  Recent advancements in pulse laser technology have facilitated the
exploration of non-equilibrium spectroscopy of electronic states in the
presence of strong electric fields across a broad range of photon energies. The
Keldysh crossover serves as an indicator that distinguishes between excitations
resulting from photon absorption triggered by near-infrared multicycle pulses
and those arising from quantum tunneling induced by terahertz pulses. Using
time-dependent density-matrix renormalization group, we investigate the
emergence of the Keldysh crossover in a one-dimensional (1D) Mott insulator. We
find that the Drude weight is proportional to photo-doped doublon density when
a pump pulse induces photon absorption. In contrast, the Drude weight is
suppressed when a terahertz pulse introduces doublons and holons via quantum
tunneling. The suppressed Drude weight accompanies glassy dynamics with
suppressed diffusion, which is a consequence of strong correlations and
exhibits finite polarization decaying slowly after pulse irradiation. In the
quantum tunneling region, entanglement entropy slowly grows logarithmically.
These contrasting behaviors between the photon-absorption and quantum tunneling
regions are a manifestation of the Keldysh crossover in 1D Mott insulators and
provide a novel methodology for designing the localization and symmetry of
electronic states called subcycle-pulse engineering.
