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Photonics Research

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

Chunyuan Bai, Chun Huang, Zhongyao Yue, Xinhua Xie, Xiao jing Liu, ruixin li, Peng Peng

Published May 12, 2026• 0 citations
Publisher

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.

DOI
10.1364/prj.591630

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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.
Abstract The quantum dynamics of a system of Rb atoms, modeled by a V -type three-level system interacting with intense probe and pump pulses, are studied. The time-delay-dependent transient-absorption spectrum of an intense probe pulse is thus predicted, simulating pump-probe experiments in which this is preceded or followed by a strong pump pulse. Numerical results are interpreted in terms of an analytical model based on interaction operators , which quantify the transformation undergone by the system under the action of an intense pulse. The oscillating features of the resulting transient-absorption spectra, due to the coupling of several excited states, are thus interpreted in terms of the atomic population and phase changes imposed by the pump and probe pulses. Strong-field-induced phases and their influence on the resulting transient-absorption spectra are thereby investigated for different values of pump and probe intensities and frequencies, focusing on the atomic properties which are encoded in the absorption line shapes for positive and negative time delays.
The quantum dynamics of a system of Rb atoms, modeled by a V-type three-level system interacting with intense probe and pump pulses, are studied. The time-delay-dependent transient-absorption spectrum of an intense probe pulse is thus predicted, when this is preceded or followed by a strong pump pulse. Numerical results are interpreted in terms of an analytical model, which allows us to quantify the oscillating features of the resulting transient-absorption spectra in terms of the atomic populations and phases generated by the intense pulses. Strong-field-induced phases and their influence on the resulting transient-absorption spectra are thereby investigated for different values of pump and probe intensities and frequencies, focusing on the atomic properties which are encoded in the absorption line shapes for positive and negative time delays.
The evolution of a V-type three-level system is studied, whose two resonances are coherently excited and coupled by two ultrashort laser pump and probe pulses, separated by a varying time delay. We relate the quantum dynamics of the excited multilevel system to the absorption spectrum of the transmitted probe pulse. In particular, by analyzing the quantum evolution of the system, we interpret how atomic phases are differently encoded in the time-delay-dependent spectral absorption profiles when the pump pulse either precedes or follows the probe pulse. This scheme is experimentally applied to atomic Rb, whose fine-structure-split 5s (2)S{1/2}→5p(2)P{1/2} and 5s(2)S_{1/2}→5p(2)P{3/2} transitions are driven by the combined action of a pump pulse of variable intensity and a delayed probe pulse. The provided understanding of the relationship between quantum phases and absorption spectra represents an important step towards full time-dependent phase reconstruction (quantum holography) of bound-state wave packets in strong-field light-matter interactions with atoms, molecules, and solids.
The transient-absorption spectrum of a VVV-type three-level system is investigated, when this is periodically excited by a train of equally spaced, $\ensuremath{\delta}$-like pump pulses as, e.g., from an optical-frequency-comb laser. We show that, even though the probe pulse is not assumed to be much shorter than the pump pulses, light-induced states appear in the absorption spectrum. The frequency- and time-delay-dependent features of the absorption spectra are investigated as a function of several laser control parameters, such as the number of pump pulses used, their pulse area, and the pulse-to-pulse phase shift. We show that the frequencies of the light-induced states and the time-delay-dependent features of the spectra contain information on the action of the intense pulses exciting the system, which can thus complement the information on light-imposed amplitude and phase changes encoded in the absorption line shapes.
We study transients in resonantly coupled three-level systems. Two of the levels are coupled by intense two-photon pumping, and the third one provides a probing level. Adiabatically eliminated levels appear in effective system parameters. Experimentally, this model is applicable, e.g., in resonantly enhanced parametric and harmonic generation in atomic vapors. A distinguishing feature in the system is the appearance of ac Stark shifts in the probe spectra when pumping at the two-photon resonance. The nutation signal is shown to give information on the effective two-photon Rabi frequency and on the decay rate of the dipole-forbidden two-photon coherence. Free-induction decay is strongly dominated by population-induced effects; Raman-like peaks introduced by Stark shifts may show large instantaneous gain. Gaussian pump pulses are investigated to investigate effects arising from the temporal sweeping of the two-photon Rabi flipping and the modulation of the Stark shifts. The results also provide relevant information on the gain dynamics of coherently pumped lasers.
Abstract The ultrafast dynamic evolution of an atomic system under medium-strength laser fields is studied by performing transient absorption measurement. An analytical model developed from perturbation theory with a modified transition dipole moment is presented to explain the spectral features of the multilevel system. By fitting the measured absorption spectra to the model, the system’s dynamic evolution is quantified by different amplitude and phase modulation factors in the pump-probe and probe-pump scenarios. This study provides a way to understand laser-matter interaction in the transition area between the strong-field and weak-field regimes.