Subcycle optical pulse is of great importance for ultrafast science and technology. While a narrower pulse can offer a higher temporal resolution, so far the pulse width has not reached the limit of half an optical cycle. Here we propose to generate deep-sub-cycle (i.e., less than half a cycle, sub-half-cycle) ultrafast optical pulses that break the half-cycle limit via inverse Compton scattering of a deep-subwavelength-confined THz optical driving field by relativistic electrons. Quantitatively, based on a deep-subwavelength-confined 0.4-THz 1-ps pulsed driving field, we obtain a 0.095-cycle 3.6-fs-width mid-infrared femtosecond pulse with a peak frequency of ∼26 THz (∼38 fs in optical cycle) by using a 3-MeV 1-fs electron bunch, and a 0.091-cycle 180-as-width visible attosecond pulse with a peak frequency of ∼0.49 PHz (∼2.0 fs in optical cycle) by using a 15-MeV 50-as electron bunch, respectively. Below the optical damage threshold of the sapphire prisms used for field confinement, the single-pulse photon number can exceed ${10}^5$. Such pulses may open opportunities for studying light-matter interaction on the deep-sub-cycle level, and pave a way to unprecedented optical technology ranging from temporal super-resolution optical microscopy and spectroscopy to unconventional atom/molecule polarization and manipulation.

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• Compton scattering
• Few-cycle light pulses
• Light-matter interaction
• Nanophotonics
• Ultrafast optics
• Ultrashort pulses
• XUV generation