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 novel Coherent Loop-based Integrated Modulating and Beamsplitting System (CLIMBS). Coupled with an octave-spanning multiple-plate continuum (MPC) source, CLIMBS enables broadband, phase-coherent measurements with attosecond-level time delay precision. Its Sagnac-inspired, nearly common-path geometry provides exceptional long-term phase stability without active feedback, eliminating beam walk-off and preserving beam pointing during delay scans. Delay calibration using spectrally resolved interferometric fringes yielded a wedge angle in excellent agreement with the designed geometry, confirming precise, linear coherence time control. The MPC technique generates broadband excitation pulses spanning 550--980 nm and temporally compressed to 3.7 fs. This bright, few-cycle source enables simultaneous interrogation of widely separated electronic and vibronic transitions, with high temporal and spectral resolution, allowing 2DES to capture vibronic cross peaks, energy-transfer pathways, and undistorted ground-state bleaching (GB), stimulated emission (SE), and excited-state absorption (ESA) features across a broad spectral window. System performance was benchmarked on chlorophyll-a in methanol, where the excitation bandwidth fully covers the $Q_x$ and $Q_y$ bands, ensuring distortion-free spectra. The nearly collinear configuration of CLIMBS eliminates beam walk-off during delay scanning, supports ultrabroadband few-cycle 2DES enabled by the high-brightness MPC source, and maintains attosecond-level phase stability, providing a simple and robust platform for high-fidelity multidimensional spectroscopy.