We have achieved researches based on the TDDFT (Time-dependent density- functional theory) for quantum dynamics of fermion many-body systems. We have been developing a real-time and real-space computational method to solve the time-dependent Kohn-Sham equation, the basic equation of the TDDFT, and have been applying it to interactions of light with various many-fermion systems including atomic nuclei, which are bound by strong interaction, and molecules and solids as many-electron systems bound by Coulomb force.

Systematic calculations for optical response of atomic nuclei

The nuclear giant dipole resonance is a collective excitation of proton-neutron relative motion and provides an important clue for the understanding of the nuclear structure. The understanding of the optical absorption of neutron-rich nuclei is also significant to understand the synthesis of elements in the rapid process which takes place in the explosive supernova. We have achieved systematic calculations of the nuclear optical responses from light to medium mass nuclei. The right figure shows the optical response calculation for ⁸⁰Se nucleus. The two peak feature reflects the deformation of the nucleus. In the bottom figure, we show the systematic calculation of the nuclear optical responses up to Ni isotopes.

First-principles calculation for the interaction of high-intensity and ultrashort laser pulse with matter

When the magnitude of the electric field of the applied laser pulse is comparable to the electric field which binds electrons in matters, there occur a variety of phenomena reflecting nonlinear electron dynamics. We have been investigating the electron-ion dynamics by solving the time-dependent Kohn-Sham equation in real-time. As an example of such studies, we show a simulation of optical dielectric breakdown.

The right figure shows the electron dynamics following the radiation of the laser pulse of 1 x 10¹⁵W/cm² intensity and 40 femto second duration. The electric fields are shown in (a), the applied laser pulse (blue line) and the total electric field including polarization (red line). At first stage, two curves are proportional with each other, with the dielectric constant as a proportional coefficient. As the laser intensity increases, the phase difference starts to appear. At the final stage, the two electric fields are out of phase. The electron excitation energy (b) and the number of excited electrons (c) show a rapid increase, in coincidence with the occurrence of the phase difference. We can thus identify the occurrence of the optical breakdown in time domain.

In the right-down figure, energy transfer from the laser pulse to the electrons is shown as a function of the intensity of the laser pulse. Two photon are required to excite electrons across the bandgap, and the calculated result at low intensity is consistent with the two photon absorption. At around the laser intensity of 7x10¹⁴W/cm², a rapid increase of the energy absorption is observed. This is regarded as the theoretical threshold for the optical breakdown.

We will develop a universal computational approach for quantum dynamics of fermion many-body systems which is useful for systems of both atomic nuclei bound by strong interaction and materials bound by Coulombic interaction. In nuclear physics, we examine the accuracy of the energy functionals proposed for atomic nuclei and develop a framework to incorporate the pairing interaction. These efforts will contribute much to explore the properties of unstable nuclei. In material sciences, we develop the first-principles framework describing dynamics of electrons and ions induced by the ultrashort laser pulse, and contribute to elucidate mechanisms of a variety of nonlinear electron dynamics in femto- and atto-second time-scale, such as the high harmonic generation, coherent phonon, and Coulomb explosion.