Spatial Phase Coherence in Femtosecond Coherent Raman Scattering

Conventional femtosecond coherent laser spectroscopy predominantly focuses on the temporal phase coherence through time- or frequency-resolved methods. In this Letter, we suggest an alternative experimental framework based on spatial phase coherence. The intrinsic spectral dispersion of wave vectors in femtosecond pulses and sample dimensions exceeding the laser wavelength creates a compelling basis to establish spatial phase coherence as a novel approach to femtosecond laser spectroscopy. Using rotational Raman coherence in air as a case study, we analyze the transverse spatial distribution of the third-order signal generated by the rotational wave packet. Our findings reveal apparent temporal shifts and distortions in time-resolved signals that arise in conventional measurements lacking sensitivity to spatial phase coherence. Moreover, we demonstrate that spatial phase coherence can serve as a useful tool for thermometric applications, showcasing its sensitivity to temperature variations. These discoveries open new avenues in femtosecond laser spectroscopy, including an alternative single-shot detection scheme, a new form of Raman coherence imaging, and molecular species quantification during overlapping fractional revivals.