Phase matching alters spatial multiphoton processes in dense atomic ensembles

TL;DR

This paper investigates how phase matching, influenced by the spatial extent of dense atomic vapors, significantly impacts multiphoton processes like four-wave mixing, leading to improved efficiency and control.

Contribution

The study introduces a simple theoretical model that accounts for spatial properties and phase matching effects in dense atomic vapors, validated by experiments.

Findings

Phase matching critically affects multiphoton process efficiency.

The model accurately predicts frequency shifts and beam directions.

Experimental optimization improves four-wave mixing efficiency.

Abstract

Multiphoton processes in dense atomic vapors such as four-wave mixing or coherent blue light generation are typically viewed from single-atom perspective. Here we study the surprisingly important effect of phase matching near two-photon resonances that arises due to spatial extent of the atomic medium within which the multiphoton process occurs. The non-unit refractive index of the atomic vapor may inhibit generation of light in nonlinear processes, significantly shift the efficiency maxima in frequencies and redirect emitted beam. We present these effects on an example of four-wave mixing in dense rubidium vapors in a double-ladder configuration. By deriving a simple theory that takes into account essential spatial properties of the process, we give precise predictions and confirm their validity in the experiment. The model allows us to improve on the geometry of the experiment and engineer more efficient four-wave mixing.