Frequency Tunable Phononic Crystal Flat Lens for Subwavelength Imaging

In this paper, we present a thickness-contrast based flat lens for subwavelenth imaging in an aluminum plate. The lens is made of phononic crystal (PC) with a triangular lattice arrangement of through holes drilled over an aluminum plate. Subwave-length imaging is achieved by exploiting the concept of negative refraction of A0 plate mode for the optical dispersion branch of the PC. The wavenumbers are matched at a design frequency by creating a step change in the thickness of the PC-lens and host plate. The thickness-contrast results in refractive index of minus one at the interface of the lens and host plate. Negative refraction-based lens overcomes the diffraction limit and enables focusing of flexural waves in an area less than a square wavelength. We validate the flat lens design at a single design frequency through numerical simulations and experiments. Further, we numerically demonstrate the tunability of the lens design over a broadband frequency range by modifying the thickness-contrast between the lens and host plate. The proposed frequency tunable design is promising for many applications such as ultrasonic inspection, tetherless energy transfer, and energy harvesting, where the localization of wave energy in a small spot is desirable.

Children with COVID-19 Who Manifest Febrile Seizure

The COVID-19 pandemic is a challenge for all medical personnel in the world. Various studies have been conducted to gain more knowledge about SARS-CoV-2, but studies in the pediatric population are still very limited. We report a case of a boy aged two years and seven months who came to the hospital with an atypical generalized seizure for less than 5 minutes and immediately regained consciousness after the seizure. Other symptoms included fever, productive cough, rhinorrhea, and shortness of breath. The X-ray showed a well-defined homogeneous consolidation in the upper right lobe and a small spot in both lungs which consistently showed top right lobar pneumonia and bronchopneumonia. From the SARS-CoV-2 nucleic acid test, positive results were obtained on the third day of hospitalization. The patient received antiseizure therapy, antibiotics, and other supportive therapies by Indonesian Pediatrician Association (IDAI) guidelines. During treatment, the patient responded well to the treatment given, with no other seizure episodes. A negative result on the SARS-CoV-2 nucleic acid test was obtained after twelve days of hospitalization as well as improvements of the lungs as seen from the X-ray.

Retinal factors of visual sensitivity in the human fovea

Humans direct their gaze towards visual objects of interest such that the retinal images of fixated objects fall onto the fovea, a small anatomically and physiologically specialized region of the retina displaying highest visual fidelity. One striking anatomical feature of the fovea is its non-uniform cellular topography, with a steep decline of cone photoreceptor density and outer segment length with increasing distance from its center. We here assessed in how far the specific cellular organization of the foveola is reflected in visual function. Increment sensitivity to small spot visual stimuli (1 x 1 arcmin, 543 nm light) was recorded psychophysically in 4 human participants at 17 locations placed concentric within a 0.2-degree diameter around the preferred retinal locus of fixation with adaptive optics scanning laser ophthalmoscopy based microstimulation. While cone density as well as maximum outer segment length differed significantly among the four tested participants, the range of observed threshold was similar, yielding an average increment threshold of 3.3 ± 0.2 log10 photons at the cornea. Thresholds were correlated with retinal eccentricity, as well as cone density and outer segment length. Biophysical simulation allowed to develop a model of foveal sensitivity based on these parameters, explaining at least 37% of the observed threshold variability. Based on high reproducibility in replicate testing, the residual variability is assumed to be caused by individual cone and bipolar cell weighting at the specific target locations.

An experimental study of focused very high energy electron beams for radiotherapy

Very high energy electron (VHEE) beams have been proposed as an alternative radiotherapy modality to megavoltage photons; they penetrate deeply without significant scattering in inhomogeneous tissue because of their high relativistic inertia. However, the depth dose distribution of a single, collimated VHEE beam is quasi-uniform, which can lead to healthy tissue being overexposed. This can be largely overcome by focusing the VHEE beam to a small spot. Here, we present experiments to demonstrate focusing as a means of concentrating dose into small volumetric elements inside a target. We find good agreement between measured dose distributions and Monte Carlo simulations. Focused radiation beams could be used to precisely target tumours or hypoxic regions of a tumour, which would enhance the efficacy of radiotherapy. The development of new accelerator technologies may provide future compact systems for delivering these focused beams to tumours, a concept that can also be extended to X-rays and hadrons.

Microsaccades mediate perceptual alternations in Monet’s “Impression, sunrise”

Troxler fading, the perceptual disappearance of stationary images upon sustained fixation, is common for objects with equivalent luminance to that of the background. Previous work showed that variations in microsaccadic rates underlie the perceptual vanishing and intensification of simple stimuli, such as Gabor patches. Here, we demonstrate that microsaccade dynamics also contribute to Troxler fading and intensification during the viewing of representational art. Participants fixated a small spot while viewing either a Gabor patch on a blank background, or Monet’s painting “Impression, Sunrise.” They continuously reported, via button press/release, whether the Gabor patch, or the sun in Monet’s painting, was fading versus intensifying, while their eye movements were recorded with high precision. Microsaccade rates peaked before reports of increased visibility, and dropped before reports of decreased visibility or fading, both when viewing Gabor patches and Monet’s sun. These results reveal that the relationship between microsaccade production and the reversal and prevention of Troxler fading applies not only to the viewing of contrived stimuli, but also to the observation of “Impression, Sunrise.” Whether or not perceptual fading was consciously intended by Monet, our findings indicate that observers’ oculomotor dynamics are a contributor to the cornerstone of Impressionism.