  We introduce a general procedure to extract the wave function of
quasiparticles in AC-driven quantum conductors. By incorporating the
Bloch-Messiah reduction into the scattering theory approach to quantum
transport, we construct the many-body state from the scattering matrix, within
which the wave function of quasiparticles can be extracted. We find that two
kinds of quasiparticles can be excited, while both of them are superpositions
of particle and hole states in the Fermi sea. Due to the electron number
conservation, the quasiparticles are always created in pairs, resulting in
particle-hole pair excitations in quantum conductors. We apply our approach to
compare these particle-hole pairs excited in perfectly transparent quantum
contacts and mesoscopic capacitors with single-level quantum dot. For the
quantum contacts, multiple pairs can be excited, while the excitation
probability of each pair increases as the strength of the driving field
increasing. The particle(hole) components are always in phase (180 degrees out
of phase) with the driving field. These features agree with the results
obtained via the extended Keldysh-Green's function technique. For the
mesoscopic capacitors, only one particle-hole pair can be excited, while the
corresponding excitation probability undergoes an oscillation as the strength
of the driving field increasing. A nonzero phase delay between the
particle(hole) component and the driving field exists, which can be tuned via
the dot-lead coupling in the mesoscopic capacitor. These features are very
distinct from the case of quantum contacts, which can be attributed to the
finite dwell time of electrons in the mesoscopic capacitor. We also find that
our approach can also offer a general way to extract the wave function of
quasiparticles from the Wigner function, making it helpful for the signal
processing of relevant experiments in electron quantum optics.
