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Two Negatives Make a Positive: Major Breakthrough in EUV Technology at NTHU
Recently a major breakthrough in optoelectronics research at NTHU has solved two problems which have been a bottleneck for over a decade. As part of a multinational project, a research team led by Prof. Chen Ming-Chang of NTHU’s Institute of Photonics Technology (IPT) has produced a form of extreme ultraviolet (EUV) light which is very small but very bright, making it possible to observe matter at the nanometer scale and to increase the speed of nanoscale research in the semiconductor and biomedical industries. This innovative breakthrough was recently published in the prestigious journal Optica.
 
According to Chen, "Human beings live in a world of seconds, but in the nanometer world time is calculated in attoseconds (10-18 seconds), so if you want to see how electrons, atoms, molecules, and materials work in the nanometer world, you need a faster and brighter light source." Chen further explains that the spatial resolution of imaging is determined by the wavelength of illumination light, so that the shorter the wavelength, the smaller the structures that can be seen. Thus over the last 20 years researchers have been trying to develop the technology for using high-order harmonic generation (HHG) to produce table-top ultrashort EUV pulses which obtains temporal and spatial resolutions down to attoseconds and nanometers, respectively.
 
But over the past decade, scientists were unable to effectively enhance the brightness of 18 nm ultraviolet light, due to the "phase mismatch" problem. Because infrared and extreme ultraviolet light run at different phase velocities, resulting in low up-conversion yield of EUV beam. Chen’s solution was to use a small iris to change the photon phase, resulting in a brighter 18 nm attosecond EUV.
 
His PhD student, Huang Pei-Chi, gives one simple example, explaining the “phase mismatch” problem. The way that light propagate is very similar to the walk way of human being. The walking speed for a child should be slower than that of an adult. He compares the phase mismatch to a three-legged race in which the infrared ray (the adult) and UV light (the child) team up, but due to their differing step length they can’t walk in tandem to reach the finish line.
 
Taiwan has a National Synchrotron Radiation Research Center that can provide EUV light for research, but the light-emitting device is as large as 2 to 3 sports fields, making it difficult to be accessible. Thus when Chen moved his research career to NTHU in 2013, he spent the first nine months designing and developing a HHG source. The main purpose of “table-top EUV device" is to probe ultrafast dynamics in nano-world.
 
Three years ago, while Chen and his master’s student Sun Hung-Wei and his Ph.D. student Huang Pei-Chi were conducting a HHG, they aligned the laser beam through an simple iris and found accidently that by adjusting the aperture size of the iris they could precisely control the EUV’s bandwidth and central wavelength. This was the breakthrough everyone had been looking for, since the simple iris actually increased the up-conversion yield of 18 nm more than one order of magnitude. Chen says that intense infrared ray basically first induces defocusing effect spatially, decreasing the laser peak intensity (the first negative effect for HHG) and then ruins the phase matching condition of HHG generation temporally (the second negative effect for HHG). Surprisingly, a proper defocusing effect of infrared ray which can be precisely controlled by the iris, actually helps compensate the phase mismatch between the infrared ray and the converted EUV light. In short, like chemistry, mixing these two negative elements actually produces bright HHG, while a simple iris acts as an essential catalyst helping this mixing effect.
 
Also using the analogy of the three-legged race, Huang Pei-Chi says that if you want to increase the brightness of EUV light, you have to invite more infrared ray (adults) to participate in this game. Definitely, doing so makes the game more difficult. And that’s where the iris comes in, since it acts as “coach” shouting out directions so that the infrared ray and EUV (adults and children) coordinate their footsteps, thereby solving the problem of phase matching and increasing the brightness of HHG. Significantly, comparing to Synchrotron, the size of Chen’s light source system is only about two meters long and more accessible to users.
 
EUV has been widely used in scientific research in such areas as materials, electronics, biology, medicine, physics, chemistry, chemical engineering, geology, archeology, energy, environmental protection, and micro-mechanics. Especially, EUV has been the key technology for the next generation high-volume manufacturing of semiconductor devices. The table-top EUV light source will definitely make great impacts in industries.
 

Prof. Chen Ming-Chang, Institute of Photonics Technology, NTHU.

Prof. Chen Ming-Chang, Institute of Photonics Technology, NTHU.


Chen and his Ph.D. student Huang Pei-Chi demonstrating the EUV system.

Chen and his Ph.D. student Huang Pei-Chi demonstrating the EUV system.


Ph.D. student Huang Pei-Chi demonstrating how to boost the up-conversion efficiency of HHG.

Ph.D. student Huang Pei-Chi demonstrating how to boost the up-conversion efficiency of HHG.

 

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