Molecular motors driven by laser pulses: Role of molecular chirality and photon helicity

Abstract

The results of a theoretical study on molecular motors driven by laser pulses are presented. The roles of molecular chirality and photon helicity in determination of their unidirectional rotation are clarified. An expression for an instantaneous angular momentum of motors driven by lasers in the density matrix formalism was derived. Assuming randomly oriented molecular motors, the initial distribution-averaged instantaneous angular momentum in the dipole approximation was obtained. Taking into account parity inversion symmetry of molecular motors in the averaged instantaneous angular momentum, it is shown that the directions of the averaged instantaneous angular momentum of (R)- and (S)-chiral molecular motors are opposite, but that the magnitudes are the same. This is independent of polarization of laser fields. That is, the chiral motors driven by a linearly polarized optical field creates a unidirectional motion in a molecular fixed frame. On the other hand, the direction of rotation in the laboratory fixed frame is decided by a circularly polarized laser regardless of its molecular chirality. A simple example of real chiral molecular motors is used to demonstrate the interplay of molecular chirality and photon helicity in determination of their unidirectional rotation. The internal rotation of the CHO group plays the role of the engine of the motor. The time evolution of the rotational wave packets of the molecular motors driven by linearly or circularly polarized laser pulses was numerically evaluated and the dynamical behaviors were analyzed. Effects of temperature on the instantaneous angular momentum of the molecular motors are presented as well.