Homegrown solution for a synchrotron light source
Physicists at ISU and Ames Laboratory have developed a new type of spectrometer that brings all the advantages of a large scale synchrotron facility to a campus lab setting. In the process they have developed a US supply of unique non-linear materials that show great potential in the areas of atomic physics, microelectronics and the defense industry. The spectrometer was described in a paper published in Review of Scientific Instruments and covered by Ames Lab Press Release, Newswise and Laser Focus World.
Many material properties are governed by the behavior of the electrons that roam the lattice of ions. Electrons in a solid are very selective. They inhabit particular energy levels for a given momentum and there are never more than two electrons sharing the same state. Electrons fill energy and momentum levels up to the Fermi energy; the highest occupied state. Electrons residing near the Fermi energy are the most important ones because they respond to external stimuli such as electric fields or temperature gradients. They are therefore responsible for many fascinating and useful phenomena such as high temperature superconductivity (used in ultrahigh resolution medical Magnetic Resonance Imaging), magneto resistance (utilized in high capacity hard disks) and the amazing properties of materials like graphene or topological insulators that will form the basis for future electronic devices.
To study the properties of electrons in solids, ISU professor and Ames Lab scientist Adam Kaminski uses a technique called Angle Resolved Photoemission Spectroscopy. “In a photoemission angle-resolved spectrometer, we map the band structure (i. e. values of energy and momentum that can be occupied by electrons) and find the properties of the most important electrons in a material.” Kaminski explains. “By shining UV light of much higher energy than visible light on the sample some of the electrons are ejected and we can measure their energy and momentum directly. We then apply the principle of conservation of energy and momentum to find the initial state of the electron before it escaped”. To help clarify the ARPES method, Kaminski compares it to the medical technique of magnetic resonance imaging. “With an MRI machine, physicians use X-rays or gamma rays to image the inside of the body and see organs like the heart and kidneys. With ARPES, we see a 3D image of the bands of electrons directly,” he says, “Electrons don’t have a well-defined spatial distribution, but they do have very well-defined momentum. So instead of working in real space like physicians do, we’re working in momentum space with ARPES. We can see what the bands look like and which states are occupied, i.e. where the electrons are in momentum space and what they are doing.”
The recent closure of two of the four synchrotron facilities in the US where ARPES experiments are performed has meant longer waiting and less time to perform experiments. So Kaminski and his students, who admittedly knew little about lasers, set about finding a way to make a tunable high photon energy light source. In researching the literature, they found that such a tunable laser had been suggested, but had never been used in an ARPES system. The laser used a potassium beryllium fluoroborate (KBBF) crystal to quadruple the frequency of infrared laser, converting photons to the required “vacuum UV” range. Obtaining such a crystal wasn’t easy. Kaminski found that China, the only source for the KBBF crystals, had embargoed their export. However, he found a research group at Clemson University that was led by Profs. Kolis and McMillen. They were able to grow the crystals he needed. He was also able to obtain funding through the DOE Office of Science and equipment fund of Ames Laboratory to build the new system. As an extra bonus, the crystal growth and preparation was picked up by a company Advanced Photonic Crystals L. L. C. that will make them commercially available in the US for applications such as UV photo lithography, spectral analysis and defense.
Kaminski’s system uses a pair of lasers. The first acts as a pump for the second one. The resulting beam consists of very short pulses (one quadrillionth of a second) and very high (400 kW) peak power. The beam is directed into a vacuum chamber that contains lenses, mirrors and the above mentioned “magic” crystal. This process quadruples the energy of the photons. By tuning the wavelength of the second laser and rotating the crystal one can tune the energy of the produced UV photons. The beam is then focused on the sample in an ultra-high vacuum chamber and a connected electron analyzer measures the electrons emitted from the sample.
“The development of a laboratory-based solution was really important,” Kaminski said. “Our beam is smaller, the photon flux is higher by one or two orders of magnitude, and the energy resolution is better by a factor of 5 compared to the best synchrotron facilities.” That translates into significantly better data. “Practical advantages are also important,” Kaminski said, “we don’t have book a limited amount of time on a beam line and then wait weeks or months to take the measurements. And because we’re not limited to a fixed time frame, we can do more complicated experiments that aren’t possible with a synchrotron beam line,” he continued. “This is particularly important when training students. They get to do hands on work in the lab, rather than watch beamline scientist performing the measurements for them.”
This work was supported by the DOE Office of Science.
Ames Laboratory is a U.S. Department of Energy Office of Science national laboratory operated by Iowa State University. Ames Laboratory creates innovative materials, technologies and energy solutions. We use our expertise, unique capabilities and interdisciplinary collaborations to solve global problems. DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.