Simultaneously-Measured Mid-Infrared Refractive Indices of GaAs/AlGaAs

We present our results for a simultaneous measurement of the refractive indices of Gallium Arsenide (GaAs) and Aluminum Gallium Arsenide (Al$_\mathrm{x}$Ga$_\mathrm{1-x}$As) in the spectral region from $2.0$ to $7.1\,\mathrm{\mu}\mathrm{m}$ ($5000$ to $1400\,\mathrm{cm^{-1}}$). These values are obtained from a monocrystalline thin-film multilayer Bragg mirror of excellent purity (background doping $\leq 1 \times 10^{-14}\,\mathrm{cm^{-3}}$), grown via molecular beam epitaxy. To recover the refractive indices over such a broad wavelength range, we fit a dispersion model for each material. For that, we measure both a photometrically accurate transmittance spectrum of the Bragg mirror via Fourier-transform infrared spectrometry and the individual physical layer thicknesses of the structure via scanning electron microscopy. To infer the uncertainty of the refractive index values, we estimate relevant measurement uncertainties and propagate them via a Monte-Carlo-type method. This method conclusively yields propagated relative uncertainties on the order of $10^{-4}$ over the measured spectral range for both GaAs and Al$_{0.929}$Ga$_{0.071}$As. The fitted model can also approximate the refractive index for MBE-grown Al$_\mathrm{x}$Ga$_\mathrm{1-x}$As for $0 \leq x \leq 1$. These updated values will be essential in the design and fabrication of next-generation active and passive optical devices in a spectral region which is of high interest in many fields, e.g., laser design and cavity-enhanced spectroscopy.Comment: 20 pages, 5 figures, submitted to PR

Transmission-dominated mid-infrared supermirrors with finesse exceeding 200 000

We fabricate and characterize substrate-transferred single-crystal mirror coatings with 9.33 $\pm$ 0.17 ppm of transmittance and 4.27 $\pm$ 0.52 ppm of excess optical loss, corresponding to a transmission-loss dominated reflectance of 99.9986% at 4.45 $\mu$m. For the first time, a cavity finesse > 200 000 is achieved in the mid-infrared.Comment: Sept 21: Minor revisions to conform to 2-page length requirement including abbr. refs.; Figure font sizes increase

Direct frequency comb measurement of OD + CO → DOCO kinetics

The kinetics of the hydroxyl radical (OH) + carbon monoxide (CO) reaction, which is fundamental to both atmospheric and combustion chemistry, are complex because of the formation of the hydrocarboxyl radical (HOCO) intermediate. Despite extensive studies of this reaction, HOCO has not been observed under thermal reaction conditions. Exploiting the sensitive, broadband, and high-resolution capabilities of time-resolved cavity-enhanced direct frequency comb spectroscopy, we observed deuteroxyl radical (OD) + CO reaction kinetics and detected stabilized trans-DOCO, the deuterated analog of trans-HOCO. By simultaneously measuring the time-dependent concentrations of the trans-DOCO and OD species, we observed unambiguous low-pressure termolecular dependence of the reaction rate coefficients for N_2 and CO bath gases. These results confirm the HOCO formation mechanism and quantify its yield

Mid-infrared interference coatings with excess optical loss below 10 ppm

Low excess optical loss, combined absorption and scatter loss, is a key performance metric for any high-reflectance coating technology and is currently one of the main limiting factors for the application of optical resonators in the mid-infrared spectral region. Here we present high-reflectivity substrate-transferred single-crystal GaAs/AlGaAs interference coatings at a center wavelength of 4.54 µm with record-low excess optical loss below 10 parts per million. These high-performance mirrors are realized via a novel microfabrication process that differs significantly from the production of amorphous multilayers generated via physical vapor deposition processes. This new process enables reduced scatter loss due to the low surface and interfacial roughness, while low background doping in epitaxial growth ensures strongly reduced absorption. We report on a suite of optical measurements, including cavity ring-down, transmittance spectroscopy, and direct absorption tests to reveal the optical losses for a set of prototype mirrors. In the course of these measurements, we observe a unique polarization-orientation-dependent loss mechanism which we attribute to elastic anisotropy of these strained epitaxial multilayers. A future increase in layer count and a corresponding reduction of transmittance will enable optical resonators with a finesse in excess of 100,000 in the mid-infrared spectral region, allowing for advances in high-resolution spectroscopy, narrow-linewidth laser stabilization, and ultrasensitive measurements of various light–matter interactions

Thickness uniformity measurements and damage threshold tests of large-area GaAs/AlGaAs crystalline coatings for precision interferometry

Precision interferometry is the leading method for extremely sensitive measurements in gravitational wave astronomy. Thermal noise of dielectric coatings poses a limitation to the sensitivity of these interferometers. To decrease coating thermal noise, new crystalline GaAs/AlGaAs multilayer mirrors have been developed. To date, the surface figure and thickness uniformity of these alternative low-loss coatings has not been investigated. Surface figure errors, for example, cause small angle scattering and thereby limit the sensitivity of an interferometer. Here we measure the surface figure of highly reflective, substrate-transferred, crystalline GaAs/AlGaAs coatings with a custom scanning reflectance system. We exploit the fact that the reflectivity varies with the thickness of the coating. To increase penetration into the coating, we used a 1550 nm laser on a highly reflective coating designed for a center wavelength of 1064 nm. The RMS thickness variation of a two inch optic was measured to be 0.41 ± 0.05 nm. This result is within 10% of the thickness uniformity, of 0.37 nm RMS, achieved with ion-beam sputtered coatings for the aLIGO detector. We additionally measured a lower limit of the laser induced damage threshold of 64 MW/cm2 for GaAs/AlGaAs coatings at a wavelength of 1064 nm. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen

Comparing breast cancer mortality rates before-and-after a change in availability of screening in different regions: Extension of the paired availability design

BACKGROUND: In recent years there has been increased interest in evaluating breast cancer screening using data from before-and-after studies in multiple geographic regions. One approach, not previously mentioned, is the paired availability design. The paired availability design was developed to evaluate the effect of medical interventions by comparing changes in outcomes before and after a change in the availability of an intervention in various locations. A simple potential outcomes model yields estimates of efficacy, the effect of receiving the intervention, as opposed to effectiveness, the effect of changing the availability of the intervention. By combining estimates of efficacy rather than effectiveness, the paired availability design avoids confounding due to different fractions of subjects receiving the interventions at different locations. The original formulation involved short-term outcomes; the challenge here is accommodating long-term outcomes. METHODS: The outcome is incident breast cancer deaths in a time period, which are breast cancer deaths that were diagnosed in the same time period. We considered the plausibility of the basic five assumptions of the paired availability design and propose a novel analysis to accommodate likely violations of the assumption of stable screening effects. RESULTS: We applied the paired availability design to data on breast cancer screening from six counties in Sweden. The estimated yearly change in incident breast cancer deaths per 100,000 persons ages 40–69 (in most counties) due to receipt of screening (among the relevant type of subject in the potential outcomes model) was -9 with 95% confidence interval (-14, -4) or (-14, -5), depending on the sensitivity analysis. CONCLUSION: In a realistic application, the extended paired availability design yielded reasonably precise confidence intervals for the effect of receiving screening on the rate of incident breast cancer death. Although the assumption of stable preferences may be questionable, its impact will be small if there is little screening in the first time period. However, estimates may be substantially confounded by improvements in systemic therapy over time. Therefore the results should be interpreted with care

Direct frequency comb measurement of OD + CO → DOCO kinetics

The kinetics of the hydroxyl radical (OH) + carbon monoxide (CO) reaction, which is fundamental to both atmospheric and combustion chemistry, are complex because of the formation of the hydrocarboxyl radical (HOCO) intermediate. Despite extensive studies of this reaction, HOCO has not been observed under thermal reaction conditions. Exploiting the sensitive, broadband, and high-resolution capabilities of time-resolved cavity-enhanced direct frequency comb spectroscopy, we observed deuteroxyl radical (OD) + CO reaction kinetics and detected stabilized trans-DOCO, the deuterated analog of trans-HOCO. By simultaneously measuring the time-dependent concentrations of the trans-DOCO and OD species, we observed unambiguous low-pressure termolecular dependence of the reaction rate coefficients for N_2 and CO bath gases. These results confirm the HOCO formation mechanism and quantify its yield

Violence is rare in autism : when it does occur, is it sometimes extreme?

A small body of literature has suggested that, rather than being more likely to engage in offending or violent behaviour, individuals with autism spectrum disorder (ASD) may actually have an increased risk of being the victim rather than the perpetrator of violence (Sobsey et al., 1995). There is no evidence that people with ASD are more violent than those without ASD (Im, 2016). There is nevertheless a small subgroup of individuals with ASD who exhibit violent offending behaviours and our previous work has suggested that other factors, such as adverse childhood experiences, might be important in this subgroup (Allely et al., 2014). Fitzgerald (2015) highlights that school shootings and mass killings are not uncommonly carried out by individuals with neurodevelopmental disorders, with frequent evidence of warning indicators. The aim of the present review is to investigate this in more detail using the 73 mass shooting cases identified by Mother Jones (motherjones.com) in their database for potential ASD features. This exercise tentatively suggests evidence of ASD in six of 73 included cases (8%) which is ten times higher when compared to the prevalence of ASD found in the general population worldwide (motherjones.com). The 8% figure for individuals with ASD involved mass killings is a conservative estimate. In addition to the six cases which provide the 8% figure, there were 15 other cases with some indication of ASD. Crucially, ASD may influence, but does not cause, an individual to commit extreme violent acts such as a mass shooting episode