  We study photoelectron angular distributions (PADs) near the ionization
threshold with a newly developed Coulomb quantum-orbit strong-field
approximation (CQSFA) theory. The CQSFA simulations present an excellent
agreement with the result from time-dependent Schr\"odinger equation method. We
show that the low-energy fan-shaped structure in the PADs corresponds to a
subcycle time-resolved holographic structure and stems from the significant
influence of the Coulomb potential on the phase of the forward-scattering
electron trajectories, which affects different momenta and scattering angles
unequally. For the first time, our work provides a direct explanation of how
the fan-shaped structure is formed, based on the quantum interference of direct
and forward-scattered orbits.
