P3 Interactions of Molecules with Twisted Light

The team of subproject P3 “Interactions of Molecules with Twisted Light” will develop a theory for interactions of molecules with twisted light based on a novel approach: the effective charge density model. The goal is to advance the fundamental theory of molecules in the presence of electromagnetic fields carrying orbital angular momentum (OAM) and to provide direct theory support required for the successful realization of the P2 subproject.

First, a general theory will be developed in the language of vector spherical harmonics and irreducible tensor operators. Both classical and quantum fields will be considered. The effects of the hyperfine structure and the importance of the Born-Oppenheimer and paraxial approximations will be assessed.

The team of P3 will calculate the transition frequencies and probabilities for CS and OCS molecules interacting with twisted light. With this, optimal protocols to observe transitions with |Dm| > 1 in experiments of P2 can be suggested. The relative magnitudes of the electric dipole (E1), E2, and M1 transitions and the applicability of twisted light for precision measurements will be assessed. As a next step, protocols to align, trap, cool, and focus molecules using off-resonant vortex beams can be developed.

The theory will enable the successful realization of the P2 subproject and thereby pave the way for the precision spectroscopy of small molecules using the light OAM. Furthermore, we aim to advance the fundamental understanding of molecule-field interactions in the presence of OAM of light.

Spectroscopy of small molecules is a mature field with a long history both in experiment and theory. Modeling of molecule–light interactions rests on two pillars: transition selection rules and independence of the internal degrees of freedom (rotational, vibrational, electronic) from molecular translational motion. Both of these assumptions stem from the fact that the spatial variations of the electromagnetic field on molecular length-scales can usually be neglected and it can be treated as a plane wave. While this allows to drastically simplify the theoretical modeling of molecular spectra and experimental assignment of spectroscopic lines, using light with spatial structure such as OAM can potentially offer clear advantages.

This is expected to open new avenues not only in precision spectroscopy, but also in controlling cold molecules and ultracold chemistry. The goal is to establish a general theory framework both for resonant laser fields (with applications to precision spectroscopy) and far-of-resonant fields (with applications for molecular alignment, slowing and trapping).

Apart from providing the theory support for P2, the team of P3 will have regular interactions with the theory group of subproject P6. The expectation is that the description of OAM beams, their interactions with charge distributions and the applicability of various approximations can be done jointly in the context of the molecular and nuclear spectroscopies.

Although there are several papers on the theory side of vortex beams and matter interaction, most of them rely on point-charge models, where the positions of all the nuclei and electrons are explicitly given by radius-vectors. This makes the geometry derivations quite cumbersome, which makes it hard to understand where exactly does the OAM transfer takes place. Therefore, there is a need for effective models relying upon a simplified description of molecular charge density, its change due to rotations and vibrations and its perturbations due to electromagnetic fields of OAM.

Team of P3

The sub-project P3 “Interactions of Molecules with Twisted Light” is led by Mikhail Lemeshko at the Institute of Science and Technology Austria (ISTA) in Klosterneuburg. ISTA is a new PhD-granting institution dedicated to cutting-edge interdisciplinary research, that provides excellent conditions for implementing this subproject. For the realization of this subproject 2 PhD students (3.5 years each) and one postdoc (of 4 years) positions will be filled.

One of the PhDs will focus on developing the general theory of OAM-light—molecule interactions with the main focus on experiments of the Heckl group. The other PhD will focus on interactions of OAM-light far detuned from any resonances with molecules. As opposed to the spectroscopy applications, this direction deals with manipulation of molecules with fields.

The postdoc position will be either held by one person or split in two 2-year positions, in which case the second PD will continue the work of the first one. The main focus of the PD is to co-supervising the PhDs along with the PI and taking a leading role in the theoretical developments of their projects and in the interactions with experimentalists. Apart from direct involvement in the PhD projects, the PD will address riskier and more theoretically involved aspects of OAM-light—matter interactions.