  A theory for light-induced current by strong optical pulses in
molecular-tunneling junctions is described. We consider a molecular bridge
represented by its highest occupied and lowest unoccupied levels, HOMO and
LUMO, respectively. We take into account two types of couplings between the
molecule and the metal leads: electron transfer that gives rise to net current
in the biased junction and energy transfer between the molecule and
electron-hole excitations in the leads. Using a Markovian approximation, we
derive a closed system of equations for the expectation values of the relevant
variables: populations and molecular polarization that are binary, and exciton
populations that are tetradic in the annihilation and creation operators for
electrons in the molecular states. We have proposed an optical control method
using chirped pulses for enhancing charge transfer in unbiased junctions where
the bridging molecule is characterized by a strong charge-transfer transition.
An approximate analytical solution of the resulting dynamical equation is
supported by a full numerical solution. When energy transfer between the
molecule and electron-hole excitations in the leads is absent, the optical
control problem for inducing charge transfer with linearly chirped pulse can be
reduced to the Landau-Zener transition to a decaying level. When chirp is fast
with respect to the rate of the electron transfer, the Landau theory is
recovered. The proposed control mechanism is potentially useful for developing
novel opto-electronic single-electron devices with optical gating based on
molecular nanojunctions.
