Attosecond‐Stable Two‐Dimensional Spectroscopy by a Sagnac‐Based Modulating System and a Sub‐4‐fs Continuum Source

We present a two‐dimensional electronic spectroscopy (2DES) platform driven by a Coherent Loop‐based Integrated Modulating and Beamsplitting System (CLIMBS). Combined with an octave‐spanning multiple‐plate continuum (MPC) source, the system enables ultrabroadband, phase‐coherent measurements with attosecond‐level delay precision. Its Sagnac‐inspired, nearly common‐path geometry provides excellent long‐term phase stability without active feedback while eliminating beam walk‐off during delay scans. Delay calibration using spectrally resolved interferometric fringes confirmed precise, linear coherence‐time control. The MPC source generates 550–980 nm excitation pulses compressed to 3.7 fs, enabling simultaneous excitation of widely separated electronic and vibronic transitions with high temporal and spectral resolution, and supporting distortion‐free measurements of ground‐state bleaching, stimulated emission, excited‐state absorption, and vibronic cross peaks. The system's performance was evaluated using chlorophyll a in methanol, where the excitation bandwidth fully covers the and bands, enabling distortion‐free 2DES measurements across the electronic manifold. This simple and robust platform provides high‐fidelity ultrabroadband 2DES for multidimensional spectroscopy.