Amplitude and phase modulations in ultrafast transient absorption induced by a strong probe

Abstract

Transient absorption spectroscopy (TAS) conventionally employs a weak probe pulse to passively interrogate pump-induced coherences in a quantum system. Here, we demonstrate a distinct regime in which an intense probe pulse actively controls and interrogates quantum coherences. Using time-dependent perturbation theory, we derive an analytical expression for the absorption cross section that incorporates probe-induced Rabi and Raman effects. Consequently, the system response becomes explicitly probe-intensity dependent, leading to pronounced modifications of both the absorption amplitude and phase. These features are in excellent agreement with density-matrix calculations. We further validate the theory experimentally using TAS of atomic rubidium, where systematic variations of the probe intensity and pump-probe delay directly reveal these predicted features. Our results establish intense probe pulses as a direct and effective tool for manipulating quantum coherences without auxiliary control fields.