  An all-optical method of ultrafast spin rotation is put forward to precisely
manipulate the polarization of relativistic charged particle beams of leptons
or ions. In particular, laser-driven dense ultrashort beams are manipulated via
single-shot interaction with a co-propagating moderate temporally asymmetric
(frequency-chirped or subcycle THz) laser pulse. Using semi-classical numerical
simulations, we find that in a temporally asymmetrical laser field, the spin
rotation of a particle can be determined from the flexibly controllable phase
retardation between its spin precession and momentum oscillation. An initial
polarization of a proton beam can be rotated to any desired orientation (e.g.,
from the common transverse to the more useful longitudinal polarization) with
extraordinary precision (better than 1\%) in tens of femtoseconds using a
feasible frequency-chirped laser pulse. Moreover, the beam qualities, in terms
of energy and angular divergence, can be significantly improved in the rotation
process. This method has potential applications in various areas involving
ultrafast spin manipulation, like laser-plasma, laser-nuclear and high-energy
particle physics.
