Experimental estimates of the photon background in a potential light-by-light scattering study

High power short pulse lasers provide a promising route to study the strong field effects of the quantum vacuum, for example by direct photon-photon scattering in the all-optical regime. Theoretical predictions based on realistic laser parameters achievable today or in the near future predict scattering of a few photons with colliding Petawatt laser pulses, requiring single photon sensitive detection schemes and very good spatio-temporal filtering and background suppression. In this article, we present experimental investigations of this photon background by employing only a single high power laser pulse tightly focused in residual gas of a vacuum chamber. The focal region was imaged onto a single-photon sensitive, time gated camera. As no detectable quantum vacuum signature was expected in our case, the setup allowed for characterization and first mitigation of background contributions. For the setup employed, scattering off surfaces of imperfect optics dominated below the residual gas pressures of 1×10-4mbar. Extrapolation of the findings to intensities relevant for photon-photon scattering studies is discussed.

Two-Dimensional Photoacoustic/Ultrasonic Endoscopic Imaging Based on a Line-Focused Transducer

Existing acoustic-resolution photoacoustic/ultrasonic endoscopy (PA/USE) generally employs a point-focused transducer for ultrasound detection, which is only sensitive in its focal region, thus the lateral resolution and sensitivity drop dramatically when the targets move far from its focus. Even if a dynamic focusing algorithm is applied, the sensitivity out of the transducer focus is still much lower than that in the focus in ultrasonic imaging mode. In this work, we propose an acoustic-resolution PA/USE with a line-focused transducer to realize automatic focusing for the first time. In comparison to a point-focused transducer, the line-focused transducer emits a more uniform sound field, causing the original signal intensity and signal-to-noise ratio (SNR) of the front and rear targets to be closer in the radial direction, which is beneficial for improving target signal uniformity in ultrasonic imaging. Simultaneously, we improved the resolution of the defocus area by modifying a prior work of back-projection (BP) reconstruction algorithm typically used in point-focused transducer based PAE and applying it to line-focused PA/USE. This combined approach may significantly enhance the depth of field of ultrasonic imaging and the resolution of the defocus zone in PA/US imaging, compared to the conventional method. Sufficient numerical simulations and phantom experiments were performed to verify this method. The results show that our method can effectively improve the lateral resolution in the image’s defocused region to achieve automatic focusing and perfectly solve the defect of the target signal difference in the far-focus region in ultrasonic imaging, while also enhancing the image SNR and contrast. The proposed method in this paper lays foundations for the realization of photoacoustic/ultrasonic combined endoscopy with enhanced lateral resolution and depth of field, which can potentially benefit a many of biomedical applications.

FOCUS SHAPING OF CYLINDRICALLY POLARIZED VORTEX BEAMS BY A LINEAR AXICON

Focus shaping of cylindrically polarized vortex beams (CPVBs) by linear axicon is studied theoretically. Vector diffraction theory has been used to derive the expressions of the light field in the focal region. It is shown that a different intensity distribution in the focal region can be obtained by adjusting the topological charge, the polarization rotation angle and the numerical aperture maximal angle. A focal spot, a dark channel and a flat-topped shapes are formed by choosing proper values of parameters. A controllable polarization state of dark channel is obtained. The different focal region shapes may find wide applications such as material processing and optical tweezers.

Cavitation threshold pressure of focused ultrasound observed with sonochemiluminescence

Spatial distribution of sonochemiluminescence (SCL) from an argon-saturated luminol solution was measured in a focused sound field at 1 MHz in a standing-wave configuration. The SCL distribution was confined to pre-focal region at acoustic powers lower than 0.9 W, and was not located at the focus but at a few mm pre-focal side at a threshold for SCL inception. The threshold pressure amplitude for SCL inception was 3.6 atm at the focus, which value was obtained with a background-oriented schlieren method. The method is based on the broadening of multiple slits due to an optical deflection caused by ultrasound, and the broadening width measured provides an acoustic pressure amplitude. A qualitative image of the focused sound field was also obtained.

Mitigating Visual Discomfort on Head Mounted Displays using Estimated Gaze Dependent Depth of Field

<p>Virtual Reality (VR) applications on Head Mounted Displays (HMDs) are now more common and accessible for personal viewing than before with the introduction of consumer-level devices like the Oculus Rift. However, exposure to VR applications on HMDs results in significant discomfort for the majority of people, the severity of which can both increase or decrease after repeated exposures. This is disadvantageous for the development and adoption of VR, as a long adaptation period cannot be relied on for making Virtual Environments palatable. Symptoms of discomfort caused by the viewing of content on VR devices including HMDs are historically described as “Simulator Sickness” and include eye fatigue, headaches, nausea and sweating; symptoms very similar to those experienced by sufferers of motion sickness. We refer to the specific subset of Simulator Sickness Symptoms caused by visual stimuli as symptoms of “Visual Discomfort”. A conflict between accommodation and vergence depth cues on stereoscopic displays is known to be a significant cause of visual discomfort. This report describes a psychophysical evaluation used for judging the effectiveness of dynamic Depth of Field (DoF) blurring on reducing visual discomfort caused by initial exposure to stereoscopic content on HMDs. Our DoF implementation adjusts the focal region of stereoscopic content based on an estimation of users’ view vectors in real time and is realised in a commercial game engine. Participants report a significant reduction of visual discomfort using a simulator sickness questionnaire when DoF blurring is enabled. On average, a 34% reduction in our sickness measure is observed, indicating that dynamic DoF blurring is an effective rendering technique for reducing visual discomfort.</p>

Mitigating Visual Discomfort on Head Mounted Displays using Estimated Gaze Dependent Depth of Field

<p>Virtual Reality (VR) applications on Head Mounted Displays (HMDs) are now more common and accessible for personal viewing than before with the introduction of consumer-level devices like the Oculus Rift. However, exposure to VR applications on HMDs results in significant discomfort for the majority of people, the severity of which can both increase or decrease after repeated exposures. This is disadvantageous for the development and adoption of VR, as a long adaptation period cannot be relied on for making Virtual Environments palatable. Symptoms of discomfort caused by the viewing of content on VR devices including HMDs are historically described as “Simulator Sickness” and include eye fatigue, headaches, nausea and sweating; symptoms very similar to those experienced by sufferers of motion sickness. We refer to the specific subset of Simulator Sickness Symptoms caused by visual stimuli as symptoms of “Visual Discomfort”. A conflict between accommodation and vergence depth cues on stereoscopic displays is known to be a significant cause of visual discomfort. This report describes a psychophysical evaluation used for judging the effectiveness of dynamic Depth of Field (DoF) blurring on reducing visual discomfort caused by initial exposure to stereoscopic content on HMDs. Our DoF implementation adjusts the focal region of stereoscopic content based on an estimation of users’ view vectors in real time and is realised in a commercial game engine. Participants report a significant reduction of visual discomfort using a simulator sickness questionnaire when DoF blurring is enabled. On average, a 34% reduction in our sickness measure is observed, indicating that dynamic DoF blurring is an effective rendering technique for reducing visual discomfort.</p>

A Guide to Sexing Salamanders of The Central Appalachians, USA

Documenting the sex of individuals encountered during wildlife research and monitoring activities is important for understanding and tracking changes in populations. However, sexing salamanders can be particularly difficult because secondary sex characters are often subtle or only visible during the breeding season, and guidance on species-specific sex determination is lacking from most field guides. The purpose of this guide is to provide a reference to assist biologists in the Central Appalachian region with identifying sex of live adult salamanders. In the main text we provide summary tables and figures to serve as concise references in the field. In Text S1 (Supplemental Material) we provide individual species accounts that contain concise yet comprehensive information for each species based on the published literature, as well as many images depicting sexually dimorphic characters. Our focal region encompasses partial or entire distributions for 56 species of salamanders in five families (Ambystomidae, Cryptobranchidae, Plethodontidae, Proteidae, and Salamandridae). We identified seven morphological characters that are strongly sexually dimorphic and useful for sexing live, non-anesthetized, adult salamanders in the field, with males of individual species exhibiting one to five of the characters. We identified &gt;20 additional characters that are weakly sexually dimorphic, difficult to distinguish in the field, or species-specific. Our guide serves as a synthesis of sexually dimorphic characters available for salamanders in Central Appalachia, and we anticipate it will have broad value for researchers, monitoring programs, and salamander enthusiasts in eastern and central North America.

Global range expansion history of pepper (Capsicum spp.) revealed by over 10,000 genebank accessions

Genebanks collect and preserve vast collections of plants and detailed passport information, with the aim of preserving genetic diversity for conservation and breeding. Genetic characterization of such collections has the potential to elucidate the genetic histories of important crops, use marker–trait associations to identify loci controlling traits of interest, search for loci undergoing selection, and contribute to genebank management by identifying taxonomic misassignments and duplicates. We conducted a genomic scan with genotyping by sequencing (GBS) derived single nucleotide polymorphisms (SNPs) of 10,038 pepper (Capsicum spp.) accessions from worldwide genebanks and investigated the recent history of this iconic staple. Genomic data detected up to 1,618 duplicate accessions within and between genebanks and showed that taxonomic ambiguity and misclassification often involve interspecific hybrids that are difficult to classify morphologically. We deeply interrogated the genetic diversity of the commonly consumed Capsicum annuum to investigate its history, finding that the kinds of peppers collected in broad regions across the globe overlap considerably. The method ReMIXTURE—using genetic data to quantify the similarity between the complement of peppers from a focal region and those from other regions—was developed to supplement traditional population genetic analyses. The results reflect a vision of pepper as a highly desirable and tradable cultural commodity, spreading rapidly throughout the globe along major maritime and terrestrial trade routes. Marker associations and possible selective sweeps affecting traits such as pungency were observed, and these traits were shown to be distributed nonuniformly across the globe, suggesting that human preferences exerted a primary influence over domesticated pepper genetic structure.