Quasi-bound states in periodically driven scattering

TL;DR

This paper introduces a method to analyze quasi-bound states in periodically driven quantum systems, revealing how external driving influences wavefunctions, decay rates, and resonances in scattering problems with mixed symmetries.

Contribution

The authors develop an explicit expansion technique for scattering eigenfunctions under periodic driving, applicable to complex geometries and nonperturbative regimes.

Findings

Eigenfunctions exhibit asymptotic dressing with partial waves.

Decay rates depend nonmonotonically on drive strength.

Strong driving causes resonant interactions between bound states.

Abstract

We present an approach for obtaining eigenfunctions of periodically driven time-dependent Hamiltonians. Assuming an approximate scale separation between two spatial regions where different potentials dominate, we derive an explicit expansion for scattering problems with mixed cylindrical and spherical symmetry, by matching wavefunctions of a periodic linear drive in the exterior region to solutions of an arbitrary interior potential expanded in spherical waves. Using this method we study quasi-bound states of a square-well potential in three dimensions subject to an axial driving force. In the nonperturbative regime we show how eigenfunctions develop an asymptotic dressing of different partial waves, accompanied by large periodic oscillations in the angular momentum and a nonmonotonous dependence of the decay rate on the drive strength. We extend these results to the strong driving regime near a resonant intersection of the quasi-energy surfaces of two bound states of different symmetry. Our approach can be applied to general quantum scattering problems of particles subject to periodic fields.