Stimulated Raman adiabatic passage into continuum

TL;DR

This paper introduces a STIRAP-based technique for nearly complete ionization of a discrete state into a continuum, enhanced by a laser-induced continuum structure to overcome spontaneous emission losses.

Contribution

It presents a novel method combining STIRAP with laser-induced continuum structures to improve ionization efficiency into a continuum.

Findings

Counterintuitive pulse ordering enhances ionization signal.

Embedding a discrete state into the continuum creates coherence, enabling STIRAP-like transfer.

The method significantly reduces population loss due to spontaneous emission.

Abstract

We propose a technique which produces nearly complete ionization of the population of a discrete state coupled to a continuum by a two-photon transition via a lossy intermediate state whose lifetime is much shorter than the interaction duration. We show that using counterintuitively ordered pulses, as in stimulated Raman adiabatic passage (STIRAP), wherein the pulse coupling the intermediate state to the continuum precedes and partly overlaps the pulse coupling the initial and intermediate states, greatly increases the ionization signal and strongly reduces the population loss due to spontaneous emission through the lossy state. For strong spontaneous emission from that state, however, the ionization is never complete because the dark state required for STIRAP does not exist. We demonstrate that this drawback can be eliminated almost completely by creating a laser-induced continuum structure (LICS) by embedding a third discrete state into the continuum with a third control laser. This LICS introduces some coherence into the continuum, which enables a STIRAP-like population transfer into the continuum. A highly accurate analytic description is developed and numerical results are presented for Gaussian pulse shapes.