Studies of mechanical and optical properties of thin film coatings for future gravitational wave detectors

Gravitational radiation in the form of gravitational waves was the last prediction to be verified from Einstein's general theory of relativity. Einstein suggested that when a body or bodies with an asymmetric distribution of mass accelerated, energy from the motion would create distortions in space-time which would propagate in all directions at the speed of light. Until the first confirmed observation of gravitational waves from the coalescence of two black holes, this theory had not been experimentally proven. The first gravitational wave (GW) event, `GW150914' confirmed Einstein's predictions, with the event releasing 3 solar masses worth of energy as gravitational radiation during the collision. Since this event, more than 50 confident GW events have been detected, including the first observation of an extremely rare kilonova event after the collision of two neutron stars. Gravitational waves exert fluctuating strains on space as they propagate, resulting in changes in the length of objects that they pass through. Current gravitational wave detectors use laser interferometry to measure this effect using a single laser source and beamsplitter. Two perpendicular laser beams are created and used to monitor the positions of suspended mirrors at the ends of km-scale perpendicular arms. The laser beams reflected from the mirrors are recombined at the beam splitter, creating an interference pattern. Any changes in these mirrors' position then result in a differential change in the arm length inside the detector, altering the generated interference pattern. As the expected change in differential arm length produced by a gravitational wave event is 1x10^-18m all other sources of motion, or noise, must be reduced to exceedingly low levels to maximise the sensitivity to such events. Throughout the range of frequencies to which a gravitational wave detector is sensitive too, its highest sensitivity occurs between approx 50 Hz and 150 Hz. In this frequency band, thermal noise stemming from thermal vibrations in the materials used to create highly reflecting mirror coatings for each test mass limits the sensitivity of the detector. Each material's contributions to the level of thermal noise are proportional to its level of mechanical loss, its temperature and dimensions of the laser beam on its surface. This thesis will focus on the development of coating materials with low mechanical loss and low optical absorption, which can be used to decrease levels of thermal noise inside and increase the stability of a gravitational wave detector. As the amount of laser light absorbed into the coating layer also dictates the test mass's thermal state, a large part of this research will also focus on this aspect of coating measurement. A large part of the work in this thesis involves the first experimental verification of the so-called `multimaterial coating' principle, through testing the optical absorption and room-temperature and cryogenic mechanical loss, of an example of this type of novel coating design. Chapter 1 describes the nature of gravitational radiation and its possible sources. An introduction to the experimental interferometry techniques used in a gravitational wave detector is considered, and notable sources of noise are summarised. Chapter 2 provides a detailed summary of coating thermal noise in gravitational wave detectors. This chapter also introduces some of the recent advancements and current avenues of research in HR gravitational wave detector coatings. Chapter 3 is an account of work carried out by the author at the LIGO Livingston Observatory to develop a technique for monitoring the absorption of the detector mirrors in situ. By studying the resonant frequencies of coated test masses in a gravitational wave detector, a relationship between frequency and the change in test mass temperature by laser heating can be produced. If the total laser power and the level of optical absorption of the coated optic are known, predictions of how its resonant frequencies will change can be modelled using finite element analysis (FEA). If the optical absorption of the coating material at the time of deposition was known, the shift in resonant frequencies of the test mass in response to laser heating could be used to predict any changes in the absorption of the optic. Chapter 4 discusses the experimental Photothermal Common-path Interferometry (PCI) technique used by the author to measure the optical absorption of thin-film coatings. This technique is used to study the peculiar changes in optical absorption of tantalum pentoxide Ta2O5 coatings after heat treatment in a laboratory atmosphere and under a low vacuum. Implementation of a polarisation stabilisation and power correction system into the PCI techniques has allowed for 2-dimensional absorption maps to produce these samples. In this chapter, measurements of a novel multimaterial coating designed to decrease coating thermal noise and optical absorption are studied. The material is subjected to two different heat treatment studies, and its optical absorption is characterised using 1064 nm, 1550 nm and 2000 nm laser light. In Chapter 5, the methods and techniques used to measure the mechanical loss (internal friction) of coating materials are introduced. Throughout this chapter, the development of an automated measurement technique used to characterise the mechanical loss diameter = 3" (76.2 mm), t = 2. 6 mm is discussed and compared to existing measurement techniques. Chapter 6 describes the use of the technique developed in Chapter 5 to study the mechanical loss of the same novel multimaterial coating as a function of heat-treatment temperature. These measurements allow the level of coating thermal noise produced by each multimaterial coating to be calculated using the methods described in Chapter 2. The development of the gentle nodal support described in Chapter 5 is continued in Chapter 7, upgrading the apparatus to function at cryogenic temperatures (80 K<T<293 K). By creating an automated gentle nodal support that operates at cryogenic temperatures, the mechanical loss of coating materials can be characterised for third-generation gravitational wave detector applications. This chapter describes the development of the CryoGeNS system and the characterisation of uncoated diameter = 2" (50.8 mm), t=360um crystalline silicon disks. Chapter 8 details cryogenic mechanical loss measurements of the prototype multimaterial coating, carried out using the CryoGeNS nodal support. Coating loss calculated from each disk is compared to measurements of the same samples carried out in a second commercial cryostat and cryogenic measurements of cSi cantilevers coated in the same materials. This allows the level of thermal noise improvement of these coatings to be calculated as a function of temperature

Knockout of ERK5 causes multiple defects in placental and embryonic development

BACKGROUD: ERK5 is a member of the mitogen activated protein kinase family activated by certain mitogenic or stressful stimuli in cells, but whose physiological role is largely unclear. RESULTS: To help determine the function of ERK5 we have used gene targeting to inactivate this gene in mice. Here we report that ERK5 knockout mice die at approximately E10.5. In situ hybridisation for ERK5, and its upstream activator MKK5, showed strong expression in the head and trunk of the embryo at this stage of development. Between E9.5 and E10.5, multiple developmental problems are seen in the ERK5-/- embryos, including an increase in apoptosis in the cephalic mesenchyme tissue, abnormalities in the hind gut, as well as problems in vascular remodelling, cardiac development and placental defects. CONCLUSION: Erk5 is essential for early embryonic development, and is required for normal development of the vascular system and cell survival

Knots and Particles

Using methods of high performance computing, we have found indications that knotlike structures appear as stable finite energy solitons in a realistic 3+1 dimensional model. We have explicitly simulated the unknot and trefoil configurations, and our results suggest that all torus knots appear as solitons. Our observations open new theoretical possibilities in scenarios where stringlike structures appear, including physics of fundamental interactions and early universe cosmology. In nematic liquid crystals and 3He superfluids such knotted solitons might actually be observed.Comment: 9 pages, 4 color eps figures and one b/w because of size limit (color version available from authors

A system-on-chip microwave photonic processor solves dynamic RF interference in real time with picosecond latency

Radio-frequency interference is a growing concern as wireless technology advances, with potentially life-threatening consequences like interference between radar altimeters and 5G cellular networks. Mobile transceivers mix signals with varying ratios over time, posing challenges for conventional digital signal processing (DSP) due to its high latency. These challenges will worsen as future wireless technologies adopt higher carrier frequencies and data rates. However, conventional DSPs, already on the brink of their clock frequency limit, are expected to offer only marginal speed advancements. This paper introduces a photonic processor to address dynamic interference through blind source separation (BSS). Our system-on-chip processor employs a fully integrated photonic signal pathway in the analogue domain, enabling rapid demixing of received mixtures and recovering the signal-of-interest in under 15 picoseconds. This reduction in latency surpasses electronic counterparts by more than three orders of magnitude. To complement the photonic processor, electronic peripherals based on field-programmable gate array (FPGA) assess the effectiveness of demixing and continuously update demixing weights at a rate of up to 305 Hz. This compact setup features precise dithering weight control, impedance-controlled circuit board and optical fibre packaging, suitable for handheld and mobile scenarios. We experimentally demonstrate the processor's ability to suppress transmission errors and maintain signal-to-noise ratios in two scenarios, radar altimeters and mobile communications. This work pioneers the real-time adaptability of integrated silicon photonics, enabling online learning and weight adjustments, and showcasing practical operational applications for photonic processing

Demonstration of the multimaterial coating concept to reduce thermal noise in gravitational-wave detectors

Thermal noise associated with the mechanical loss of current highly reflective mirror coatings is a critical limit to the sensitivity of gravitational-wave detectors. Several alternative coating materials show potential for reducing thermal noise, but cannot be used due to their high optical absorption. Multimaterial coatings have been proposed to enable the use of such materials to reduce thermal noise while minimizing their impact on the total absorption of the mirror coating. Here we present experimental verification of the multimaterial concept, by integrating aSi into a highly reflective SiO2 and Ta2O5 multilayer coating. We show a significant thermal noise improvement and demonstrate consistent optical and mechanical performance. The multimaterial coating survives the heat treatment required to minimize the absorption of the aSi layers, with no adverse effects from the different thermomechanical properties of the three materials

Quasi-monocrystalline silicon for low-noise end mirrors in cryogenic gravitational-wave detectors

Mirrors made of silicon have been proposed for use in future cryogenic gravitational-wave detectors, which will be significantly more sensitive than current room-temperature detectors. These mirrors are planned to have diameters of ≈50 cm and a mass of ≈200 kg. While single-crystalline float-zone silicon meets the requirements of low optical absorption and low mechanical loss, the production of this type of material is restricted to sizes much smaller than required. Here we present studies of silicon produced by directional solidification. This material can be grown as quasi-monocrystalline ingots in sizes larger than currently required. We present measurements of a low room-temperature and cryogenic mechanical loss comparable with float-zone silicon. While the optical absorption of our test sample is significantly higher than required, the low mechanical loss motivates research into further absorption reduction in the future. While it is unclear if material pure enough for the transmissive detector input mirrors can be achieved, an absorption level suitable for the highly reflective coated end mirrors seems realistic. Together with the potential to produce samples much larger than ≈50 cm, this material may be of great benefit for realizing silicon-based gravitational-wave detectors

How do parents experience being asked to enter a child in a randomised controlled trial?

<p>Abstract</p> <p>Background</p> <p>As the number of randomised controlled trials of medicines for children increases, it becomes progressively more important to understand the experiences of parents who are asked to enrol their child in a trial. This paper presents a narrative review of research evidence on parents' experiences of trial recruitment focussing on qualitative research, which allows them to articulate their views in their own words.</p> <p>Discussion</p> <p>Parents want to do their best for their children, and socially and legally their role is to care for and protect them yet the complexities of the medical and research context can challenge their fulfilment of this role. Parents are simultaneously responsible for their child and cherish this role yet they are dependent on others when their child becomes sick. They are keen to exercise responsibility for deciding to enter a child in a trial yet can be fearful of making the 'wrong' decision. They make judgements about the threat of the child's condition as well as the risks of the trial yet their interpretations often differ from those of medical and research experts. Individual pants will experience these and other complexities to a greater or lesser degree depending on their personal experiences and values, the medical situation of their child and the nature of the trial. Interactions at the time of trial recruitment offer scope for negotiating these complexities if practitioners have the flexibility to tailor discussions to the needs and situation of individual parents. In this way, parents may be helped to retain a sense that they have acted as good parents to their child whatever decision they make.</p> <p>Summary</p> <p>Discussing randomised controlled trials and gaining and providing informed consent is challenging. The unique position of parents in giving proxy consent for their child adds to this challenge. Recognition of the complexities parents face in making decisions about trials suggests lines for future research on the conduct of trials, and ultimately, may help improve the experience of trial recruitment for all parties.</p

Structural neuroimaging correlates of allelic variation of the BDNF val66met polymorphism.

BACKGROUND: The brain-derived neurotrophic factor (BDNF) val66met polymorphism is associated with altered activity dependent secretion of BDNF and a variable influence on brain morphology and cognition. Although a met-dose effect is generally assumed, to date the paucity of met-homozygotes have limited our understanding of the role of the met-allele on brain structure. METHODS: To investigate this phenomenon, we recruited sixty normal healthy subjects, twenty in each genotypic group (val/val, val/met and met/met). Global and local morphology were assessed using voxel based morphometry and surface reconstruction methods. White matter organisation was also investigated using tract-based spatial statistics and constrained spherical deconvolution tractography. RESULTS: Morphological analysis revealed an "inverted-U" shaped profile of cortical changes, with val/met heterozygotes most different relative to the two homozygous groups. These results were evident at a global and local level as well as in tractography analysis of white matter fibre bundles. CONCLUSION: In contrast to our expectations, we found no evidence of a linear met-dose effect on brain structure, rather our results support the view that the heterozygotic BDNF val66met genotype is associated with cortical morphology that is more distinct from the BDNF val66met homozygotes. These results may prove significant in furthering our understanding of the role of the BDNF met-allele in disorders such as Alzheimer's disease and depression

Essential versus accessory aspects of cell death: recommendations of the NCCD 2015

Cells exposed to extreme physicochemical or mechanical stimuli die in an uncontrollable manner, as a result of their immediate structural breakdown. Such an unavoidable variant of cellular demise is generally referred to as ‘accidental cell death’ (ACD). In most settings, however, cell death is initiated by a genetically encoded apparatus, correlating with the fact that its course can be altered by pharmacologic or genetic interventions. ‘Regulated cell death’ (RCD) can occur as part of physiologic programs or can be activated once adaptive responses to perturbations of the extracellular or intracellular microenvironment fail. The biochemical phenomena that accompany RCD may be harnessed to classify it into a few subtypes, which often (but not always) exhibit stereotyped morphologic features. Nonetheless, efficiently inhibiting the processes that are commonly thought to cause RCD, such as the activation of executioner caspases in the course of apoptosis, does not exert true cytoprotective effects in the mammalian system, but simply alters the kinetics of cellular demise as it shifts its morphologic and biochemical correlates. Conversely, bona fide cytoprotection can be achieved by inhibiting the transduction of lethal signals in the early phases of the process, when adaptive responses are still operational. Thus, the mechanisms that truly execute RCD may be less understood, less inhibitable and perhaps more homogeneous than previously thought. Here, the Nomenclature Committee on Cell Death formulates a set of recommendations to help scientists and researchers to discriminate between essential and accessory aspects of cell death