Attosecond Fourier Transform Spectroscopy

Attosecond metrology reveals the sub-cycle temporal evolution of field-driven systems by their interaction with attosecond pulses. However, the large bandwidth of attosecond pulses couples the contribution of multiple quantum states into the optical response, hindering the identification of their individual dynamics. Here, we establish attosecond Fourier transform spectroscopy, integrating the enhanced spectral resolution and signal-to-noise ratio of Fourier transform spectroscopy with the sub-cycle temporal resolution of attosecond metrology. We first demonstrate a background-free measurement of the field-free energy spectrum of helium, obtained by transmitting a phase-locked attosecond pulse pair through a helium sample. This measurement resolves spectral features that are hidden in absorption measurements recorded with a single attosecond light source. Advancing to the field-driven regime, our scheme identifies the contribution of individual quantum states and their transient interaction with the field. Attosecond Fourier transform spectroscopy establishes the ability to reveal meV scale physics on attosecond timescales, opening the door to resolving rich quantum dynamics in field-driven systems with unprecedented precision.