How virtual photons alter atomic X-ray spectra

Control out of the vacuum, virtually

Certain X-ray optical properties of metal atoms can be controlled with the help of virtual photons. This has been demonstrated for the first time by a DESY research team at PETRA III, by using the highly brilliant radiation from this X-ray light source at DESY. In the journal Physical Review Letters they report on how the X-ray spectra of metal atoms can be controlled by virtual photons. This opens up new possibilities for specifically modifying the X-ray optical properties of materials.
So-called virtual photons play an important role in the interaction of light and matter. This is quite remarkable because they do not exist in the classical sense. Virtual photons are created in the vacuum out of nothing and then disappear again after an extremely short time. If these photons interact during their short existence with the electrons of an atom, the binding energies of the electrons shift ever so slightly.

>Read more on the PETRA III website at DESY

Image: Experimental setup to measure the collective Lamb shift at tantalum.
Credit: DESY, Haber et al.

Towards X-ray transient grating spectroscopy at SwissFEL

The high brilliance of new X-ray sources such as X-ray Free Electron Laser opens the way to non-linear spectroscopies.

These techniques can probe ultrafast matter dynamics that would otherwise be inaccessible. One of these techniques, Transient Grating, involves the creation of a transient excitation grating by crossing X-ray beams on the sample. Scientists at PSI have realized a demonstration of such crossing by using an innovative approach well suited for the hard X-ray regime. The results of their work at the Swiss Free Electron Laser have been published in the journal Optics Letters.
Non-linear optics is an important field of physics where the non-linear response of matter in extreme electromagnetic fields is studied and exploited for novel applications. It has been widely used for creating new laser wavelengths (Sum/Difference Frequency Generation – S/DFG) as well as for studying a variety of properties such as charge, spin, magnetic transfer as well as heat diffusion. A broad class of non-linear spectroscopy is Four Wave Mixing (FWM) where three laser beams are overlapped in space and time in a sample and a fourth beam with different wavelength and angle is detected, background free. This allows studying specific transitions and selectively excite the sample tuning the incoming beams’ wavelength while studying their dynamics by controlling the relative time delays between the laser pulses. Transient Grating (TG) spectroscopy is a special case of degenerate FWM where two of the incoming beams have the same wavelength and are crossed at the sample creating an interference pattern, or transient grating, which excites the sample as long as the field is present. When the TG impinges on the material, its index of refraction is locally perturbed and electrons exposed to the radiation are excited. These electrons then transfer their extra energy to the lattice and the heat is then dissipated by the system. A third beam, delayed with respect to the pump TG, probes the dynamics of this excitation.

>Read more on the SwissFEL at PSI website

Image: Layout depicting the experimental conditions at the Alvra experimental station. (Find all the details here)

World record: Fastest 3D tomographic images at BESSY II

An HZB team has developed an ingenious precision rotary table at the EDDI beamline at BESSY II and combined it with particularly fast optics.

This enabled them to document the formation of pores in grains of metal during foaming processes at 25 tomographic images per second – a world record.

The quality of materials often depends on the manufacturing process. In casting and welding, for example, the rate at which melts solidify and the resulting microstructure of the alloy is important. With metallic foams as well, it depends on exactly how the foaming process takes place. To understand these processes fully requires fast sensing capability. The fastest 3D tomographic images to date have now been achieved at the BESSY II X-ray source operated by the Helmholtz-Zentrum Berlin.

Dr. Francisco Garcia-Moreno and his team have designed a turntable that rotates ultra-stably about its axis at a constant rotational speed. This really depends on the highest precision: Any tumbling around the rotation axis or even minimal deviations in the rotation speed would prevent the reliable calculation of the 3D tomography. While commercially available solutions costing several hundred thousand euros allow up to 20 tomographic images per second, the Berlin physicists were able to develop a significantly cheaper solution that is even faster. ”My two doctoral students at the Technische Universität Berlin produced the specimen holders themselves on the lathe”, says Garcia-Moreno, who not only enjoys working out solutions to tricky technical problems, but possesses a lot of craftsman skill himself as well. Additional components were produced in the HZB workshop. In addition, Garcia-Moreno and his colleague Dr. Catalina Jimenez had already developed specialized optics for the fast CMOS camera during the preliminary stages of this work that allows even for simultaneous diffraction. This makes it possible to record approximately 2000 projections per second, from which a total of 25 three-dimensional tomographic images can be created.

>Read more on the BESSY II at Helmholtz-Zentrum Berlin (HZB) website

Image: Experimental setup is composed of a fast-rotation stage, an IR heating lamp (temperature up to 800 °C), a BN crucible transparent to X-rays, a 200-μm thick LuAG:Ce scintillator, a white-beam optical system, and a PCO Dimax CMOS camera. The incident (red) and transmitted (green) X-ray beams as well as the light path from the scintillator to the camera (blue) are shown.
Credit: HZB

Scientists have a new way to gauge the growth of nanowires

In a new study, researchers from the U.S. Department of Energy’s Argonne and Brookhaven National Laboratories observed the formation of two kinds of defects in individual nanowires, which are smaller in diameter than a human hair.

These nanowires, made of indium gallium arsenide, could be useful for a wide range of applications in a field scientists have termed optoelectronics, which encompasses devices that work by converting light energy into electrical impulses. Fiber optic relays are a good example.

The effectiveness of these devices, however, can be affected by tiny defects in their components. These defects, which can change both the optical and electronic properties of these materials, interest scientists who seek to tailor them to boost the functionality of future optoelectronics, including materials that will be able to manipulate quantum information.

>Read more on the NSLS-II website and the Advanced Photon Source website

Image: Argonne and Brookhaven researchers observed two kinds of defects forming in individual nanowires, depicted here. These nanowires are smaller in diameter than a human hair.
Credit: Megan Hill/Northwestern University

A comparison of the etch mechanisms of germanium and silicon

Time multiplexed, deep reactive ion etching (DRIE) is a standard silicon microfabrication technique for fabricating MEMS sensors, actuators, and more recently in CMOS development for 2.5D and 3D memory devices.

At CHESS, we have adopted this microfabrication technique to develop novel x-ray optics called,Collimating Channel Arrays  (CCAs) [1], for confocal x-ray fluorescence microscopy (CXRF). Because the first CCA optics were fabricated from silicon substrates, the range of x-ray fluorescence energies for which they could be used, and hence the elements they could be used to study, was limited. Unwanted x-rays above about 11 keV could penetrate through the silicon, showing up as background and interfering with the measurement.

To solve the background problem, germanium substrates were used to fabricate the CCA optics. Germanium, which is much denser and therefore x-ray opaque than silicon, is also etch compatible with the fluorine etch chemistry for silicon DRIE. However, small differences in etch behavior between germanium and silicon can cause big differences in the outcome. Here, Genova et al JVST B [2] report a systematic comparison of  the etch mechanisms of silicon and germanium, performed with the Plasma Therm Versaline deep silicon etcher at the Cornell NanoScale Science & Technology Facility (CNF). The etch rates of silicon and germanium were compared by varying critical parameters in the DRIE process, especially the applied power and voltage used for each of 3 steps in the etch process,  on custom-designed wafers with a variety of features with systematically varying dimensions.

>Read more on the CHESS website

Image: (extract, full image here) SEM of high aspect ratio (>13:1) etched features in Si at 3.7 μm/min (a) and Ge at 3.4 μm/min (b)

Scientists demonstrate unparalleled phase control of free-electron laser pulses

Double flashes with attosecond precision

Thanks to a smart mirror scientists can control the phase of X-rays from DESY’s free-electron laser FLASH with attosecond precision. The feat enables new investigations of the interactions of light and matter, as the team headed by DESY scientist Tim Laarmann reports in the journal Nature Communications. An attosecond is a billionth of a billionth of a second. The phase indicates at which point in its rapid oscillation a light wave is at a given point in time or space. Phase-sensitive measurements are important to gain insight of light-matter interactions and require phase-controlled pulses. Although phase control is an established technique in optics, the soft X-rays generated by FLASH oscillate a hundred times faster than visible light, requiring a hundred times better precision.

The scientists have now demonstrated phase control and interferometric autocorrelation at FLASH using pulse pairs created with a smart split-and-delay unit. The successful transfer of a powerful optical method towards short wavelengths paves the way towards utilization of advanced nonlinear methodologies even at partially coherent free-electron lasers that rely on self-amplified spontaneous emission (SASE). Free-electron lasers (FEL) are driven by powerful particle accelerators and produce laser-like light pulses by sending bunches of fast electrons through a magnetic slalom course.

>Read More