In the traditional scheme of extreme ultraviolet (XUV) attosecond pulse pump and infrared (IR) femtosecond laser probe, modifications of spectral signatures in the attosecond transient absorption (ATA) spectra are usually achieved by variations in the attosecond pulse waveform, time delay between the two fields, and IR-field intensity. However, the effect of the carrier-envelope phase (CEP) of the IR laser is neglected. Here, we theoretically demonstrate that the CEP modulation of the laser-dressed ATA spectra in a singly excited helium atom depends on the IR pulse area. Using a full multilevel model and a simplified three-level model, we find that the absorption line shape of the $2p$ state exhibits a pronounced periodic modulation as a function of the CEP exclusively when the pulse area reaches $\pi$. In contrast, this modulation becomes negligible at other pulse areas. Analytical derivations based on the $\delta$-pulse approximation reveal that this effect originates from the complete population transfer between the bright $2p$ state and the dark $3d$ state at specific IR pulse areas. We further confirm the robustness of this pulse-area-dependent feature against variations in IR laser duration, central wavelength detuning, and attochirp of the XUV pulse. Finally, we show that the CEP modulation remains robustly observable when considering macroscopic propagation effects in a gas medium. We expect that such CEP sensitivity of the ATA spectra could be used as an all-optical diagnostic tool for absolute CEP measurement.

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• Atomic & molecular processes in external fields
• High-order harmonic generation
• Optical transient phenomena
• Ultrafast phenomena
• Attosecond laser spectroscopy
• Ultrafast pump-probe spectroscopy