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arXiv:2604.05261

Nonlinear signal enhancement of strongly-coupled molecules in pump-probe experiments

Alexander M. McKillop, Marissa L. Weichman

physics.chem-phquant-phSubmitted Apr 6, 2026• Announced Apr 8, 2026• 0 citations
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Abstract

Nonlinear spectroscopy is widely used to study the transient dynamics of molecules under strong light-matter coupling, though it remains unclear to what extent uncoupled intracavity molecules obscure signals from the strongly-coupled species of interest. Pump or probe fields resonant in the strongly-coupled spectral region will preferentially interact with cavity-coupled molecules, but can exhibit severe optical artifacts due to wave interference in the cavity. On the other hand, non-resonant pump or probe fields having wavelengths at which the cavity mirrors are highly transmissive propagate as traveling waves along the cavity axis, interacting with both coupled and uncoupled intracavity molecules. Here, we quantify the contributions of signals from strongly-coupled and uncoupled populations in simulated experiments with different resonant and non-resonant pump-probe configurations. We find that while resonant schemes maximize selectivity for the signals of strongly-coupled molecules, non-resonant schemes retain surprisingly high sensitivity to these signals while remaining less susceptible to optical artifacts.

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Nonlinear spectroscopy is widely used to study the transient dynamics of molecules under strong light-matter coupling, though it remains unclear to what extent uncoupled intracavity molecules obscure signals from the strongly-coupled species of interest. Pump or probe fields resonant in the strongly-coupled spectral region will preferentially interact with cavity-coupled molecules, but can exhibit severe optical artifacts due to wave interference in the cavity. On the other hand, non-resonant pump or probe fields having wavelengths at which the cavity mirrors are highly transmissive propagate as traveling waves along the cavity axis, interacting with both coupled and uncoupled intracavity molecules. Here, we quantify the contributions of signals from strongly-coupled and uncoupled populations in simulated experiments with different resonant and non-resonant pump-probe configurations. We find that while resonant schemes maximize selectivity for the signals of strongly-coupled molecules, non-resonant schemes retain surprisingly high sensitivity to these signals while remaining less susceptible to optical artifacts.
A nonlinear spectroscopic investigation of a strongly coupled atom-cavity system is presented. A two-field pump-probe experiment is employed to study nonlinear structure as the average number of intracavity atoms is varied from $\overline{N}\ensuremath{\approx}4.2$ to $\overline{N}\ensuremath{\approx}0.8$. Nonlinear effects are observed for as few as 0.10.10.1 intracavity pump photons. A detailed semiclassical simulation of the atomic beam experiment gives reasonable agreement with the data for $\overline{N}\ensuremath{\gtrsim}2$ atoms. The simulation procedure accounts for fluctuations in atom-field coupling which have important effects on both the linear and nonlinear probe transmission spectra. A discrepancy between the simulations and the experiments is observed for small numbers of atoms $(\overline{N}\ensuremath{\lesssim}1)$. Unfortunately, it is difficult to determine if this discrepancy is a definitive consequence of the quantum nature of the atom-cavity coupling or a result of the severe technical complications of the experiment.
We introduce cavity enhanced ultrafast transient absorption spectroscopy, which employs frequency combs and high-finesse optical cavities. % The schematic of apparatus is shown in Figure 1. Sub-100 fs pulses with a repetition rate of 90 MHz are generated by a home-built Ytterbium fiber laser. The amplified light has a power up to 10 W, which is used to pump an optical parametric oscillator, followed by second-harmonic generation(SHG) that converts the wavelength from near-IR to visible. A pump comb at 530 nm is separately generated by SHG. Both pump and probe combs are coupled into high-finesse cavities. Compared to the conventional transient absorption spectroscopy method, the detection sensitivity can be improved by a factor of (Fπ)2∼105\left(\frac{\mathcal{F}}{\pi} \right)^2 \sim 10^5(πF​)2∼105, where F\mathcal{F}F is the finesse of cavity. This ultrasensitive technology enables the direct all-optical dynamics study in molecular beams. We will apply the cavity enhanced ultrafast transient absorption spectroscopy to investigate the dynamics of visible chromophores and then extend the wavelength to mid-IR to study vibrational dynamics of small hydrogen-bonded clusters. %\begin{wrapfigure}{I}{1\textwidth} % \centering % \includegraphics[width = 0.7\textwidth]{CETAS_details.eps} %\caption{Schematic of cavity enhanced ultrafast transient absorption spectroscopy apparatus. An amplified fiber laser frequency comb is converted to visible light and pump and probe combs are coupled to high-finesse optical resonators. Transmission of the probe light is measured.} %\end{wrapfigure}
Abstract Measurements of ultrahigh-fidelity absorption spectra can help validate quantum theory, engineer ultracold chemistry, and remotely sense atmospheres. Recent achievements in cavity-enhanced spectroscopy using either frequency-based dispersion or time-based absorption approaches have set new records for accuracy with uncertainties at the sub-per-mil level. However, laser scanning or susceptibility to nonlinearities limits their ultimate performance. Here we present cavity buildup dispersion spectroscopy (CBDS), probing the CO molecule as an example, in which the dispersive frequency shift of a cavity resonance is encoded in the cavity’s transient response to a phase-locked non-resonant laser excitation. Beating between optical frequencies during buildup exactly localizes detuning from mode center, and thus enables single-shot dispersion measurements. CBDS can yield an accuracy limited by the chosen frequency standard and measurement duration and is currently 50 times less susceptible to detection nonlinearity compared to intensity-based methods. Moreover, CBDS is significantly faster than previous frequency-based cavity-enhanced methods. The generality of CBDS shows promise for improving fundamental research into a variety of light–matter interactions.
We study unidirectional three-wave spectroscopy for which one pump and two probe waves beat in a nonlinear medium at the frequencies ±Δ with respect to the pump. Field transmission spectra change as the fields propagate through the medium due to probe variations in both amplitude and phase. In a homogeneously broadened two-level system, the rates of change in sidemode amplitudes and phases are determined by the effectiveness of the population pulsations in coupling the fields. For cases in which the population difference lifetime T1 is comparable with the dipole lifetime T2, the strong coupling between the sidemodes and the pump leads to fast amplitude change, while strong coupling between the two sidemodes leads to fast phase change. When the pump Rabi flopping frequency Ω ≅ Δ and Δ ≅ 1/T1, the population pulsations couple the two sidemodes most efficiently, causing the sidemode phases relative to the pump phase to vary rapidly. When Ω ≅ 2Δ, the pump field scatters strongly off the population pulsations, thereby amplifying the sidemode fields. When Ω ≫ Δ, the population pulsations become too weak to couple the fields significantly, and phase, amplitude, and spectral features in general change very slowly.