The secret of X-ray science - like so much else - is in the timing. Scientists at the U.S. Department of Energy's (DOE) Argonne National Laboratory have created a new way of manipulating high-intensity X-rays, which will allow researchers to select extremely brief but precise X-ray bursts for their experiments (, "X-ray photonic microsystems for the manipulation of synchrotron light"). 
Scientists at Argonne have created a new way of manipulating high-intensity X-rays, which will allow researchers to select extremely brief but precise X-ray bursts for their experiments. This schematic of their microelectromechanical device consisting of a small oscillating mirror illustrates the reflection of an incoming X-ray at a particular critical angle. (Image courtesy Daniel Lopez/Argonne National Laboratory) The new technology, developed by a team of scientists from Argonne's Center for Nanoscale Materials (CNM) and the Advanced Photon Source (APS), involves a small microelectromechanical system (MEMS) mirror only as wide as a few hairs. MEMS are microscale devices fabricated using silicon wafers in facilities that make integrated circuits. The MEMS device acts as an ultrafast mirror reflecting X-rays at precise times and specific angles. "Extremely compact devices such as this promise a revolution in our ability to manipulate photons coming from synchrotron light sources, not only providing an on-off switch enabling ultrahigh time-resolution studies, but ultimately promising new ways to steer, filter, and shape X-ray pulses as well," said Stephen Streiffer, Associate Laboratory Director for Photon Sciences and Director of the Advanced Photon Source. "This is a premier example of the innovation that results from collaboration between nanoscientists and X-ray scientists." The device that the Argonne researchers developed essentially consists of a tiny diffracting mirror that oscillates at high speeds. As the mirror tilts rapidly back and forth, it creates an optical filter that selects only the X-ray pulses desired for the experiment. Only the light that is diffracted from the mirror goes on to hit the sample, and by adjusting the speed at which the MEMS mirror oscillates, researchers can control the timing of the X-ray pulses. According to Argonne nanoscientist Daniel Lopez, one of the lead authors on the paper, the device works because of the relationship between the frequency of the mirror's oscillation and the timing of the positioning of the perfect angle for the incoming X-ray. "If you sit on a Ferris wheel holding a mirror, you will see flashes of light every time the wheel is at the perfect spot for sunlight to hit it. The speed of the Ferris wheel determines the frequency of the flashes you see," he said. "The Argonne team's work is incredibly exciting because it creates a new class of devices for controlling X-rays," added Paul Evans, a professor of materials science at the University of Wisconsin-Madison. "They have found a way to significantly shrink the optics, which is great because smaller means faster, cheaper to make, and much more versatile." In the future, the MEMS devices could split an X-ray pulse into even tinier, faster, and more precise slices by oscillating the device many millions of times a second, according to Argonne emeritus scientist Gopal Shenoy. "It will herald a new era of dramatically new and improved kinds of X-ray experiments," he said. "The advantage of this new device is that it provides a very cheap way to generate and manipulate X-rays, and it can be adapted to virtually any X-ray facility in the world that already exists," Lopez said. "The successful application of the MEMS technology to manipulate an X-ray beam at very high frequencies will certainly lead to further, more elaborate X-ray optical schemes for studying the structure and dynamics of matter at atomic length and time scales," added Edgar Weckert, the director of photon science at DESY, a German synchrotron research facility. "This work is a very interesting first step of the MEMS application to X-ray optics. I am looking forward to the progression of the technology and its applications in wider fields at next-generation light sources," said Tetsuya Ishikawa, the director of the RIKEN SPring-8 Center in Japan. These include newly planned light source facilities such as the Advanced Photon Source Upgrade. "Such small sources and tiny MEMS devices form an ideal combination to make 3-D X-ray ultrafast movies with nanometer resolution," added Jin Wang, a senior scientist at the APS and one of the lead authors.
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Scientists at Argonne have created a new way of manipulating high-intensity X-rays, which will allow researchers to select extremely brief but precise X-ray bursts for their experiments. This schematic of their microelectromechanical device consisting of a small oscillating mirror illustrates the reflection of an incoming X-ray at a particular critical angle. (Image courtesy Daniel Lopez/Argonne National Laboratory) The new technology, developed by a team of scientists from Argonne's Center for Nanoscale Materials (CNM) and the Advanced Photon Source (APS), involves a small microelectromechanical system (MEMS) mirror only as wide as a few hairs. MEMS are microscale devices fabricated using silicon wafers in facilities that make integrated circuits. The MEMS device acts as an ultrafast mirror reflecting X-rays at precise times and specific angles. "Extremely compact devices such as this promise a revolution in our ability to manipulate photons coming from synchrotron light sources, not only providing an on-off switch enabling ultrahigh time-resolution studies, but ultimately promising new ways to steer, filter, and shape X-ray pulses as well," said Stephen Streiffer, Associate Laboratory Director for Photon Sciences and Director of the Advanced Photon Source. "This is a premier example of the innovation that results from collaboration between nanoscientists and X-ray scientists." The device that the Argonne researchers developed essentially consists of a tiny diffracting mirror that oscillates at high speeds. As the mirror tilts rapidly back and forth, it creates an optical filter that selects only the X-ray pulses desired for the experiment. Only the light that is diffracted from the mirror goes on to hit the sample, and by adjusting the speed at which the MEMS mirror oscillates, researchers can control the timing of the X-ray pulses. According to Argonne nanoscientist Daniel Lopez, one of the lead authors on the paper, the device works because of the relationship between the frequency of the mirror's oscillation and the timing of the positioning of the perfect angle for the incoming X-ray. "If you sit on a Ferris wheel holding a mirror, you will see flashes of light every time the wheel is at the perfect spot for sunlight to hit it. The speed of the Ferris wheel determines the frequency of the flashes you see," he said. "The Argonne team's work is incredibly exciting because it creates a new class of devices for controlling X-rays," added Paul Evans, a professor of materials science at the University of Wisconsin-Madison. "They have found a way to significantly shrink the optics, which is great because smaller means faster, cheaper to make, and much more versatile." In the future, the MEMS devices could split an X-ray pulse into even tinier, faster, and more precise slices by oscillating the device many millions of times a second, according to Argonne emeritus scientist Gopal Shenoy. "It will herald a new era of dramatically new and improved kinds of X-ray experiments," he said. "The advantage of this new device is that it provides a very cheap way to generate and manipulate X-rays, and it can be adapted to virtually any X-ray facility in the world that already exists," Lopez said. "The successful application of the MEMS technology to manipulate an X-ray beam at very high frequencies will certainly lead to further, more elaborate X-ray optical schemes for studying the structure and dynamics of matter at atomic length and time scales," added Edgar Weckert, the director of photon science at DESY, a German synchrotron research facility. "This work is a very interesting first step of the MEMS application to X-ray optics. I am looking forward to the progression of the technology and its applications in wider fields at next-generation light sources," said Tetsuya Ishikawa, the director of the RIKEN SPring-8 Center in Japan. These include newly planned light source facilities such as the Advanced Photon Source Upgrade. "Such small sources and tiny MEMS devices form an ideal combination to make 3-D X-ray ultrafast movies with nanometer resolution," added Jin Wang, a senior scientist at the APS and one of the lead authors.
The interface between a donor and acceptor material determines the efficiency at which photogenerated electrons (–) and holes (+) are separated to produce a current in a photovoltaic device. (Image: Keisuke Tajima, RIKEN Center for Emergent Matter Science) Improving the efficiency of solar cells requires an intimate understanding of what happens when light strikes a material. Most semiconductor-based photovoltaic devices have essentially the same operating principle: light absorbed by the active material creates an electron and a positively charged counterpart, known as a hole. For a current to flow, these two charges must move in opposite directions (Fig. 1), which is difficult to achieve in organic semiconductors. One solution is to combine two different materials, referred to as a donor and an acceptor, so that a molecular heterojunction exists at the interface. The energy offset between the molecular orbitals of the two materials helps to separate the photogenerated charge carriers. A surprising experimental observation, however, is that the process is more efficient than expected by theory, with some organic photovoltaic devices exhibiting a quantum efficiency close to 100%. “A simple calculation estimates that the binding energy is much stronger than thermal energy at room temperature,” explains Keisuke Tajima from the RIKEN team. “Several theories have been proposed to rationalize this paradox, but the debate is still ongoing.” Previous studies had looked at various mechanisms for weakening the attraction between charge pairs at the interface, but experimental studies provided contradictory results. The results of these studies suggest, however, that the secret lies in the molecular detail at the donor–acceptor interface. Tajima, working with his colleagues from the RIKEN Center for Emergent Matter Science and the University of Tokyo, attempted to get a better understanding of this problem by carefully controlling the energy levels in the vicinity of donor–acceptor interfaces. Using a contact film transfer method, the researchers were able to stick the donor and acceptor films together at room temperature without using organic solvents to give the well-defined interfaces needed for the experiment. “Our results show that the energetics of the first molecular monolayer at the donor–acceptor interfaces are of vital importance for charge generation and recombination processes in an organic photovoltaic cell, indicating that the solar cell efficiency can be significantly improved,” explains Tajima. “The next big challenge is to implement these interfaces precisely at the molecular level in bulk heterojunction structures, preferably through molecular self-organization. This could push the envelope of organic photovoltaic performance in the future.”
Each of the nano-lanterns highlights the expression of different genes in mouse embryonic stem cells. (© PNAS) Previous attempts to use luminescent proteins for imaging foundered because their light is usually too dim to track rapid changes inside cells. Instead, Yasushi Okada, Akira Takai and colleagues from the RIKEN Quantitative Biology Center, in collaboration with the team of Takeharu Nagai from Osaka University, looked to nature for inspiration. The sea pansy Renilla reniformis contains an enzyme called Renilla luciferase (RLuc) that helps to oxidize a molecule called coelenterazine—a chemical reaction that produces a flash of blue light. A few years ago, Nagai’s team coupled a mutant form of RLuc with a yellow-green fluorescent protein called mVenus to produce a luminescent imaging agent. Building on this work, Okada and his colleagues created two more coelenterazine-oxidizing luminescent agents using the same approach by coupling RLuc with cyan and orange fluorescent proteins. The resulting nano-lanterns are about 20 times brighter than natural RLuc. “The luminescent probes are bright enough to be detected with an iPhone camera,” says Okada. To demonstrate their technique, the team attached nano-lanterns to cellular structures such as lysosomes to track their movements over several minutes. They also monitored the concentration of calcium ions in cells, using nano-lanterns with a mutant RLuc that was split in two such that the lanterns only shone when a calcium ion joined the two halves together. Finally, the researchers used their nano-lanterns to study gene expression. By adding the genetic codes for the nano-lanterns next to three different genes inside mouse cells, the team was able to produce a distinct color signature when those genes were expressed. In just ten seconds, this revealed patterns of gene expression across a colony of cells (Fig.). The nano-lanterns are still about 100 times dimmer than conventional fluorescent probes, and rely on a steady supply of coelenterazine to keep glowing, but Okada believes that these limitations are challenges that will be overcome. “Another goal is to create red nano-lanterns that could be useful for deep-tissue imaging,” he notes. The team is now developing microscopes for super-resolution luminescence imaging, and using the nano-lanterns to further analyze gene expression within single cells.