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Utilizing Light in the Nanoworld
RIKEN Discovery Research Institute
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| Using light to see the nanoworld | |||
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The nanoworld cannot be imaged by light-that has been the "common knowledge." One nanometer (nm) is equal to one-billionth of a meter, or the size of several atoms. On the other hand, the light that can be perceived (visible light) is a kind of electromagnetic wave with a wavelength of approximately 400 to 700 nm. It has been the common belief that things measuring less than half the wavelength of visible light cannot, in principle, be seen with light. Electromagnetic waves are divided by frequency (number of oscillations per second) into radio waves, infrared rays, visible light, ultraviolet rays, x-rays, and gamma rays, in ascending order. As the frequency increases, the wavelength shortens. This is the reason why shorter waves such as X-rays are used to visualize the nanoworld. It has long been considered impossible to shorten the wavelength of electromagnetic waves sufficiently to visualize smaller worlds unless the frequency of such waves is increased. Kawata says, however, that this common knowledge can be refuted. "One possible approach is to decrease the velocity of light. Decreasing the velocity of light without changing the number of wave oscillations per second would result in a shorter wavelength." However, can light be slowed down? "Light travels approximately 300,000 kilometers per second. This, however, is the velocity obtained in a vacuum. The velocity of light decreases approximately 23% in water and about 34% in glass." Other situations can be created in which the velocity of light decreases. For example, if light is shone on a piece of metal with an aperture smaller than the light's wavelength, a very small amount of light will come out from the other side of the aperture. The light's velocity will have decreased and its wavelength shortened. This type of light occurs only in close vicinity to the metal's surface and is therefore known as "near-field light." In 1972, British researchers successfully applied this principle to observe a 0.5-mm structure using a radio wave with a wavelength of 30 mm. In 1985, a method of observing images by means of near-field light coming from a metal-covered aperture approximately 100 nm in diameter at the tip of an optical fiber was proposed. However, near-field light coming out from such a small aperture is rather dim. Near-field light of a shorter wavelength cannot be obtained unless the aperture size is further reduced. If so reduced, the near-field light becomes increasingly dim. "In 1992, I had spent months trying to come up with a way to obtain brighter near-field light and discussed this everyday at my university. Then one morning, I was struck with an idea while driving to the university." The idea was to aim an intense laser beam to a very small particle of metal or the sharply pointed tip of a metal needle. Intense near-field light is thus produced under the needle. About 70% of the near-field optical microscopes currently available in the world are of the metal needle type. This method invented by Kawata is now the world's standard in the relevant field. |
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| Seeing the nanoworld in color | |||
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If we have electron microscopes and other tools to explore the nanoworld, why do we need to use light to see the nanoworld? "This is because under an electron microscope, the world is monochrome, but under light, the world is in color," Kawata answers. The principle of electron microscopy is that an image is formed as gradations in density according to the intensity of electron rays passing through the sample. Color images obtained using an electron microscope are products of artificial coloring. Optical microscopes enable us to see the world in color. However, cells look transparent when seen under an optical microscope. Accordingly, the technology to investigate cell structures and the functions of proteins and other ingredients therein has been developed using a variety of dyes and fluorescent substances. This technology serves as a powerful tool to support advances in medicine and life sciences. Kawata and members of his laboratory developed a microscope capable of examining the structures and functions of living cells in their natural colors without staining them, using a specially designed picosecond (one-trillionth of a second) laser. This new microscope has already been commercialized in a venture business started by Kawata and his members. How can the colors of cells, which are essentially transparent, be seen without using dyes? "The molecules that constitute the cellular organelles and proteins oscillate constantly. Upon exposure to light, these molecules emit a kind of light known as Raman scattered light, which has a wavelength (color) that differs slightly from that of the light applied. This is because the wavelength shifted due to molecular oscillation. Although these color differences are imperceptible to the human eye, the most advanced optical technology enables us to perceive them. The color differences result from a variation in the mode of oscillation, depending on the molecular species. Hence, we can distinguish different kinds of molecules according to their colors." Recently, Kawata's laboratory succeeded in distinguishing the four bases of DNA according to color differences in Raman scattered light. Kawata's method is a powerful tool in evaluating and analyzing nanomaterials as well. For example, different thicknesses (diameters) of carbon nanotubes (nano-sized tubes formed by carbon atoms) can be distinguished by color differences (Figure 1). This is because tubes of different diameters produce different frequencies of carbon molecular structure. "We have already achieved a resolution of 10 nm, equivalent to one-fiftieth the wavelength of light. However, this is only halfway toward fulfilling my dream. I want to spend the rest of my research life improving the resolution to one-fiftieth the current level, or approximately 0.3 nm." Each hydrogen atom measures approximately 0.1 nm in diameter. With a resolution of approximately 0.3 nm, different atoms and molecules can be distinguished by their colors in the nanoworld.
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| Smallest laser sculptured bull featured in Guinness World Records | |||
Kawata succeeded in making a microscopic sculpture of a bull measuring 8 μm (0.008 mm) long using laser beams (Figure 2). Light enables us to not only see things but also process them. "My family was indifferent to my work even when my achievements were published in Nature and other well-known journals, both in Japan and abroad. However, when my microscopic bull appeared as the world's smallest laser sculpture in the 2004 edition of Guinness World Records, they finally thought highly of me (laughter)."The bull was sculpted from an acrylic resin that sets upon exposure to ultraviolet rays. In accordance with data stored in a computer on the contours of a real bull, a near-infrared laser was irradiated for an extremely short period of 100 femtoseconds (one-ten trillionth of a second) to define the contours of the microscopic sculpture. Finally, the surrounding resin, which remained unset, was washed off to reveal a three-dimensional bull. The bull had details measuring only 50 nm long. How could we create a sculpture with a precision of 50 nm using near infrared rays-which have a wavelength of 800 nm, longer than that of visible light-and how could we set the resin, which normally sets upon exposure to ultraviolet rays, using-near infrared rays? Light behaves as a particle does. This particle is known as a photon. Under normal conditions, any substance exposed to light absorbs one photon. However, when extremely intense light, which has a large number of photons, is used, two photons are absorbed at one time. In this state, the energy of two photons of infrared rays reaches a level equivalent to that of one photon of ultraviolet rays, thus enabling us to set the resin. "This is another method of shortening the wavelength of light without changing its frequency. Do the multiplication for light. Because a wave can be expressed by a cosine function, its wavelength shortens as two cosines are multiplied." |
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| Three-dimensional optical memory device | |||
Light can also be used to write and retrieve information three-dimensionally. "In 2000, U.S. President Bill Clinton proposed the creation of a memory device the size of a sugar cube to store all the information in the nation's Library of Congress. However, there was no one who carried out specific work to realize this proposal. We have actually made an optical memory about the size of a cube of sugar that can store information written three-dimensionally," says Kawata (Figure 3)"Optical memory devices that are currently available, including compact discs (CDs) and digital versatile discs (DVDs), suffer the limitation of information being written only on their surfaces," Kawata points out. "What is called blue-ray discs and HD-DVDs, both generically referred to as new-generation DVDs, can write and store information at very high densities on their surfaces using short-wavelength light, such as blue lasers. However, the method will not be able to improve much further. As the density of the information written on the disc increases, noise from dust likewise increases to the extent where the information will be irretrievable because the dust particles will be larger than the particles of disc material. Information should be written three-dimensionally in multiple layers rather than two-dimensionally at higher densities." Kawata says that further advances in the technology of three-dimensional information writing would lead to an optical memory device capable of storing one terabyte (1,000 gigabytes) of data in its sugar-cube-sized body. This is equivalent to more than 200 units of the currently available 4.7-Gb DVD and several tens of units of the new-generation DVD under development. "I believe this three-dimensional optical memory device will be out on the market within five years, provided that companies devote themselves to commercializing it," says Kawata confidently. |
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| Nanomachines that combat cancer cells | |||
"It took three hours to fabricate the microscopic bull statue featured in Guinness World Records. Now, it would take about 13 minutes to do the same thing. Recently, our laboratory succeeded in making 1,000 microscopic bull statues at one time using 1,000 light spots produced simultaneously with a specially designed lens."At the Nanophotonics Laboratory, Kawata and his colleagues applied the technology used to simultaneously make the 1,000 microscopic bulls and created a nanostructure with an array of metal antennae. Working the technology to fabricate a nanomachine the size of a red blood cell would make scenes from the science fiction movie Fantastic Voyage a reality. The microscopic bull, by the way, is the size of a single red blood cell. "Although it is impossible to reduce the size of physicians and deliver them into blood vessels as depicted in the movie, a nanomachine the size of a single blood cell would enable us to explore everywhere in the body, from the heart to the lungs and even the brain. The smallest blood vessel is as thin as a red blood cell in diameter. Injected into the blood vessel of a fingertip, the nanomachine would make its way in the bloodstream to a lesion. Receiving light transmitted from outside the body, the nanomachine would emit a laser to fight the cancer cells." The next question is how to transmit light to the nanomachine from outside the body? "We use near infrared rays. Visible light is quickly absorbed in superficial layers, whereas near infrared rays go into the body." Through trial and error, Kawata and his colleagues have created a pacemaker that is rechargeable using extracorporeally applied near infrared rays, thus obviating the need to replace batteries. Current pacemakers require wearers to undergo surgery to replace batteries every several years. "Light is gentle to the body and is useful as a source of energy to drive nanomachines in our bodies. In the nanoworld, the amount of energy does not necessarily have to be large; nanomachines can operate fully as long as a very small amount of thermal or optical energy is available." |
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| Pioneering plasmonics | |||
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Kawata served as a professor in the Graduate School of Engineering at Osaka University and was the head of the University's Frontier Research Center, founded in 2001 (until March 2004). The center aims at developing new disciplines and new industries that go beyond the borders separating different academic fields as well that between industry and university. Kawata, as a research leader in nanotechnology both in Japan and abroad, established the Nanophotonics Laboratory at the Discovery Research Institute of RIKEN in 2002. "During the first two years of my service at RIKEN, I prohibited those under me from publishing their findings, telling them not to write papers and refrain from giving presentations. I said that we should take our time doing our work. Even if they make a groundbreaking discovery, they should keep the fact a secret for a while. Being busy writing papers and filing patent applications are of no pleasure to researchers. The point of evaluating papers should be in their content rather than their number. I instructed them to store energy before embodying their work. Then, after two years, they all began getting desirable results." Kawata says that he wants to do two things at RIKEN. "One of my own goals is to advance research into nanoscale science by means of the photon; this is my lifework. The other is to pioneer a new discipline we call plasmonics." When light is applied to metal, the electrons in the metal oscillate as a whole. Scaling down a metal structure to the order of nanometers limits the space available for oscillation, causing the electrons' behavior to change from that found on larger scales. Plasmonics is the field of research into this behavior of electrons as a whole. For example, near-field light is emitted by a sharply pointed metal needle upon exposure to light, and a laser is emitted by the nanoscale structure of metal. Both of these are phenomena found in plasmonics. "There have been discussions on the possible interaction of light applied to the nanoscale structure of metal and the electrons in the metal causing various unusual phenomena, including light curving in the direction opposite to its ordinary direction. However, there are no methods that demonstrate such phenomena. Owing to the compilation of technical knowledge on optics and nanoscience, it has finally become possible to conduct experimental investigations of the potential of plasmonics. We presented a paper in November 2004 demonstrating that the structure emits a laser upon exposure to light and the other science in cyclic structure fabrication (Figure 4).This is what I want to do at RIKEN. Plasmonics promises to provide our society with new materials that would facilitate the use of light in the nanoworld." According to Kawata, "the 20th century was the age of Edison." In other words, technologies that relied on the use of electrons supported science, technology, and society in the last century. "It is certain, however, that the times are seeing a shift from the electron to the photon," says Kawata definitely. For example, conventional telephones, which depend on copper wires, are being replaced by mobile phones, which employ optical fibers or electronic waves, and television displays are changing from cathode-ray tubes, which employ electron rays, to liquid crystals. "This is because light is friendly to the body and, in addition, can easily be utilized without the need of a vacuum like those of cathode-ray tubes. However, it has been impossible to utilize long-wavelength light in the nanoworld. If we can learn how to freely manipulate photons in the nanoworld, the transition from the electron to the photon will accelerate. At RIKEN, many of my colleagues are engaged in research into light. I would like to go along with them to pioneer the new field of optics and nanoscience at RIKEN, a world center of scientific research." 
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Listening to what a chicken embryo says about the rules of body formation
RIKEN Kobe Institute |
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| Meeting Dr. Le Douarin | |||
After receiving a doctoral degree in Japan in 1988, Takahashi began studying chicken embryos at the Developmental Biology Laboratory at Centre National de la Recherche Scientifique (CNRS) in France. She worked there under the guidance of the institute's director, Dr. Nicole Le Douarin, an authority of worldwide fame in developmental biology for studies in neural crest cells (Figure 1). "She was very strict but quite august! If we skimped on our own work even a little, everyone would receive a stern rebuke from her. At times like that, I was struggling just to survive," says Takahashi in retrospect."However, she is a woman of learning and has a broad view of things. Because I believed that any work that meets her approval would be honored worldwide, I was always searching for something interesting. If I found one that I thought she would like, I would take it to her. More often than not, unfortunately, my efforts would receive harsh criticism. I clearly remember on one occasion her telling me, "Yoshiko, don't do what everyone else is doing. Do what no one else is doing!" I found my way where I should go. |
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| Elucidating the mechanisms behind morphological boundary formation | |||
ES cells, removed from an early embryo, are universal cells capable of differentiating into any tissue or organ. Currently, there is amazing research being done in regeneration medicine that aims to differentiate ES cells into particular types of cells, which are then cultured and applied to medicine. "I think ES cells have great potential in medical practices. However, studies of cell differentiation in laboratory dishes are a thing of the past for me. I want to go beyond it. We researchers of basic science are required to constantly challenge ourselves to find new ideas," says Takahashi. She is now tackling the issue of three-dimensional morphogenesis in the body. "The proliferation and differentiation of cells will produce no more than a mass of cells. A normally functioning body cannot be formed unless cells differentiate into a predetermined shape at a predetermined site in a particular pattern." One of the themes to which Takahashi has paid special attention in morphogenesis concerns the question of how the morphological boundary between tissues is formed. "Although how the boundary is formed is a new theme and not mentioned in textbook, it represents an essential issue of morphogenesis. Without a boundary, there would not be any shape. Then, how can we study the mechanisms of boundary formation using a constantly changing embryo? I directed my attention to a phenomenon known as somitic segmentation." The somite is a kind of tissue from which bones and muscles originate. Although the backbone consists of a number of discrete bones, such as the first and second cervical vertebrae, it is essentially a product of segmentation in which a series of somites split up one by one. In a chicken embryo, somites split up in order, starting from the head, at constant intervals every 90 minutes (Figure 2). "Because the embryo changes constantly and is difficult to control, we must set up a sophisticated experiment to get successful results from its studies. For any phenomenon that occurs only once during genesis, we cannot determine whether it is an inevitable consequence or a chance occurrence even if experimental conditions are altered variously. Somitic segmentation, which occurs repeatedly in the body, serves as an excellent experimental system that enables us to understand how morphological boundaries are formed." Researchers at our laboratory conducted daily experiments on embryos for two years, and finally determined that the cells posteriorly adjoining the prospective segmentation site play a critical role." In an attempt to confirm the potential of such cells for boundary formation separating somites, Takahashi and her colleagues transplanted them to other sites. Surprisingly, fissures formed at non-segmentation sites, where segmentation does not occur under normal conditions. "A fissure forms as these cells direct the just anteriorly adjoining boundary-forming cells to 'go away.' We designated this type of fissure-making activity a 'segmentor.'" Furthermore, Takahashi and her colleagues examined the genes that function during segmentation to clarify the molecular mechanisms behind segmentation. The on/off boundary for the expression of a gene called L-fringe was identified as the segmentor site. Subsequently, they applied the electroporation technique to confirm the actual functioning of this gene to induce segmentation. Electroporation is a method of transferring a gene into a cell by using electricity to force the cell to express the gene, a method that had been traditionally used with bacteria, cultured cells, and other easily handleable materials. Takahashi and her colleagues were the first ever to apply this method to the somites of a living embryo. Using the method, they succeeded in expressing the L-fringe in cells that had not begun segmentation. They transplanted those cells (L-fringe expressed) to the unsegmented somites of other embryos. A boundary for the on-off switching of the L-fringe was thus created artificially, resulting in segmentation at that boundary (Figure 3). Because the L-fringe is known to regulate the function of a protein called Notch, Takahashi conducted similar experiments using Notch and found it to be capable of inducing segmentation. Thus, they demonstrated that segmentor functions are under the control of the L-fringe and Notch using their own experimentation system. Takahashi believes that there are common rules of morphogenesis in body formation among the discrete tissues and organs. "Some of the genes functioning in somitic segmentation could also function in, for example, boundary formation that divides the brain into the forebrain and the midbrain. I want to overview the entire individual on the basis of findings in a single tissue." Takahashi extends her views to biological evolution. "We are now focusing on communication among cells and among tissues in chicken embryogenesis; in the near future, common rules of genesis beyond the barrier of species will become evident. For example, developmental processes are much alike among chickens, mice, and humans. However, differences do exist. What, then, is the reason for these differences? This poses a major problem in discussing biological evolution. To know the reason, we must first know what are shared. I want to explore the common rules of genesis. I think that clarifying the rules would unveil what remains unknown." |
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| Elucidating the mechanisms behind morphological changes in the cell | |||
A study result published by Takahashi and others in 2004 was played by the media as leading to the elucidation of the mechanisms of cancer metastasis.Cancer mostly develops in cells that are regularly arranged in the form of sheets, such as in the mucosal membranes of the gastrointestinal tract and respiratory organs. These cells are called epithelial cells. A fear of cancer resides in epithelial cells cancerating into atypical cells (mesenchymal cells), departing from the sheets, and migrating elsewhere in the bloodstream. These morphological changes in the cell are of paramount importance to body formation in the developmental stage. While cells migrate to predetermined sites, some turning into epithelial cells and others into mesenchymal cells, morphogenesis in body formation proceeds. What are the mechanisms behind morphological changes in a cell? Cells are supported from the inside by a fiber structure known as a cytoskeleton. Experiments using cultured cells have revealed that morphological changes in cells involve a mechanism in which actin, a protein used in the forming of the fibrous arrangement of the cytoskeleton, is controlled under the direction of a group of genes known as the Rho family. "However, almost nothing was known about the relationship between cell morphological changes occurring in the body and the Rho family. A majority of cell biologists culture cells in laboratory dishes. Their experiments using cultured cells revealed new genes one by one. In the meantime, developmental biologists handle cells in situ in the actual body. At present, there is little connection between the two sectors. Last year's publication of our achievements correlated them to each other." The present study was also conducted in the context of somitic segmentation. While pre-somite cell essentially comprises mesenchymal cells prior to segmentation, some of them, at sites where segmentation takes place, turn into epithelial cells and wrap mesenchymal cells (Figure 4). Thus, the somite consists of only two kinds of cells: epithelial and mesenchymal. "This is the key point," emphasizes Takahashi. "Genesis is a very complex series of biological phenomena. It is the researcher's task to discover an easily analyzable phenomenon of simple mechanism and employ it as an original experimental system." Takahashi and her colleagues introduced the GFP gene, which generates fluorescence substances, into somites by electroporation to distinguish the different shapes of cells and found that the GFP gene is incorporated randomly in both mesenchymal and epithelial cells (Figure 5). "With this in mind, we activated or suppressed the functions of the genes of the Rho family. As a result, suppressing the function of the Cdc42 gene resulted in the preferred formation of epithelial cells (Figure 6)." The function of Cdc42 determines cell morphology. Why, then, is the function of Cdc42 suppressed at segmented sites? Takahashi hypothesizes that a secretory factor from the ectoderm, a tissue outside the somite, is likely to penetrate segmented cells as a result of boundary formation in somites and, hence, induces Cdc42 inactivation.
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| Challenging the riddle of cell migration | |||
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"Now, I'm interested in cell migration."
In the process of genesis, cells migrate to a predetermined site, stop, and join themselves to the cells already there, thus forming a particular tissue. Alternatively, discrete cells gather like a stack of bricks to form a tube. Cells in sheet form also show major action. For example, our brain comprises numerous inflated and/or complexly folded tubes formed by sheets of cells that have become round. "It is the dynamic movements of cells that allow the formation of our beautifully complex body. It is quite interesting, isn't it?" Owing to experiments using cultured cells or the knock-out technique, which suppresses the function of a particular gene by gene manipulation, and other studies, genes that play key roles in cell migration and morphogenesis in body formation have been discovered one after another. "However, regarding the issue of cell migration, for instance, it is still unknown when and where a particular gene functions as well as what is happening in the body. The greatest drawback of today's life science resides in the lack of research in the situation. This is because there are only a few researchers able to directly explore the inside of a living body. I'm confident that I'm one of the few researchers who are most aware of the issue." What is Takahashi's approach to elucidating the rules of cell migration? "Our body has predetermined routes of cell migration. In somite, in particular, the route of migration of neural crest cells, which are destined to form peripheral nerves, is already known. Somite cells and migrating neural crest cells remain in mutual communication when determining where to change their orientation and where to stop. At a site where migrating cells stop, somite cells generate a 'stop' signal. To elucidate the rules of cell migration, we investigate the communication between migrating cells and somite cells." |
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| Listening to the voices of embryos | |||
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Takahashi, from the viewpoint of a female scientist, says, "When I was a university student, I was the only female in my department. In those days, Japan was a completely androcentric society. Recently, however, a social system is emerging that enables women to develop and exercise their abilities to the fullest. Unfortunately, however, female researchers themselves are not as persistent as expected. Many talented women leave their workplace unnoticed. I recognize the diverse lifestyles women have, but I hope female scientists will be free to enjoy their research activities more." "I think I'm very strict with my students. I want them to build up their courage and, above all, nurture logic. The beautiful voices of the embryo cannot be heard with vague ideas. Secure logic is essential." Takahashi continues, "Being a professional researcher is, of course, not always enjoyable. This is true for all occupations. Keeping highly motivated as a professional is impossible unless one constantly finds something new and interesting. For its sake, I listen to the voices of the embryo. I can understand what the cell is thinking. I want the members of my laboratory to talk to the embryo and listen to it as I do." Takahashi says that she always becomes serious when she sees a photograph of Dr. Le Douarin. "I, both as a researcher and an educator, have not yet surpassed her. I want to find more interesting things and encourage my students. Dr. Le Douarin is a preceptor who always leads me where to go."
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X-Ray Free-Electron Laser Project SCSS Exploiting Material and Life Sciences in the 21st Century
Interview with Tsumoru Shintake, Chief Scientist, Ph. Dr. |
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| Potential of X-ray free-electron laser | ||
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-- What is the difference between the X rays now produced at SPring-8 and the X-ray free-electron laser (XFEL) targeted in the SCSS project? Shintake: At SPring-8, approximately one billion electrons are revolving in bundles in its ring. An electron emits light when its trajectory is deflected by a magnetic force. This type of light is called "synchrotron radiation." As the speed of the electron is increased, the wavelength of the radiation shortens. SPring-8 is capable of producing radiation in a broad range of wavelengths and, in particular, of producing radiation of the world's highest brilliance in the wavelength range from vacuum ultraviolet rays to X rays. SCSS can produce X rays 100,000 times as intense as the X rays from SPring-8. Although radiation is emitted by each electron, in microscopic view, the longitudinal positions of one billion electrons are distributed randomly in one electron bunch (electron cloud); overlaps of crests and troughs of each light wave result in counteraction and attenuated light. Provided that electron positions are aligned for each interval of light wavelength, crest-crest and trough-trough overlaps occur, resulting in extremely intense light (Figure 1). This type of light is called coherent light. Laser is a type of light in this state. SCSS is an apparatus for accelerating electrons, aligning their positions, and producing XFEL. -- What is expected from XFEL? Shintake: Our ultimate goal in the SCSS project is to elucidate the three-dimensional structures of proteins without crystallizing them. Proteins exhibit a variety of functions with steric forms (structures). In understanding the functions of proteins, it is important that their structures should be analyzed at resolutions on an atomic scale, and such analysis requires the use of X rays. However, if ordinary X rays are applied to a protein, only a portion of the dose will collide with atoms in the protein and scatter, with almost all passing through the analyte protein. For this reason, it is now common practice to make a crystal containing a large number of regularly arranged protein molecules, and expose it to X rays. Although each protein molecule emits weak scattered light, we can collect the scattered light from an array of many protein molecules, and convert the data using a computer to determine the three-dimensional structure of the protein. At SPring-8, we have successfully applied this method with remarkable results in analyzing the steric structures of a number of various proteins. However, only 20 to 30% of proteins are crystallizable. Because the remaining 70 to 80% of proteins are uncrystllizable, their structures cannot be examined using this method. In the XFEL, the wavelets of X-ray from electrons coherently overlap to produce very strong peak power, in place of arranging protein molecules. The XFEL of the SCSS project, 100,000 times as intense as the X rays from SPring-8, would enable us to explore the steric structure of a protein using a single molecule thereof, without crystallizing the protein. Furthermore, this method is also expected to enable extensive examination of the inside of a living cell on an atomic scale. The XFEL of the SCSS project can also be used like a camera flash to examine phenomena that persist for very short times-in the order of several tens of femtoseconds with resolutions on an atomic scale. The prefix "femto-" refers to one quadrillionth (10-15). Although a femtosecond laser based on visible light is already available, its resolution remains above atomic levels. Using XFEL, it is possible to finely visualize the instantaneous movement of electrons involved in chemical reactions and the process of crystal growth during the making of materials. XFEL would help major advances in chemistry and material science, hence leading to the development of new materials with innovative functions.
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| What is an X-ray free-electron laser? | ||
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-- How do you produce XFEL?Shintake: The currently available laser of the shortest wavelength is an excimer laser, which is used to expose the circuits of central processing units (CPU) in computers, having a wavelength of 157 to 248 nm (1 nm is one-billionth of a meter). On the other hand, our ultimate target wavelength in the SCSS project is 0.1 nm. The wavelength must therefore be shortened by as much as 1000-fold. An X-ray laser of 0.1-nm wavelength represents an unexplored field. Hence, we will employ a free-electron laser (FEL). When electrons are run between two arrays of magnets positioned with alternating N and S poles, the electrons are deflected by the magnetic force, emitting intense radiation (Figure 2). This apparatus is called an "undulator." Mirrors are placed at each end of an undulator about one meter long, and an electron beam is applied to each reciprocal light path. Electron positions are aligned for each interval of the wavelength of the light due to the light-electron interaction, with a laser emitted after about 100 repeated cycles. This is a free-electron laser. Now, we can produce lasers over the wavelength range from radio waves to ultraviolet rays using this method. However, as the wavelength shortens, the mirrors' reflectance decreases, approaching to zero at wavelengths of less than 80 nm. Therefore, the mirror-based method is not promising for the realization of an XFEL of 0.1-nm wavelength. -- Is there any method not using a mirror? Shintake: In the first half of the 1990s, US and German researchers proposed that if "100 reciprocal light paths in an undulator one meter long produces a laser, then it is possible to produce a short-wavelength XFEL by extending the total length of the undulator to 100 meters." This will obviate the need for a mirror to reflect the light. This is called self-amplified of spontaneous emission free-electron laser (SASE-FEL). However, to run light and electrons together and allow them to interact with each other, the 100-meter undulator must be constructed precisely in a straight line. The required precision is such that the transverse trajectory error should not exceed approximately 10 μm (0.01 mm) over the 100 meters of the undulator. Compare this with the thickness of a hair, which measures about 50 μm in diameter. They thought it would be impossible to build an undulator straight over a distance of 100 meters to such high precision. However, the technology to handle nano-size structures saw remarkable advances as a result of the high-energy physics project for the "Linear Collider," a new-generation large linear accelerator under development with international cooperation. I have been engaged in the development of nano-size measuring technology as an invited researcher for that project in the US. Now, at last, we have acquired a firm technical footing for the realization of XFEL. Japan, the US, and Germany are racing with each other to become the first to realize XFEL. |
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| Unique technology and craftsmanship underlying SCSS | ||
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-- What sort of apparatus is the SCSS? Shintake: SCSS is an apparatus about one kilometer long comprising an electron gun, which produces electrons, a linear accelerator, which accelerates the electrons to form an electron beam, and an undulator, which produces XFEL from the electron beam (Figure 3). The total length of our SCSS is only a fraction of the size of the apparatuses built in the US and German projects. The "C" in SCSS stands for "compact." The size reduction results in reduced construction costs. -- How can you achieve such size reduction? Shintake: Because our own unique technologies are employed for the undulator and the linear accelerator. For the undulator, the width of each magnet was reduced to 15 mm, one-third that of a conventional undulator, as suggested by chief scientist Hideo Kitamura of the Coherent Synchrotron Light Source Physics Laboratory. The total length of the undulator can thus be shortened, and cutting energy for its operation. It should be noted, however, that we cannot create a magnetic force field that deflects electrons unless the upper and lower magnets are closely positioned relative to each other in proportion to the reduction in magnet width. However, it has been impossible to position closely the upper and lower magnets because of the vacuum pipe used to run the electrons between the upper and lower magnets. To resolve the problem, Kitamura devised a groundbreaking method that explodes the conventional knowledge, in which the magnets are housed in a vacuum vessel with no vacuum pipe. This is the in-vacuum undulator. Kitamura established the in-vacuum undulator technology through extensive efforts, including coating the powder-sintered magnets with titanium nitride to prevent gas emission. For the linear accelerator, we will employ C-band technology, which was developed in the "Linear Collider" project as a result of joint research by myself and Hiroshi Matsumoto, Assistant Professor of the High Energy Accelerator Research Organization (KEK), when I was working there. -- What is this C-band? Shintake: The C-band is a frequency band of a type of radio wave known as microwave. Ordinary electron linear accelerators employ microwaves in the 2.8-GHz S-band to accelerate the electrons. I have developed a technology using the 5.7-GHz C-band. Doubling the frequency results in doubled acceleration efficiency, thus enabling us to halve the total length of the accelerator. However, the increased frequency calls for higher working precision for the acceleration tube, through which the electrons run. The acceleration tube is fabricated by machining copper on a turning lathe, and this step requires craftsmanship. Additionally, since any stain in the acceleration tube would cause an electric discharge, the tube must be worked and installed in an extremely clean state. To this end, a lot of technical know-how is required. We have maintained a close connection with the factory in charge of manufacturing the tube for two decades. The copper material we use is of the world's highest quality. A copper refiner in Japan produces purified copper with an electrical conductivity equivalent to 105% of the world standard. This is proof of its high purity. I think Japan is wonderful! -- You have also developed a new technology for the electron gun, haven't you? Shintake: The electrons coming out from the electron gun must be parallel because of the shortness of the undulator. Bearing in mind the source of ions using lanthanum boride (LaB6) as electron emitter that I developed when I was a graduate student, I attempted to use cerium boride (CeB6) monocrystals of 3 mm diameter as the electron source (thermal cathode). It should be noted, however, that the electron gun of an ordinary accelerator is heated at about 900°C, whereas CeB6 must be heated to an ultrahigh temperature of 1450°C. The electric power required to achieve this temperature is about 10 times as much as that for the ordinary accelerator, and causes the filament in the ordinary heater to burn out. Hence, we adopted the ultrahigh-temperature heater used to produce silicon single-crystals. When I had a plant tour to a semiconductor factory, I stumbled upon this heater technology. Our electron gun set a new world record in the parallel emission of the electrons coming out. -- What are the important points in keeping the apparatus straight with a 10 μm precision? Shintake: One point resides in the platform that supports the apparatus. The platform is one meter high; its coefficient of thermal expansion is of concern. As the air temperature changes by one degree, the length of the platform, if made of iron, increases or decreases by 20 μm. Hence, we use a special ceramic material known as cordierite. Its coefficient of thermal expansion is one-twentieth that of iron. From the beginning, cordierite was also used for the power transmission line insulators. -- The accelerator requires a variety of technologies, doesn't it? Shintake: Yes. To build a new accelerator, all relevant technologies must be reconsidered comprehensively. In these two decades, I have pursued new technologies through visits to actual production sites right across Japan, from Hokkaido to Kyushu. For example, hearing that the sales of liquid crystals were growing remarkably, I hurried to visit a production line, where I learned of a subcontract plant, to which I then made a plant tour. Where money concentrates, a new technology exists. Japan's production sites are wonderful. There are a lot of technologies based on excellent craftsmanship. I realize that Japan cannot stand as a nation without craftspeople. Craftsmanship is necessary for both the acceleration tube and the ceramic material I have just mentioned. The accelerator relies largely on their art.
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-- Finally, please describe the current trends in the US and Europe and the prospects for the SCSS project. Shintake: All the three sectors aim at a wavelength of 0.1 nm. In Germany, the TESLA project is ongoing at Deutsches Elektronen-Synchrotron in der Helmholtz-Gemeinschaft (DESY), where a prototype apparatus 20 meters in total length is already in operation to produce radiation. The project aims at 6-nm laser to be produced in 2005. However, the planned facility to produce XFEL of 0.1-nm wavelength will be several times as large and expensive as our SCSS project. Against this background, DESY is striving to secure a budget for the TESLA project by involving the entire European Union, by trying to arrange the matter to the satisfaction of the member countries. On the other hand, we are planning to build an apparatus about 40 meters long in the premises of SPring-8, and to produce a laser of 60-nm wavelength, as the first term goal of the SCSS project, in 2005. If the requested budget is approved for Japan's national project, the equipment will be expanded to a total length of one kilometer to produce an XFEL of 0.1-nm wavelength. A strong rival may be America's Linac Coherent Light Source (LCLS) project at the Stanford Linear Accelerator Center (SLAC). This is a reformed project to use a 3-km linear accelerator built in 1967 in connection with a newly constructed undulator. The reformation will begin this year, aiming at an XFEL of 0.1-nm wavelength in September 2008. -- Are you confident about successfully fighting off the international competition? Shintake: Comparing the three runners, I think Japan is the best in technical potential. Additionally, advanced technology is required in handling the coherent X-ray laser radiation produced in these projects, and active technical development is ongoing under the leadership of Tetsuya Ishikawa, the chief scientist of the Coherent X-Ray Optics Laboratory. A firm foothold is being established that will keep us ahead of the other countries. I think what remains lacking is budgeting. If the requested budget is approved, we will have a fair chance of success. In scientific research, Japan has always made a bad start, becoming the second runner and lagging behind other countries in making a global contribution. Being quick to commence investigations using XFEL would surely result in a world first achievement. In the context of frontier research, coming second has no meaning!  |
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