Phase-controlled transparent superluminal light propagation in a Doppler-broadened four-level N-type system

TL;DR

This paper demonstrates phase-controlled, absorption-free superluminal light propagation in a Doppler-broadened four-level quantum system, showing how group velocity can be tuned from subluminal to superluminal without incoherent fields.

Contribution

It introduces a method to control light speed in a four-level system via phase adjustments, even with Doppler broadening, without using incoherent laser fields.

Findings

Group velocity controlled from subluminal to superluminal by phase adjustment.

Doppler broadening does not prevent superluminal propagation with proper laser directions.

Nonlinear susceptibilities and phase shifts are characterized for the system.

Abstract

The propagation of a weak probe field in a four-level N-type quantum system in the presence of spontaneously generated coherence (SGC) is theoretically investigated. The optical properties of the system are studied and it is shown that the group velocity of light pulse can be controlled by relative phase of applied fields. By changing the relative phase of applied fields, the group velocity of light pulse changes from transparent subluminal to the transparent superluminal light propagation. Thus, the phase-controlled absorption-free superluminal light propagation is obtained without applying an incoherent laser fields to the system. The propagation of a weak probe light pulse is studied by solving the Maxwell's wave equation on numerical grid in space and time. Moreover, we study the third order self- and cross-Kerr susceptibility of probe field and calculate the nonlinear cross-phase shift for different values of intensity of applied fields. In addition, we take into account the effect of Doppler broadening on the light pulse propagation and it is found that a suitable choice of laser propagation directions allows us to preserve our results even in the presence of Doppler effect. It is demonstrated that by increasing the Doppler width of distribution to the room temperature, the dispersion changes from transparent subluminal to transparent superluminal light propagation which is our major motivation for this work.