Effects of thermal motion on electromagnetically induced absorption

TL;DR

This paper investigates how thermal motion, buffer-gas collisions, and Doppler effects influence electromagnetically induced absorption (EIA) in a four-level atomic system, providing both exact and approximate models to understand the spectral behavior.

Contribution

The study introduces an approximate solution for the EIA spectrum considering buffer-gas collisions and Doppler effects, offering physical insights into the spectral features and Ramsey narrowing phenomena.

Findings

VCC produce a wide pedestal at the EIA peak base.

Residual Doppler-Dicke effect broadens and weakens the central EIA peak.

Ramsey narrowing occurs for finite-width probe beams.

Abstract

We describe the effect of thermal motion and buffer-gas collisions on a four-level closed N system interacting with strong pump(s) and a weak probe. This is the simplest system that experiences electromagnetically induced absorption (EIA) due to transfer of coherence via spontaneous emission from the excited to ground state. We investigate the influence of Doppler broadening, velocity-changing collisions (VCC), and phase-changing collisions (PCC) with a buffer gas on the EIA spectrum of optically active atoms. In addition to exact expressions, we present an approximate solution for the probe absorption spectrum, which provides physical insight into the behavior of the EIA peak due to VCC, PCC, and wave-vector difference between the pump and probe beams. VCC are shown to produce a wide pedestal at the base of the EIA peak, which is scarcely affected by the pump-probe angular deviation, whereas the sharp central EIA peak becomes weaker and broader due to the residual Doppler-Dicke effect. Using diffusion-like equations for the atomic coherences and populations, we construct a spatial-frequency filter for a spatially structured probe beam and show that Ramsey narrowing of the EIA peak is obtained for beams of finite width.