Topological charges of periodically kicked molecules

TL;DR

This paper demonstrates that simple diatomic molecules subjected to periodic laser pulses can host topological charges in their rotational states, enabling the study of topological physics in gas-phase molecular systems.

Contribution

It introduces a novel approach to realize topological charges in molecular rotational states using periodic laser driving, mapping the problem onto a lattice in angular momentum space.

Findings

Discovery of Dirac cones with topological charges in molecular rotational spectra.

Topologically protected edge states can be controlled by laser parameters.

Experimental observability through molecular alignment and rotational level populations.

Abstract

We show that the simplest of existing molecules -- closed-shell diatomics not interacting with one another -- host topological charges when driven by periodic far-off-resonant laser pulses. A periodically kicked molecular rotor can be mapped onto a ''crystalline'' lattice in angular momentum space. This allows to define quasimomenta and the band structure in the Floquet representation, by analogy with the Bloch waves of solid-state physics. Applying laser pulses spaced by $1/3$ of the molecular rotational period creates a lattice with three atoms per unit cell with staggered hopping. Within the synthetic dimension of the laser strength, we discover Dirac cones with topological charges. These Dirac cones, topologically protected by reflection and time-reversal symmetry, are reminiscent of (although not equivalent to) that seen in graphene. They -- and the corresponding edge states -- are broadly tunable by adjusting the laser strength and can be observed in present-day experiments by measuring molecular alignment and populations of rotational levels. This paves the way to study controllable topological physics in gas-phase experiments with small molecules as well as to classify dynamical molecular states by their topological invariants.