Optical Imaging of Coherent Molecular Rotors

Abstract

Short laser pulses are widely used for controlling molecular rotational degrees of freedom and inducing molecular alignment, orientation, unidirectional rotation, and other types of coherent rotational motion. To follow the ultrafast rotational dynamics in real time, several techniques for producing molecular movies have been proposed based on the Coulomb explosion of rotating molecules, or recovering molecular orientation from the angular distribution of high harmonics. The present work offers and demonstrates a novel nondestructive optical method for direct visualization and recording of movies of coherent rotational dynamics in a molecular gas. The technique is based on imaging the time-dependent polarization dynamics of a probe light propagating through a gas of coherently rotating molecules. The probe pulse continues through a radial polarizer, and is then recorded by a camera. The technique is illustrated by implementing it with two examples of time-resolved rotational dynamics: alignment-antialignment cycles in a molecular gas excited by a single linearly polarized laser pulse, and unidirectional molecular rotation induced by a pulse with twisted polarization. This method may open new avenues in studies on fast chemical transformation phenomena and ultrafast molecular dynamics caused by strong laser fields of various complexities.

Rotation of polarization of light propagating through a gas of molecular super-rotors

“…Apart from the experimental demonstration of a fundamental physical effect, the presented results have several practical aspects. Polarization drag measurements may be regarded as a new diagnostic method for characterizing molecular rotation and its relaxation, an addition to the continuously expanding toolbox (e.g., [55][56][57][58][59] to name just a few). In particular, the observed nonexponential decay of the drag angle reflects the inhibited relaxation rate of the fast spinning molecular superrotors and depicts their gradual thermalization over a long time-scale.…”

Visualizing coherent molecular rotation in a gaseous medium

“…In the present work, the PAF is composed of a vortex plate acting as a radial polarizer [25][26][27][28][29] and a linear polarizer. Recently, a similar PAF was successfully applied to imaging of coherent molecular rotors [20]. Qualitatively, given a circularly polarized light at the input plane, the electric field at an output plane can be determined with the help of Jones calculus [30].…”

“…Here, we present a theoretical analysis of the recently demonstrated [20] purely optical approach allowing detecting the instantaneous orientation of the principal optical axes of laser-excited molecular gas. The approach relies on ultrafast birefringence measurement using delayed femtosecond probe pulses.…”

Manipulation of Rotational States With Varying Pulse Parameter: Achieving High Molecular Alignment and Orientation for CH3F

“…Alignment dynamics of different systems with varying individual pulse parameter such as, effect of pulse intensity was studied for the iodobenzene molecule by Lotte Holmegaard et al 16 , and for the O 2 and N 2 molecules by P. Peng 17 . Jérémy Bert et al 18 illustrated the time-resolved rotational dynamics in CO 2 gas after excitation with a single linearly polarized laser pulse, and unidirectional molecular rotation induced by a pulse with twisted polarization. Kenta Mizuse et al 19 reported high-precision time-resolved Coulomb explosion imaging of the rotational wave packets induced by a polarization-skewed double-pulse to investigate the creation process and dynamics of the packets in N 2 molecules.…”