A review of the electrical properties of semiconductor nanowires: Insights gained from terahertz conductivity spectroscopy

Accurately measuring and controlling the electrical properties of semiconductor nanowires is of paramount importance in the development of novel nanowire-based devices. In light of this, terahertz (THz) conductivity spectroscopy has emerged as an ideal non-contact technique for probing nanowire electrical conductivity and is showing tremendous value in the targeted development of nanowire devices. THz spectroscopic measurements of nanowires enable charge carrier lifetimes, mobilities, dopant concentrations and surface recombination velocities to be measured with high accuracy and high throughput in a contact-free fashion. This review spans seminal and recent studies of the electronic properties of nanowires using THz spectroscopy. A didactic description of THz time-domain spectroscopy, optical pump–THz probe spectroscopy, and their application to nanowires is included. We review a variety of technologically important nanowire materials, including GaAs, InAs, InP, GaN and InN nanowires, Si and Ge nanowires, ZnO nanowires, nanowire heterostructures, doped nanowires and modulation-doped nanowires. Finally, we discuss how THz measurements are guiding the development of nanowire-based devices, with the example of single-nanowire photoconductive THz receivers.The authors gratefully acknowledge EPSRC (UK) for research funding. H J Joyce gratefully acknowledges the Royal Commission for the Exhibition of 1851 for her research fellowship.This is the final version of the article. It first appeared from IOP via https://doi.org/10.1088/0268-1242/31/10/10300

Patients' and caregivers' experiences of driving with chronic breathlessness before and after regular low-dose sustained-release morphine: A qualitative study.

BACKGROUND:Chronic breathlessness is a disabling syndrome that profoundly impacts patients' and caregivers' lives. Driving is important for most people, including those with advanced disease. Regular, low-dose, sustained-release morphine safely reduces breathlessness, but little is known about its impact on driving. AIM:To understand patients' and caregivers' (1) perspectives and experiences of driving with chronic breathlessness; and (2) perceived impact of regular, low-dose, sustained-release morphine on driving. DESIGN:A qualitative study embedded in a pragmatic, phase III, randomised, placebo-controlled trial of low-dose, sustained-release morphine (⩽32 mg/24 h) for chronic breathlessness. Semi-structured interviews were conducted immediately after participants withdrew or completed the randomised, placebo-controlled trial. Informed by grounded theory, a constant comparative approach to analysis was adopted. SETTING/PARTICIPANTS:Participants were recruited from an outpatients palliative care service in Adelaide, Australia. Participants included patients (n = 13) with severe breathlessness associated with chronic obstructive pulmonary disease and their caregivers (n = 9). RESULTS:Participants were interviewed at home. Eleven received morphine 8-32 mg. Three themes emerged: (1) independence; (2) breathlessness' impact on driving; and (3) driving while taking regular, low-dose, sustained-release morphine. CONCLUSION:Driving contributed to a sense of identity and independence. Being able to drive increased the physical and social space available to patients and caregivers, their social engagement and well-being. Patients reported breathlessness at rest may impair driving skills, while the introduction of sustained-release morphine seemed to have no self-reported impact on driving. Investigating this last perception objectively, especially in terms of safety, is the subject of ongoing work

Modulation of terahertz polarization on picosecond timescales using polymer-encapsulated semiconductor nanowires

© OSA 2017. We exploit the photoconductivity of semiconductor nanowires to achieve ultrafast broad-bandwidth modulation of THz pulses. A modulation depth of -8 dB was exhibited by a polarizer consisting of 14 layers of nanowires encapsulated in polymer

Nanowires in Terahertz Photonics: Harder, Better, Stronger, Faster

By virtue of their quasi one-dimensional geometries, III-V semiconductor nanowires present unique capabilities for terahertz photonic devices. Ultrafast terahertz polarisation modulators and miniature terahertz photoconductive detectors are two examples of such nanowire-based devices. By the same token, terahertz methods such as terahertz conductivity spectroscopy offer unparalleled insight into the electronic processes that dictate the performance of nanowire-based devices

Finite-size and correlation-induced effects in Mean-field Dynamics

The brain's activity is characterized by the interaction of a very large number of neurons that are strongly affected by noise. However, signals often arise at macroscopic scales integrating the effect of many neurons into a reliable pattern of activity. In order to study such large neuronal assemblies, one is often led to derive mean-field limits summarizing the effect of the interaction of a large number of neurons into an effective signal. Classical mean-field approaches consider the evolution of a deterministic variable, the mean activity, thus neglecting the stochastic nature of neural behavior. In this article, we build upon two recent approaches that include correlations and higher order moments in mean-field equations, and study how these stochastic effects influence the solutions of the mean-field equations, both in the limit of an infinite number of neurons and for large yet finite networks. We introduce a new model, the infinite model, which arises from both equations by a rescaling of the variables and, which is invertible for finite-size networks, and hence, provides equivalent equations to those previously derived models. The study of this model allows us to understand qualitative behavior of such large-scale networks. We show that, though the solutions of the deterministic mean-field equation constitute uncorrelated solutions of the new mean-field equations, the stability properties of limit cycles are modified by the presence of correlations, and additional non-trivial behaviors including periodic orbits appear when there were none in the mean field. The origin of all these behaviors is then explored in finite-size networks where interesting mesoscopic scale effects appear. This study leads us to show that the infinite-size system appears as a singular limit of the network equations, and for any finite network, the system will differ from the infinite system

Therapeutic limitations in tumor-specific CD8+ memory T cell engraftment

BACKGROUND: Adoptive immunotherapy with cytotoxic T lymphocytes (CTL) represents an alternative approach to treating solid tumors. Ideally, this would confer long-term protection against tumor. We previously demonstrated that in vitro-generated tumor-specific CTL from the ovalbumin (OVA)-specific OT-I T cell receptor transgenic mouse persisted long after adoptive transfer as memory T cells. When recipient mice were challenged with the OVA-expressing E.G7 thymoma, tumor growth was delayed and sometimes prevented. The reasons for therapeutic failures were not clear. METHODS: OT-I CTL were adoptively transferred to C57BL/6 mice 21 – 28 days prior to tumor challenge. At this time, the donor cells had the phenotypical and functional characteristics of memory CD8+ T cells. Recipients which developed tumor despite adoptive immunotherapy were analyzed to evaluate the reason(s) for therapeutic failure. RESULTS: Dose-response studies demonstrated that the degree of tumor protection was directly proportional to the number of OT-I CTL adoptively transferred. At a low dose of OT-I CTL, therapeutic failure was attributed to insufficient numbers of OT-I T cells that persisted in vivo, rather than mechanisms that actively suppressed or anergized the OT-I T cells. In recipients of high numbers of OT-I CTL, the E.G7 tumor that developed was shown to be resistant to fresh OT-I CTL when examined ex vivo. Furthermore, these same tumor cells no longer secreted a detectable level of OVA. In this case, resistance to immunotherapy was secondary to selection of clones of E.G7 that expressed a lower level of tumor antigen. CONCLUSIONS: Memory engraftment with tumor-specific CTL provides long-term protection against tumor. However, there are several limitations to this immunotherapeutic strategy, especially when targeting a single antigen. This study illustrates the importance of administering large numbers of effectors to engraft sufficiently efficacious immunologic memory. It also demonstrates the importance of targeting several antigens when developing vaccine strategies for cancer

Balancing the dilution and oddity effects: Decisions depend on body size

Background Grouping behaviour, common across the animal kingdom, is known to reduce an individual's risk of predation; particularly through dilution of individual risk and predator confusion (predator inability to single out an individual for attack). Theory predicts greater risk of predation to individuals more conspicuous to predators by difference in appearance from the group (the ‘oddity’ effect). Thus, animals should choose group mates close in appearance to themselves (eg. similar size), whilst also choosing a large group. Methodology and Principal Findings We used the Trinidadian guppy (Poecilia reticulata), a well known model species of group-living freshwater fish, in a series of binary choice trials investigating the outcome of conflict between preferences for large and phenotypically matched groups along a predation risk gradient. We found body-size dependent differences in the resultant social decisions. Large fish preferred shoaling with size-matched individuals, while small fish demonstrated no preference. There was a trend towards reduced preferences for the matched shoal under increased predation risk. Small fish were more active than large fish, moving between shoals more frequently. Activity levels increased as predation risk decreased. We found no effect of unmatched shoal size on preferences or activity. Conclusions and Significance Our results suggest that predation risk and individual body size act together to influence shoaling decisions. Oddity was more important for large than small fish, reducing in importance at higher predation risks. Dilution was potentially of limited importance at these shoal sizes. Activity levels may relate to how much sampling of each shoal was needed by the test fish during decision making. Predation pressure may select for better decision makers to survive to larger size, or that older, larger fish have learned to make shoaling decisions more efficiently, and this, combined with their size relative to shoal-mates, and attractiveness as prey items influences shoaling decisions

Ultrafast two-dimensional field spectroscopy of terahertz intersubband saturable absorbers

Intersubband (ISB) transitions in semiconductor multi-quantum well (MQW) structures are promising candidates for the development of saturable absorbers at terahertz (THz) frequencies. Here, we exploit amplitude and phase-resolved two-dimensional (2D) THz spectroscopy on the sub-cycle time scale to observe directly the saturation dynamics and coherent control of ISB transitions in a metal-insulator MQW structure. Clear signatures of incoherent pump-probe and coherent four-wave mixing signals are recorded as a function of the peak electric field of the single-cycle THz pulses. All nonlinear signals reach a pronounced maximum for a THz electric field amplitude of 11 kV/cm and decrease for higher fields. We demonstrate that this behavior is a fingerprint of THz-driven carrier-wave Rabi flopping. A numerical solution of the Maxwell-Bloch equations reproduces our experimental findings quantitatively and traces the trajectory of the Bloch vector. This microscopic model allows us to design tailored MQW structures with optimized dynamical properties for saturable absorbers that could be used in future compact semiconductor-based single-cycle THz sources