Nonlinear Effects in Pulse Propagation through Doppler-Broadened Closed-Loop Atomic Media

TL;DR

This paper theoretically investigates nonlinear pulse propagation in a four-level atomic system with a closed-loop configuration, revealing strong intensity-dependent refractive index effects with minimal absorption, even considering Doppler and pressure broadening.

Contribution

It introduces a detailed time-dependent analysis of nonlinear effects in closed-loop atomic media, including realistic broadening mechanisms, and demonstrates significant self-phase modulation with low absorption.

Findings

Strong intensity-dependent refractive index observed

Self-phase modulation of π achieved over a few centimeters

Minimal absorption in the nonlinear spectral range

Abstract

Nonlinear effects in pulse propagation through a medium consisting of four-level double-$\Lambda$-type systems are studied theoretically. We apply three continous-wave driving fields and a pulsed probe field such that they form a closed interaction loop. Due to the closed loop and the finite frequency width of the probe pulses the multiphoton resonance condition cannot be fulfilled, such that a time-dependent analysis is required. By identifying the different underlying physical processes we determine the parts of the solution relevant to calculate the linear and nonlinear response of the system. We find that the system can exhibit a strong intensity dependent refractive index with small absorption over a range of several natural linewidths. For a realistic example we include Doppler and pressure broadening and calculate the nonlinear selfphase modulation in a gas cell with Sodium vapor and Argon buffer gas. We find that a selfphase modulation of $\pi$ is achieved after a propagation of few centimeters through the medium while the absorption in the corresponding spectral range is small.