Attosecond Control of an Atomic Electron Cloud Using Synchrotron Radiation

Researchers at SAGA Light Source, the University of Toyama, Hiroshima University and the Institute for Molecular Science have demonstrated a method to control the shape and orientation of an electron cloud in an atom by tuning the attosecond spacing in a double pulse of synchrotron radiation. Working as a collaborative research team, Tatsuo Kaneyasu (SAGA Light Source/Institute for Molecular Science), Yasumasa Hikosaka (University of Toyama), Masahiro Katoh (Hiroshima University/Institute for Molecular Science) and co-workers have invented a way to manipulate the shape of an electron cloud in an atom using the coherent control technique with synchrotron radiation. The work, which has been published in Physical Review Letters, paves the way towards the ultrafast control of electronic systems using synchrotron radiation. Controlling and probing the electronic motion in atoms and molecules on their natural time scale of attoseconds is one of the frontiers in atomic physics and attosecond physics. Thanks to advances in laser technology, a number of attosecond experiments have been performed with ultrashort laser pulses. In contrast, this research team has presented a new route to the attosecond coherent control of electronic systems using synchrotron radiation. The potential use of undulator radiation as longitudinally coherent wave packets was demonstrated by achieving population control in the photoexcitation of helium atoms [Y. Hikosaka et al., Nature Commun. 10, 4988 (2019)]. The next challenge was the application of the polarization properties of the synchrotron radiation to coherent control. The team’s latest paper, recently published in Physical Review Letters, reports a successful observation of the control of the electron cloud in a helium atom. Pairs of left- and right-circularly polarized radiation wave packets were generated using two helical undulators. The duration of each wave packet pair was a few femtoseconds, and the extreme ultraviolet radiation was used to irradiate helium atoms. With this technique they succeeded in controlling the shape and orientation of the electron cloud, formed as a coherent superposition state, by tuning the time delay between the two wave packets on the attosecond level. In contrast to standard laser technology, there is no technical restriction on the extension of this method to shorter and shorter wavelengths. This new capability of synchrotron radiation not only helps scientists to study ultrafast phenomena in atomic and molecular processes, but may also open up new applications in the development of functional materials and electronic devices in the future.

Reference: “Controlling the Orbital Alignment in Atoms Using Cross-Circularly Polarized Extreme Ultraviolet Wave Packets” by T. Kaneyasu, Y. Hikosaka, M. Fujimoto, H. Iwayama and M. Katoh, 3 December 2019, Physical Review Letters.

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Multiple Scattering In Scanning Helium Microscopy

Using atom beams to image the surface of samples in real space is an emerging technique that delivers unique contrast from delicate samples. Here, we explore the contrast that arises from multiple scattering of helium atoms, a specific process that plays an important role in forming topographic contrast in scanning helium microscopy (SHeM) images. A test sample consisting of a series of trenches of varying depths was prepared by ion beam milling. SHeM images of shallow trenches (depth/width < 1) exhibited the established contrast associated with masking of the illuminating atom beam. The size of the masks was used to estimate the trench depths and showed good agreement with the known values. In contrast, deep trenches (depth/width > 1) exhibited an enhanced intensity. The scattered helium signal was modeled analytically and simulated numerically using Monte Carlo ray tracing. Both approaches gave excellent agreement with the experimental data and confirmed that the enhancement was due to localization of scattered helium atoms due to multiple scattering. The results were used to interpret SHeM images of a bio-technologically relevant sample with a deep porous structure, highlighting the relevance of multiple scattering in SHeM image interpretation. Scanning helium microscopy (SHeM) is a nascent technology that scans a narrow beam of low energy neutral helium atoms over a surface, to produce images of materials without any possibility of beam damage.1–4 The technique can be applied widely and has particular applications in imaging delicate samples, which are difficult to measure using existing techniques.5 Examples include the imaging of insulators, polymers, and biological materials, all of which can be done without coatings or other preparation. As the technique becomes used more broadly, it is crucial to have a good understanding of the image formation process. Here, we report on the significant role that multiple scattering plays in the contrast observed in SHeM images. We use a test sample with simple, well defined, topography. By comparing experimental images with quantitative contrast modeling, we obtain a clear understanding of the process. We also show how quantitative topographic information can be extracted without making any assumptions about the atom-surface interaction. These insights are then used to understand and interpret helium images of a bio-technologically relevant sample in its native state. Contrast in scanning helium microscopy has similarities with the origins of contrast in scanning electron microscopy, both of which involve rastering a focused or collimated beam across the sample and the collection of a fraction of the backscattered signal. In the case of helium atoms, a narrow spot can be generated via simple pinhole collimation, as used in the current work;3 via diffractive focusing with a Fresnel zone plate or similar;6–8 or through the use of atom mirrors.9–12 Since the local surface position and orientation affect the resulting distribution of scattered particles, topographic contrast is evident in both cases. The scattering geometries mean that images appear as if they are illuminated from the direction of the detector, and when a point on the sample is occluded from the detector, typically by a convex region of sample structure, “masked” regions are formed in the image. However, there are also significant differences between the angular distributions of scattered electrons and helium atoms, due to the underlying differences in interaction with surfaces,13 and the different relative detector sizes; helium detectors only cover a relatively small fraction of solid angle. These issues have a significant effect on contrast formation and image interpretation. In current SHeM instruments, the incident beam is typically at [Math Processing Error] to the sample normal and illuminates a small region on the surface, corresponding to a particular pixel in the image. Atoms scattered through a total angle of approximately [Math Processing Error] reach the detector and are counted to give the pixel intensity. There are three primary contributions to topographic contrast, which have been discussed in the literature and are useful to distinguish here. First, height contrast arises primarily from a change in the proportion of the scattered signal that is detected using a fixed position detector;14,15 however, such contrast is weak and only appears over large changes in the height.14–16 Second, angular orientation contrast14,16 occurs when the local orientation of the sample changes the portion of the scattering distribution that enters the detector aperture. For the largely diffuse scattering that occurs from unprepared surfaces, higher intensity is expected when the local surface is orientated toward the detector. Finally, masking, due to the detector being occluded from the illuminated spot on the sample, gives very strong contrast as primary scattered atoms cannot be detected. Masking is independent of the atom-surface interaction14–17 but is related to the underlying surface topography, thus enabling quantitative topographic information to be extracted. A particularly important feature of SHeM is that since there is essentially no possibility of sample penetration by, or adsorption of, the probe particles, incident helium atoms can undergo multiple scattering. Given atoms travel along straight line paths, there is a limited probability of them reaching the detector entrance aperture after a single scattering event ([Math Processing Error] in our current arrangement). However, by scattering from the sample multiple times, atoms can reach the detector indirectly, and thus, multiple scattering makes a further contribution to topographic contrast. We also note that multiple scattering can also provide weak diffuse illumination, a process that has been previously noted15,17 but not examined in detail. A SHeM image of our test sample is shown in Fig. 1, along with its corresponding surface profile. The sample consists of a set of trenches (manufactured by plasma focused ion beam milling of a silicon wafer), each with the same area but different depths. Measured depths of some of the trenches are given in Table I. The helium image clearly shows a distinction between the “shallow” trenches on the right hand side of the image, exhibiting dark regions that widen with the depth, and “deep” trenches on the left hand side of the image, showing very similar, almost uniform, contrast without distinct dark regions. The transition from shallow to deep trenches occurs at a depth/width ratio of about 1. The deep trenches appear significantly brighter than the dark regions in the shallow trenches. Looking closely, the intensity inside the deep trenches increases from left to right, and on the inside right, it even exceeds the intensity of the flat substrate.

FIG. 1. SHeM image of the test sample with the corresponding surface profile. The helium beam is incident from the right as shown, while the detector is located to the left. As a result of re-deposition during ion beam milling, the sides of the trenches are not vertical and edges of the trenches are rounded. The scale bar length is [Math Processing Error]. See the supplementary material for further details.

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Mineral Commodity Summary Publication

Mineral Commodity Summary about helium published by U.S. Geological Survey.


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Here’s Why You Don’t See Blimps Anymore

You still see planes and jets in the sky, but a blimp is a rare sighting these days.

The history of the blimp

Airships, or dirigible balloons, are lighter-than-air aircraft that operate from a lifting-gas that is less dense than the surrounding air, keeping them afloat. The three main types of airships are rigid, semi-rigid, and non-rigid. Non-rigid airships are what people commonly call blimps. Blimps rely on internal pressure to maintain their shape. Semi-rigid airships also rely on internal pressure to maintain their shape, but the structure is also supported by metal keel at the bottom of the structure. Rigid airships, also called zeppelins, have a structural framework that keeps the shape and the lifting gas is contained in one or more cells within the airship. The first steam-powered airship took its first flight in September of 1852. Fifty-one years before the Wright brothers’ first flight, blimps claimed to be the future of air transportation. Here are some other amazing inventions that have changed the world. “Blimps or Zeppelins were primarily used for military and civilian purposes, including transatlantic travel,” says Jennifer Wilnechenko, editor of the travel site, “In 1925, Goodyear Tire & Rubber Company began building airships of the blimp design. These aircraft were used for advertising and military purposes (such as surveillance and anti-submarine warfare) throughout World War II.” As time went on though, the technology of airplanes and helicopters continued to advance and started becoming the more popular mode of air transportation. The decline of airships was then accelerated after a number of crashes occurred, most notably the German airship Hindenburg, which burst into flames while landing in New Jersey in 1937, killing 35 people on board.

Why you rarely see airships at all anymore

While the Hindenburg accident marked the end of airships being a means of public transportation, they were still used at times when the ability to hover for a long period of time outweighed the need for speed and maneuverability. And, in the modern era, blimps were used for “advertising, freight transportation, tourism, camera platforms for sporting events, geological surveys, aerial observation, interdiction platforms, advertising, TV coverage, tourism, and some research purposes,” says Wilnechenko. But more recently, you pretty much never see them in the sky. The main reason you never see airships in the sky anymore is because of the huge costs it takes to build and run them. They’re very expensive to build and very expensive to fly. Airships require a large amount of helium, which can cost up to $100,000 for one trip, according to Wilnechenko. And the prices of helium keeps going up due to a world-wide helium shortage. It’s also no small feat to fly one. According to the Federal Aviation Administration, only 128 people in the United States are qualified to fly airships. And only 17 of them are paid to do it full time. On average, it takes pilots ten to 15 hours to learn how to fly a single-engine plane. But in order for a pilot to go on their first solo trip in an airship, it takes 250 to 400 hours of training. Another main reason is the technical advances of drones. They have become much more reliable in the past few years and are an easier and cheaper way to capture things from an aerial view. Today, there are about 25 blimps still in existence and only about half of them are still in use for advertising purposes. So if you ever happen to see a blimp floating up above you, know that it’s a rare sight to see. Another rare sight these days? Alarm clocks, stereos, and DVD players—just some of the things in your home that won’t exist in 10 years.

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Precise Level Measurement For Liquid Helium

With this device liquid helium levels in cryo containers up to 1.370 mm are displayed linearly. Several probes with different measuring lengths are available. The measuring instrument is adjusted to the requested measuring range in the factory. Due to a built-in rechargeable battery, the levels can be measured at any time, even in mobile applications. A charger, which can also be used as a power supply unit, is included. Connection options for a remote display and a recorder complete the package.
The device switches off automatically after max. 60 sec.

The measuring principle

The superconductor of the probe has a so-called transition temperature in the boiling range of helium, at which its electrical resistance becomes infinitely small. A constant current creates a voltage drop across the residual resistance, corresponding to the probe section not immersed in the cryo liquid. This voltage is displayed via a measuring bridge, which thus becomes a measure for the filling level.

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