Resonant and first-order dipolar interactions between ultracold molecules in static and microwave electric fields

TL;DR

This paper provides a theoretical analysis of ultracold polar molecule collisions under static and microwave electric fields, revealing how different field conditions influence dipolar interactions and collision losses.

Contribution

It systematically compares first-order and resonant dipolar interactions in ultracold molecules under various electromagnetic field conditions, highlighting the dominance of resonant interactions in microwave dressing.

Findings

Ground state molecules follow first-order dipolar interactions.

Microwave dressing leads to resonant dipolar collisions.

Resonant interactions cause structured losses and microwave shielding.

Abstract

We theoretically study collisions between ultracold polar molecules that are polarized by microwave or static electric fields. We systematically study the dependence on field strength, microwave polarization, and detuning from rotational transitions. We calculate the loss in two-body collisions that is observable experimentally and compare to the results expected for purely first-order dipolar interactions. For ground state molecules polarized by a static electric field, the dynamics are accurately described by first-order dipolar interactions. For microwave dressing, instead, resonant dipolar collisions dominate the collision process, in which molecules reorient along the intermolecular axis and interact with the full strength of the transition dipole. For red detuning, reorientation can only be suppressed at extreme Rabi frequencies. For blue detuned microwaves, resonant dipolar interactions dominate even for high Rabi frequencies, leading to microwave shielding for circular polarization and structured losses due to resonances for linear polarization. The results are presented numerically for fermionic $^{23}$Na$^{40}$K and bosonic $^{23}$Na$^{39}$K molecules.