Manufacturing Bits: July 3

X-ray holography; photoelectron spectrometers; more metrology.


X-ray holography
Using a technique called X-ray holography, a group of researchers have uncovered the phase transitions of vanadium dioxide.

X-ray holography is a promising high-resolution metrology technique. Vanadium dioxide is one of many materials that can exhibit metal or insulator properties depending on the temperature. Vanadium dioxide can switch from an insulating to a metallic phase just above room temperature.

This process, which is called phase separation, is similar to when ice turns into water during the melting phase. Liquid and solid water can coexist in separate regions when ice melts, according to researchers.

For vanadium dioxide, though, it’s still unclear how and why the phase separation occurs.

Using resonant soft X-ray holography, Max Born Institute, the Technische Universität, the Institute for Photonic Sciences, ALBA and Vanderbilt University have been able to probe the phase transitions that occur in thin films of vanadium dioxide.

X-ray holography is a coherent diffractive imaging technique. Capable of imaging samples in three dimensions, X-ray holography can image the dynamics of a process within a femtosecond or picosecond.
Using this technique, researchers explored vanadium dioxide crystal platelets. They found that the defects in the material play a key part in the phase transition process from an insulator to a metal.

Researchers also observed a third state. “Whilst some regions transformed directly from the insulating to metallic phase, others transformed into a second different insulating state before becoming metallic at higher temperatures, with the exact pathway taken depending on the defects present in the material,” according to the Max Born Institute.

X-ray hologram of VO2 during the phase transition. (Picture: MBI)

“We are really excited about the prospect of recording a movie of how the phase transition proceeds, reflecting the interplay of atomic nuclei and electrons in the different domains of a sample,” said Simon Wall, a professor from the Institute of Photonic Sciences. “Beyond vanadium dioxide, this approach will help us to fundamentally understand the properties of many intriguing materials, including for example high temperature superconductors.”

Photoelectron spectrometers
Fraunhofer and Max Planck have developed a new photoelectron spectrometer technology that can make measurements in seconds rather than hours with conventional systems.

Photoemission spectroscopy is a technology that takes measurements of electrons emitted from a sample. Using the photoelectric effect, photoemission spectroscopy determines the binding energies of electrons in a sample.

The technology makes use of a laser that works in the kilohertz range. The laser shoots high-energy photons on a sample, emitting a few thousand laser light pulses per second.

The problem? At times, there are too many electrons that are free in a given pulse. The electrons tend to repel each other, meaning it is impossible to measure them, according to Fraunhofer.

In response, Fraunhofer Institutes for Applied Optics and Precision Engineering, Fraunhofer Institute for Laser Technology and Max Planck Institute of Quantum Optics have developed the world’s first photoelectron spectrometer that works at 18 megahertz. This, in turn, enables a thousand times more pulses that strike the surface than traditional systems, thereby accelerating measurement times.

The spectrometer consists of three main components—a laser, an enhancement resonator, and a sample chamber with the spectrometer. The system uses a phase-stable titanium-sapphire laser. Using pre-amplifiers and amplifiers, the system can boost the power from 300 microwatts to 110 watts.

Main amplifier stages of the fiber laser system, where high pulse energies are generated. (Source: Fraunhofer IOF, Marco Plötner, Walter Oppel)

Initially, the system fires a phase. The pulse duration is short, but the energy of the photons is not yet sufficient. So, with the help of another component, the system increases the photon energy of the laser. Then, the laser beams are directed in the resonator. In the resonator, mirrors steer the laser light around in a circle. This, in turn, creates high-energy attosecond XUV pulses.

The result? “Certain measurements used to take five hours; we can now complete them in ten seconds,” said Oliver de Vries, scientist at Fraunhofer IOF

More metrology
The third light source has begun to operate at the European XFEL.

The European XFEL, a research organization that operates the world’s largest X-ray laser, began to operate in 2017. The European XFEL is a 3.4 kilometer-long facility, which is located mainly in underground tunnels in Germany.

The operation’s first X-ray light source enables researchers to take measurements using various instruments. This includes the SPB/SFX (single particles, clusters and biomolecules and serial femtosecond crystallography) and the FXE (femtosecond X-ray experiments).

The second source provides light for the following instruments: SQS (small quantum systems) and SCS (spectroscopy and coherent scattering). It is scheduled to start user operation in November 2018.

The third light source will provide light for the MID (materials imaging and dynamics) and HED (high energy density science) instruments, which are scheduled to start user operation in 2019

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