–1 s–1. Photodetectors utilizing annealed tin dioxide demonstrate significant improvement in photoresponsivity reaching a value of 5.2 × 103 A W–1 compared to that found in an unannealed sample at an ultraviolet wavelength of 285 nm. We demonstrate that the improvement in device performance is due to nanocrystalline changes within the oxide layers during the annealing process. This work offers a straightforward and ambient air-compatible method for depositing ultrathin, large-area semiconducting oxides as potential candidates for enabling emerging applications in transparent nanoelectronics and optoelectronics.
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ItemNo Preview AvailableMeta-optics for new approaches to all-optical image processing(SPIE, 2023-08-12)Meta-optical devices have emerged as promising candidates for all-optical image processing. These devices are of subwavelength size and have the potential to address limitations of current image processing methods including processing speed, energy requirements as well as form factor. We present experimental results demonstrating the use of thin-film absorbers and optical metasurfaces to real-time detection of edges in images and the visualisation of phase objects including human cancer cells. Furthermore, we discuss progress towards the use of meta-optics for ultra-compact wavefront recovery. The findings to be presented have potential for applications in biological live-cell imaging, ultra-compact medical diagnostic tools, and wavefront correction methods.
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ItemHigh resolution bio-imaging via inverse design of metasurfaces(SPIE, 2024-03-08)Metasurfaces with angular sensitivity have been shown to provide a platform for developing an ultra-compact phase imaging system. Their performance, however, is often limited to a narrow range of spatial frequencies. Here, we apply inverse design to design and fabricate a metasurface an asymmetric optical transfer function across a numerical aperture (NA) of 0.6. The engineered response of this device enables phase imaging of microscopic transparent objects.
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ItemTuning Single Quantum Dot Emission with a Micromirror(AMER CHEMICAL SOC, 2018-02)The photoluminescence of single quantum dots fluctuates between bright (on) and dark (off) states, also termed fluorescence intermittency or blinking. This blinking limits the performance of quantum dot-based devices such as light-emitting diodes and solar cells. However, the origins of the blinking remain unresolved. Here, we use a movable gold micromirror to determine both the quantum yield of the bright state and the orientation of the excited state dipole of single quantum dots. We observe that the quantum yield of the bright state is close to unity for these single QDs. Furthermore, we also study the effect of a micromirror on blinking, and then evaluate excitation efficiency, biexciton quantum yield, and detection efficiency. The mirror does not modify the off-time statistics, but it does change the density of optical states available to the quantum dot and hence the on times. The duration of the on times can be lengthened due to an increase in the radiative recombination rate.
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ItemThe Degradation and Blinking of Single CsPbl3 Perovskite Quantum Dots(AMER CHEMICAL SOC, 2018-06-28)We demonstrate using single molecule spectroscopy that inorganic CsPbI3 perovskite quantum dots (PQDs) undergo an irreversible, photoaccelerated reaction with water that results in a blue-shift of the photoluminescence (PL) and ultimately to complete quenching of the emission. We find that decomposition does not take place in the presence of oxygen alone but that it requires light and water. We also analyze the blinking for some stable PQDs and find a continuous distribution of emission states with a linear correlation between intensity and lifetime. We postulate that, in addition to charging and discharging processes, blinking arises from the activation and deactivation of nonradiative recombination centers in the PQDs.
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ItemArbitrary interaction quench phenomena in harmonically trapped two-body systems(American Physical Society (APS), 2024)We consider the evolution of two contact-interacting harmonically trapped particles following an arbitrary quench in interaction strength. We focus on the change in system energy, the work, associated with the quench. When quenching from any nonzero interaction strength to zero interaction strength we observe that the work done and particle separation diverge. In particular, the divergent behavior arises always and exclusively when quenching to the noninteracting regime. We demonstrate that the source of the divergence is its instantaneous nature. This validates and builds upon previous work that found divergent behavior arises when quenching from the strongly interacting limit to the noninteracting limit in both the two- and three-body cases.
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ItemWavelength dependence of nitrogen vacancy center charge cycling(American Physical Society (APS), 2024-04-01)Optically active spin qubits in wide-band-gap semiconductors exist in several charge states, though typically only specific charge states exhibit desirable spin or photonic properties. An understanding of how interconversion between different charge states occurs is important for most applications seeking to employ such defects in quantum sensing and information processing, and additionally serves as a means of testing and verifying models of the defect electronic structure. Here, we use charge-sensitive confocal imaging to study the wavelength dependence of optical carrier generation in diamonds hosting nitrogen vacancy (NV) centers, silicon vacancy (SiV) centers, and substitutional nitrogen (N). We study the generation of distinctive charge-capture patterns formed when photogenerated charge carriers are captured by photoluminescent defects, using light spanning 405-633 nm (1.96-3.06 eV). We observe distinct regimes where one- or two-photon ionization or recombination processes dominate, and a third regime where anti-Stokes mediated recombination drives weak NV charge cycling with red light. We then compare red-induced charge cycling to fast charge carrier transport between isolated single NV centers driven with green and blue light. This work reports optically mediated charge cycling processes of the NV centers, and has consequences for schemes using charge transfer to identify nonluminescent defects and photoelectric detection, where ambiguity exists as to the source of photocurrent.
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ItemQuench dynamics in the Jaynes-Cummings-Hubbard and Dicke models(IOP Science, 2024-05-01)Both the Jaynes-Cummings-Hubbard (JCH) and Dicke models can be thought of as idealised models of a quantum battery. In this paper we numerically investigate the charging properties of both of these models. The two models differ in how the two-level systems are contained in cavities. In the Dicke model, the N two-level systems are contained in a single cavity, while in the JCH model the two-level systems each have their own cavity and are able to pass photons between them. In each of these models we consider a scenario where the two-level systems start in the ground state and the coupling parameter between the photon and the two-level systems is quenched. Each of these models display a maximum charging power that scales with the size of the battery N and no super charging was found. Charging power also scales with the square root of the average number of photons per two-level system m for both models. Finally, in the JCH model, the power was found to charge inversely with the photon-cavity coupling κ.