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Major Breakthrough in X-ray Research
An international research project including a research team led by Prof. Chen Ming-chang of NTHU’s Institute of Photonics Technology has made a breakthrough in the production of a high-brightness, tabletop X-ray device. By using the extremely short wavelengths of a laser, they have succeeded in dramatically increase the conversion efficiency of high order harmonic generation (HHG), coherent X-ray, by more than a thousand times. With a brightness of 13.5 nm, the new light source is expected to play an important role in nano bio-imaging and defect inspections in the semiconductor industry. The results have been published in the December edition of Science (Science, 350, 1225 (2015)) .
According to Prof. Chen, since the 20th century X-rays have been one of the most important light sources in scientific and technological research, and have been widely used in both basic and applied scientific research in areas such as materials, electronics, biology, medicine, physics, chemistry, chemical engineering, geology, archeology, energy, environmental protection, and micro-mechanics.
The common goal of all types of lasers—the large synchrotron radiation center, the free-electron laser, right down to the table-top X-ray light sources—has been the production of a high-brightness, ultrafast, coherent X-ray for use in the development of an ultra-high, ultra-precise, spatially-resolved, time-resolved detector. Since the femtosecond-to-attosecond X-ray pulse enables the dynamics of chemical reactions, nano-materials and bio-molecular systems to be studied with unprecedented temporal and spatial resolution.
However, up to now, the brightness of HHG light source still limits its applications. Research over the past three decades has been limited by the low brightness of the X-ray, so this recent breakthrough is expected to be widely utilized in applied science and industry, such as ultrafast, nano-microscopy applications.
How do electrons move in the nanometer film? How is energy transmitted in the nanometer transistor? How is photosynthesis energy effectively stored? These are some of the research questions on which ultrafast, coherent X-rays play a critical role. This is because X-rays can pass through the structure of cells, facilitating study of how they operate, e.g. four-dimensional X-ray microscope.
The first harmonic HHG, coherent X-ray, was discovered in 1987, but since its conversion efficiency was relatively low, its practical applicability was also limited. Scientists have been trying to find a way to increase the conversion efficiency of the coherent X-ray for three decades.
In addition to Chen’s research team, the project included researchers from the University of Colorado at Boulder; the University of Salamanca, Spain; Cornell University; Temple University; and the Lawrence Livermore National Laboratory in California.
Typically, scientists used mid-IR lasers for HHG, the up-conversion efficiency is about 10-6–10-9. The present research used an ultraviolet (270 nm) laser light source to generate HHG, and found a maximum conversion efficiency of 10-3, thereby greatly enhancing the coherent X-ray flux, leading to more research applications of X-rays.
Currently, Prof. Chen’s research team is focusing its resources on laser development, student training and research on new ultrafast nano-microscopy technology. His future plans include developing research projects to enhance Taiwan’s international visibility in ultrafast lasers and table-top coherent X-ray technology. He also hopes to make Taiwan a world leader in research and development in photonics technology.
 Prof. Chen Ming-chang (center) with his research associates at the University of Colorado at Boulder, Dr. Tenio Popmintchev (right) and Dr. Dimitar Popmintchev (left).

Prof. Chen Ming-chang (center) with his research associates at the University of Colorado at Boulder, Dr. Tenio Popmintchev (right) and Dr. Dimitar Popmintchev (left).


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