We aim to observe and manipulate ultrafast electron motion using attosecond (10^-18 s) laser pulses, which is shortest optical pulse in the world. In an atom, electrons are classified into outer shell (valance-shell) and inner shell (core-shell) types. In common devices, the outer shell with a low energy band gap (a few electronvolts) is used. However, since the inner shell has a larger band gap (a few orders magnitude higher than the outer shell), the electron motion is much faster. For instance, the decay time of an excited inner-shell electron is on a scale from a few attoseconds to several hundred femtoseconds (that of an outer-shell electron is on the scale of nanosecond).

We successfully observed electron motion with the dipole response in the inner shell using the combination of an IAP (isolated attosecond pulse) and SPIDER (spectral phase interferometry for direct electric field reconstruction) method [1]. We generated the IAP with 192-as duration –one of the shortest laser pulses in the world− and this IAP can approach the time scale of the electron motion. Figure 1 shows the dipole response in neon atom (2s-3p states) induced by the IAP. The dipole response produces the coherent photoemission, which is interfered with the IAP and detected by a spectrometer. The SPIDER method can fully characterize the dipole response (the decay time, dipole phase, and periodicity of dipole oscillation). The decay time indicates 35 fs (10^-15 s), which is million times shorter than general outer-shell decay time. In addition, the periodicity of dipole oscillation corresponds to 90 as. Principally, the periodicity produces 10 PHz (10¹⁵ Hz) responses for the optical device. The achievement for inner shell with ultrafast motion may pave the way for the development of new types of ultrafast devices.