  We applied the time-resolved attosecond transient absorption spectroscopy to
systematically investigate ultrafast optical responses of condensed matter
systems. Under an intense pump pulse, absorption spectra indicate that the
non-interacting electrons of band insulators produce a field-induced redshift,
known as the dynamical Franz-Keldysh effect, as commonly expected. In contrast
to the band insulators, in Mott insulators, unconventional spectra are observed
which do not fully reflect the dynamical Franz-Keldysh effect. While it still
exhibits the fishbone-like structures mimicking the dynamical Franz-Keldysh
effect, the spectra show a negative difference absorption below the band edge,
rendering a blueshift. In addition, the decomposed difference absorption
reveals the creation and the annihilation of double occupancy mainly contribute
to the negative signal, implying that the unconventional spectra are purely
driven by the electron-correlations. These demonstrations of unconventional
responses would guide us to the correlation-embedded attosecond electron
dynamics in condensed matter systems.
