If a molecule is subject to strong laser fields, its symmetry is modified compared to the field-free case. One goal of this project is to extend the concept of molecular symmetry groups, a specific approach to describe the symmetry of molecules, to the case of strong field control. We show that the symmetry of molecules with observable torsions is reduced subject to strong laser fields and derive the selection rules the molecules have to then obey. Eventually, we will study how the symmetry reduction is reflected in the quantum dynamics of molecules, considering in particular the example of torsional control with (plasmon-enhanced) laser fields.
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We developed a design for a class of molecular rotors fixed to a silicon surface, induced by a moderately intense, linearly-polarized laser pulse. The rotor consists of an organic molecule possessing a polarizable head group that is attached via a linear component to the surface. The polarization direction in parallel to the surface plane is determined so as to maximize the torque experienced by the molecular head group and hence the duration of the ensuing rotation, while also controlling the sense of rotation. We find that the molecule continues to rotate for many rotational periods after the laser pulse turns off, before multiple scattering by the potential barrier result in dephasing. Such rotors can be realized in the laboratory using the TERS junction discussed here , where a picosecond laser pulse triggers the motion while the tip serves to enhance the incident field and spatially localize it as well as to image the (averaged) effect of the rotation.

We propose a methodology to design Wigner representations in phase spaces with nontrivial topology. Our quantization scheme can be viewed as an alternative to the Stratonovich-Weyl correspondence principle and is aimed at finding the most convenient and computationally friendly form of the equations of motion. We successfully applied the general approach to quantization of the rotations of asymmetric tops and also refined the physical meaning of the Schwinger’s oscillator model. Perhaps most interestingly, we proved that the dynamics of the quantum symmetric top is guided by classical-like equations of motion, similar to the cases of the free particle and the harmonic oscillator. This last proof carries both important fundamental content and potential to simplify numerical methods.
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An intense low frequency laser pulse can tunnel ionize atoms and molecules and drive the freed electrons in the direction of the alternating laser field. As the accelerated electrons reverse their direction and revisit the core, they may recombine with the ion core whereby their kinetic energy is converted into radiation at high harmonics (HH) of the incident frequency. One exciting feature of these high order harmonics is that they are emitted as a train of ultrashort (tens of attoseconds) duration, hence a source of ultrashort laser pulses. A second useful feature is that the emission is typically in the XUV or X-ray domain, providing, potentially table-top sources of extremely high frequency radiation. Perhaps most interestingly, it can be shown the HH contain detailed information about the electronic structure of the emitting molecule. In order to extract this information, it is necessary to align the molecule. In earlier research our group has developed an approach to align molecules using moderately-intense laser pulses. One of the attractive feature of the approach is that it can generate field free alignment after turn-off of the laser pulse. This approach was now demonstrated in many laboratories and was combined with HH generation to translate the emitted harmonics into structural information. Our most recent and ongoing research on the problem of HH generated by aligned molecules focuses on four projects: (1) Development of a classical method to compute the electronic dynamics and its combination with a quantum descriptions of the rotational dynamics to compute HH signals from large polyatomic molecules. This approach is applied to explore the way in which structural details of polyatomic molecules are reflected in HH spectra. (2) Development of a time-dependent density functional theory to study the electronic dynamics and its application in collaborative research with an experimental group. (3) Theoretical and numerical study of the ellipticity measured in HH in different instances and its fundamental origin. (4) Study of the correlation between rotational moments and electronic angular momentum transitions in the HHG process.
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