Strong-field dipole resonance. I. Limiting analytical cases

TL;DR

This paper analyzes the population dynamics of N-level dipolar systems under strong fields, identifying limiting cases where solutions simplify and revealing how field variation speed influences population transfer and resonance effects.

Contribution

It introduces analytical limiting cases for strong-field dipole resonance in N-level systems, highlighting how field variation speed affects population dynamics and resonance conditions.

Findings

Rapidly varying fields lead to sign-independent population dynamics.

Slowly varying fields enable population transfer optimization via dipole resonance.

Intermediate regimes show mixed behavior with partial sign dependence.

Abstract

We investigate population dynamics in N-level systems driven beyond the linear regime by a strong external field, which couples to the system through an operator with nonzero diagonal elements. As concrete example we consider the case of dipolar molecular systems. We identify limiting cases of the Hamiltonian leading to wavefunctions that can be written in terms of ordinary exponentials, and focus on the limits of slowly and rapidly varying fields of arbitrary strength. For rapidly varying fields we prove for arbitrary $N$ that the population dynamics is independent of the sign of the projection of the field onto the dipole coupling. In the opposite limit of slowly varying fields the population of the target level is optimized by a dipole resonance condition. As a result population transfer is maximized for one sign of the field and suppressed for the other one, so that a switch based on flopping the field polarization can be devised. For significant sign dependence the resonance linewidth with respect to the field strength is small. In the intermediate regime of moderate field variation, the integral of lowest order in the coupling can be rewritten as a sum of terms resembling the two limiting cases, plus correction terms for N>2, so that a less pronounced sign-dependence still exists.