Development of Picosecond Time-Resolved Microbeam X-ray Diffraction Technique for Investigation of Optical Recording Process

A time-resolved structural investigation of the data storage process in a digital versatile disk (DVD) or Blu-ray disk media is desired for better understanding of the fast phase-change mechanism for designing a higher performance optical recording material. Thus, the development of an in situ structural observation technique at the picosecond level has been carried out for the investigation of the fast phase-change phenomena using synchrotron radiation pulsed X-rays and synchronized femtosecond laser irradiation. Then, for Ge2Sb2Te5 (GST) and Ag3.5In3.8Sb75.0Te17.7 (AIST), the technique has been applied to a snapshot of X-ray diffraction pattern in the nanosecond crystallization process from the amorphous to the crystal phase, which corresponds to the erasing process of the DVD recording system. The key to success of the time-resolved experiment was to optimize the measurement timing system efficiency for both sufficient counting statistics, i.e., counting quantity and precision of time resolution, i.e., counting quality. Our time-resolved experimental system has sufficient accuracy, so that our previous study showed a significant difference in the crystal growth process between GST and AIST, indicating that the crystal growth process is key to fast phase change. To increase the precision of data in terms of signal-to-noise ratio and counting statistics, we developed a highly repetitive pump-probe measurement system combined with an X-ray microbeam technique providing high counting quantity, and used it to demonstrate a snapshot of an X-ray diffraction profile of GST. The measurement system has a time resolution of 50 ps and a 1 kHz repetition rate. Our development of the technique to take a snapshot for the structural visualization of phase-change phenomena with the time-resolved microbeam X-ray diffraction is described.