ECAMP 15

Enantiomer-selective methods - the separation of non-superimposable mirror-image chiral molecules - is critical for many industries relying on chiral purity. Traditional methods like circular dichroism exploit the spin angular momentum of light but their sensitivity is limited by the inherent molecular properties. Orbital angular momentum (OAM) in structured light offers a promising alternative through helical dichroism (HD), hypothesized to arise from electric-dipole–electric-quadrupole interactions. We present a rigid theoretical framework for the analysis of HD, based on molecular symmetries and rotational selection rules [1]. Using our recently developed model of molecule-light interaction Hamiltonian [2] we analyse how a chiral molecule interacts with OAM-carrying light and elucidate the microscopic mechanisms of OAM transfer to the molecular rotation. Going beyond the conventional assumptions on HD, we uncover that paraxial OAM-carrying beams, such as Laguerre-Gaussian modes, cannot transfer OAM to molecular rotations. In fact, HD only emerges from spin-orbit coupled light, such as tightly focused beams. Strikingly, even beams without OAM away from the focus, induce enantiomer-sensitive absorption profiles that can be attributed to HD. Our findings redefine the conditions for observing HD, motivating the reassessment of prior experiments and guiding future designs for chiral sensing using structured light.

[1] M. Hrast, G. M. Koutentakis, M. Maslov, and M. Lemeshko, Bottom-up Analysis of Ro-Vibrational Helical Dichroism, (Manuscript in preparation).
[2] M. Maslov, G. M. Koutentakis, M. Hrast, O. H. Heckl, and M. Lemeshko, Theory of angular momentum transfer from light to molecules, Phys. Rev. Res. 6, 033277 (2024).