School of Physics - Research Publications
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ItemOn the creation of near-surface nitrogen-vacancy centre ensembles by implantation of type Ib diamond(SPRINGER HEIDELBERG, 2023-11-28)Abstract Dense, near-surface (within $$\sim 10$$ ∼ 10 nm) ensembles of nitrogen-vacancy (NV) centres in diamond are moving into prominence as the workhorse of many envisaged applications, from the imaging of fast-fluctuating magnetic signals to enacting nuclear hyperpolarisation. Unlike their bulk counterparts, near-surface ensembles suffer from charge stability issues and reduced formation efficiency due to proximity to the diamond surface. Here we examine the prospects for creating such ensembles by implanting nitrogen-rich type Ib diamond, aiming to exploit the high bulk nitrogen density to combat surface-induced band bending. This approach has previously been successful at creating deeper ensembles, however we find that in the near-surface regime there are fewer benefits over nitrogen implantation into pure diamond substrates. Our results suggest that control over diamond surface termination during annealing is key to successfully creating high-yield near-surface NV ensembles generally and implantation into type Ib diamond may be worth revisiting once that has been accomplished. Graphical Abstract
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ItemNo Preview AvailableJADES: Probing interstellar medium conditions at z ∼ 5.5-9.5 with ultra-deep JWST/NIRSpec spectroscopy(EDP SCIENCES S A, 2023-09-12)We present emission-line ratios from a sample of 27 Lyman-break galaxies from z ∼ 5.5 − 9.5 with −17.0 < M1500 < −20.4, measured from ultra-deep JWST/NIRSpec multi-object spectroscopy from the JWST Advanced Deep Extragalactic Survey (JADES). We used a combination of 28 h deep PRISM/CLEAR and 7 h deep G395M/F290LP observations to measure, or place strong constraints on, ratios of widely studied rest-frame optical emission lines including Hα, Hβ, [O II] λλ3726, 3729, [Ne III] λ3869, [O III] λ4959, [O III] λ5007, [O I] λ6300, [N II] λ6583, and [S II] λλ6716, 6731 in individual z > 5.5 spectra. We find that the emission-line ratios exhibited by these z ∼ 5.5 − 9.5 galaxies occupy clearly distinct regions of line-ratio space compared to typical z ∼ 0 − 3 galaxies, instead being more consistent with extreme populations of lower-redshift galaxies. This is best illustrated by the [O III]/[O II] ratio, tracing interstellar medium (ISM) ionisation, in which we observe more than half of our sample to have [O III]/[O II] > 10. Our high signal-to-noise spectra reveal more than an order of magnitude of scatter in line ratios such as [O II]/Hβ and [O III]/[O II], indicating significant diversity in the ISM conditions within the sample. We find no convincing detections of [N II] λ6583 in our sample, either in individual galaxies, or a stack of all G395M/F290LP spectra. The emission-line ratios observed in our sample are generally consistent with galaxies with extremely high ionisation parameters (log U ∼ −1.5), and a range of metallicities spanning from ∼0.1 × Z⊙ to higher than ∼0.3 × Z⊙, suggesting we are probing low-metallicity systems undergoing periods of rapid star formation, driving strong radiation fields. These results highlight the value of deep observations in constraining the properties of individual galaxies, and hence probing diversity within galaxy population.
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ItemNo Preview AvailableSpread of activation and interaction between channels with multi-channel optogenetic stimulation in the mouse cochlea(ELSEVIER, 2023-12)For individuals with severe to profound hearing loss resulting from irreversibly damaged hair cells, cochlear implants can be used to restore hearing by delivering electrical stimulation directly to the spiral ganglion neurons. However, current spread lowers the spatial resolution of neural activation. Since light can be easily confined, optogenetics is a technique that has the potential to improve the precision of neural activation, whereby visible light is used to stimulate neurons that are modified with light-sensitive opsins. This study compares the spread of neural activity across the inferior colliculus of the auditory midbrain during electrical and optical stimulation in the cochlea of acutely deafened mice with opsin-modified spiral ganglion neurons (H134R variant of the channelrhodopsin-2). Monopolar electrical stimulation was delivered via each of four 0.2 mm wide platinum electrode rings at 0.6 mm centre-to-centre spacing, whereas 453 nm wavelength light was delivered via each of five 0.22 × 0.27 mm micro-light emitting diodes (LEDs) at 0.52 mm centre-to-centre spacing. Channel interactions were also quantified by threshold changes during simultaneous stimulation by pairs of electrodes or micro-LEDs at different distances between the electrodes (0.6, 1.2 and 1.8 mm) or micro-LEDs (0.52, 1.04, 1.56 and 2.08 mm). The spread of activation resulting from single channel optical stimulation was approximately half that of monopolar electrical stimulation as measured at two levels of discrimination above threshold (p<0.001), whereas there was no significant difference between optical stimulation in opsin-modified deafened mice and pure tone acoustic stimulation in normal-hearing mice. During simultaneous micro-LED stimulation, there were minimal channel interactions for all micro-LED spacings tested. For neighbouring micro-LEDs/electrodes, the relative influence on threshold was 13-fold less for optical stimulation compared electrical stimulation (p<0.05). The outcomes of this study show that the higher spatial precision of optogenetic stimulation results in reduced channel interaction compared to electrical stimulation, which could increase the number of independent channels in a cochlear implant. Increased spatial resolution and the ability to activate more than one channel simultaneously could lead to better speech perception in cochlear implant recipients.
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ItemNo Preview AvailableTiming is Everything: Stochastic Optogenetic Stimulation Reduces Adaptation in Retinal Ganglion Cells.(IEEE, 2023-07)Optogenetics gives us unprecedented power to investigate brain connectivity. The ability to activate neural circuits with single cell resolution and its ease of application has provided a wealth of knowledge in brain function. More recently, optogenetics has shown tremendous utility in prosthetics applications, including vision restoration for patients with retinitis pigmentosa. One of the disadvantages of optogenetics, however, is its poor temporal bandwidth, i.e. the cell's inability to fire at a rate that matches the optical stimulation rate at high frequencies (>30 Hz). This research proposes a new strategy to overcome the temporal limits of optogenetic stimulation. Using whole-cell current clamp recordings in mouse retinal ganglion cells expressing channelrhodopsin-2 (H134R variant), we observed that randomizing inter-pulse intervals can significantly increase a retinal ganglion cell's temporal response to high frequency stimulation.Clinical Relevance- A significant disadvantage of optogenetic stimulation is its poor temporal dynamics which prohibit its widespread use in retinal prosthetics. We have shown that randomizing the interval between stimulation pulses reduces adaptation in retinal ganglion cells. This stimulation strategy may contribute to new levels of functional restoration in therapeutics which incorporate optogenetics.
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ItemNo Preview AvailableMid-Infrared Gas Classification Using a Bound State in the Continuum Metasurface and Machine Learning(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2023-10-01)
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ItemNo Preview AvailableMeasurement of gravitational lensing of the cosmic microwave background using SPT-3G 2018 data(American Physical Society, 2023-12-12)
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ItemNo Preview AvailableLeast-square Fitting on the Circle using Optimal Control(Elsevier BV, 2023-07-01)
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ItemA novel large energy acceptance beamline for hadron therapy(JACoW, 2023)A design study is currently underway at the University of Melbourne for a large energy acceptance beamline to enable future hadron therapy modalities. As part of the TURBO project, a beam delivery system demonstrator is being developed for a DC Pelletron accelerator, which will provide 3 MeV H+ beams. Fixed Field Accelerator optics will be used to maximise momentum acceptance, with dispersion minimised at both ends of the transport line. This project aims to be the first `closed dispersion arc' with fixed fields ever constructed. As part of the design process, the input beam phase space from the Pelletron has been characterised. Our results show that the Pelletron beam can be injected into the novel transport line successfully, and Zgoubi simulations show that near-zero dispersion at each end will be achievable. This is supplemented by error studies and magnet investigations, demonstrating that beam transport can be achieved under realistic circumstances. This initial study establishes the feasibility of this beamline design and work is continuing toward further optimisation for implementation.
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ItemAtomically Thin Synaptic Devices for Optoelectronic Neuromorphic Vision(Wiley, 2023)Imaging sensors with inbuilt processing capability are expected to form the backbone of low-latency and highly energy efficient artificial vision systems. A range of emerging atomically thin materials provide opportunities to exploit their electrical and optical properties for human vision and brain inspired functions. This work reports atomically thin nanosheets of β-In2S3 which exhibit inherent persistent photoconductivity (PPC) under ultraviolet and visible wavelengths. This PPC effect enables β-In2S3-based optoelectronic devices to optically mimic the dynamics of biological synapses. Based on the material characterizations, the PPC effect is attributed to the intrinsic defects in the synthesized β-In2S3 nanosheet. Furthermore, the feasibility of adopting these atomically thin synaptic devices for optoelectronic neuromorphic hardware is demonstrated by implementing a convolutional neural network for image classification. As such, the demonstrated atomically thin nanosheets and optoelectronic synaptic devices provide a platform for scaling up complex vision-sensory neural networks, which can find many promising applications for multispectral imaging and neuromorphic computation.
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ItemNo Preview AvailableDeterministic Shallow Dopant Implantation in Silicon with Detection Confidence Upper‐Bound to 99.85% by Ion–Solid Interactions (Adv. Mater. 3/2022)(Wiley, 2022-01)In article number 2103235, David N. Jamieson and co-workers report that single-crystal silicon can be configured with arrays of single, near-surface, dopant atoms using on-chip electrodes and low-noise charge-sensitive electronics to register single-ion implants. Suitable for integration with a nanostencil scanner to localize the implants, the system makes use of the signal of electron–hole pairs generated as the ions dissipate their kinetic energy in the crystal. The system can be part of an engineering strategy for high-confidence fabrication of large-scale donor arrays to exploit donor spin ensembles in silicon devices.