Research Interest

Interaction of ultrashort laser pulses with matter - theory and simulation
(Atomic and molecular dynamics in ultrashort intense laser pulses)

Everybody knows that electrons are moving around the nucleus inside an atom. Then, who has ever observed or photographed such a motion? Nobody! Because there exists no sufficiently ultrashort flash. Now, state-of-the-art femtosecond laser technology as well as intense ultrashort extreme ultraviolet (XUV) and soft-x-ray pulse sources such as high-order harmonics (HH) and X-ray free electron lasers (XFEL) are expected to open a way to investigate such ultrafast dynamics in the attosecond (10-18 second) range. This is not only interesting as fundamental science but also may serve as a basis of advanced optical technology such as control of electronic, atomic and molecular processes, that of chemical reactions, and real-time biomolecular imaging.

We study the interaction of ultrashot laser and soft-x-ray pulses with atoms and molecules, where nonlinear optical effects and electron-electron interaction play an essential role. We also investigate novel methods to generate and measure ultrashort (attosecond) XUV and soft-x-ray pulses, mainly based on high-order harmonic generation (HHG, phenomenon in which laser light is converted to light with integer multiples of the laser frequency). The best way to study atomic and molecular dynamics under such circumstances theoretically is direct numerical solution of the time-dependent Schr?dinger equation (TDSE). We apply this method to various kinds of atoms and molecules to study, for example, above-threshold ionization (ionization process in which photoelectrons absorb further photons than the minimum necessary), high-order harmonic generation and two-photon double ionization.

Our grand goal in emerging attoscience is to observe, control and manipulate electronic motion inside atoms and electrons at will. This will greatly contribute to technological innovation such as creation of unique atomic and molecular states, and the control of chemical reactions and intra-atomic electron dynamics with the attosecond time scale.

Recent and current research topics:

  • Single attosecond pulse generation using a seed harmonic pulse train
  • Optical-field ionization by femtosecond laser pulses of time-dependent polarization
  • Temporal Young's interference experiment by attosecond double and triple soft-x-ray pulses (Fig. 1) [Phys. Rev. A 74, 023806 (2006)]
  • Above-threshold double ionization of helium with attosecond intense soft-x-ray pulses [Phys. Rev. A 72, 013407 (2005)]
  • Dramatic enhancement of high-order hamonic generation [Phys. Rev. Lett. 91, 043002 (2003)]
Fig. 1 Attosecond triple-slit experiment using soft-x-ray pulses.

Computer design of subwavelength diffractive optical elements using the finite-difference time-domain (FDTD) method

Diffractive optical elements (DOEs) are devices that can be used for precision laser beam control and shaping. With recent progress of nanotechnology, it is possible to fabricate DOEs with structures finer than optical wavelengths, namely, subwavelength structures (SWSs). We investigate the behavior of laser beams propagating in subwavelength DOEs using the FDTD simulation method and design novel devices with previously unattainable functions.

Recent and current research topics:

  • Multi-functional subwavelength (SW) DOEs
  • Design of SW-DOEs using genetic algorithm
  • Unconditionally stable Crank-Nicholson FDTD method
Fig.2 Complex focusing behavior of a laser beam incident from the left to a SW-DOE
Photon Science Center, School of Engineering, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
[e-mail] ishiken[at] (replace "[at]" by @)