Extreme Photonics
Kazutaka Nakamura, Tokyo Institute of Technology

Light has been used for seeing or probing matter in a long history of science. Laser, which is coherent light source, is now used in many places in science and technology. Recent development of laser technology enables to generate intense (up to 1015 W) and ultrashort (picosecond (10-12 s) to femtosecond (10-15 s)) pulses. The interaction between intense femtosecond laser and matter has recently made possible to generate short-pulsed light beam with very short (0.1 nm: X-rays) and long (100 µm: THz radiation) wavelength, which have yet been accessible. The new light source has opened an entirely new domain for measurement in physics, chemistry and biology. From recent pioneering studies, it has been revealed that the dynamics behavior of electrons, atoms, and molecules could directly be monitored and controlled.

Soft X-rays have been generated by focusing femtosecond laser on atoms and molecules as higher harmonic generation. The pulse width of the soft X-rays reached to attosecond (10-18 s) regime and they are expected to be used for monitor electron dynamics. Harr X-rays have been generated by using laser induced plasma and used for ultrafast time-resolved X-ray diffraction in order to study structural dynamics of matter under phase transition. This technique is applicable for monitoring atomic motion in photo-induced phenomena in biology and in folding of proteins.

When the laser beam, which pulse width is shorter than vibrational period of crystals, is irradiated on the crystal, coherent motion of lattice vibration (coherent phonon) is induced. The frequency of the vibration is in a THz range, which corresponds to a vibrational period in picosecond rage. The coherent phonon has been recently detected directly by using femtosecond time-resolved X-ray diffraction. THz radiation is simultaneously generated by coherent vibrational motions induced by femtosecond laser irradiation. The THz wave has a spectral region between microwave and infrared and can penetrate many materials such as semiconductors, plastics, textiles and bone. It is expected to use for probing physical properties of matter in fundamental science and sensing in industry.
In this session, recent topics of research using X-rays and THz radiation are presented by Prof. Rose-Petruck and Prof. Kawase, respectively.

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