Optical
pumping of solids creates a nonequilibrium electronic structure where
electrons and photons combine to form quasiparticles of dressed electronic
states. The resulting shift of electronic levels is known as the optical
Stark effect, visible as a red shift in the optical spectrum. Here
we show that in a pump–probe setup we can uniquely define a
nonequilibrium quasiparticle bandstructure that can be directly measurable
with photoelectron spectroscopy. The dynamical photon-dressing (and
undressing) of the many-body electronic states can be monitored by
pump–probe time and angular-resolved photoelectron spectroscopy
(tr-ARPES) as the photon-dressed bandstructure evolves in time depending
on the pump–probe pulse overlap. The computed tr-ARPES spectrum
agrees perfectly with the quasi-energy spectrum of Floquet theory
at maximum overlap and goes to the equilibrium bandstructure as the
pump–probe overlap goes to zero. Additionally, we show how
this time-dependent nonequilibrium quasiparticle structure can be
understood to be the bandstructure underlying the optical Stark effect.
The extension to spin-resolved ARPES can be used to predict asymmetric
dichroic response linked to the valley selective optical excitations
in monolayer transition metal dichalcogenides (TMDs). These results
establish the photon dressed nonequilibrium bandstructures as the
underlying quasiparticle structure of light-driven steady-state quantum
phases of matter
