Deep-sub-cycle ultrafast optical pulses

Sub-cycle 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 break the half-cycle limit via inverse Compton scattering in a deep-subwavelength-confined infrared or THz optical driving field, which can be converted into a deep-sub-cycle (i.e., less than half a cycle) ultrafast pulse by relativistic electrons passing through. Quantitatively, by using a deep-subwavelength-confined 0.4-THz 1-ps pulsed driving field and a 3-MeV 1-fs electron bunch, we obtain a $\sim$ 0.1-cycle femtosecond pulse with a peak frequency of $\sim$ 26 THz ($\sim$ 38 fs in optical cycle) and a pulse width of $\sim$ 3.6 fs; with the same driving field and a 30-MeV 50-as electron bunch, we obtain a 0.17-cycle attosecond pulse with a peak frequency of $\sim$ 2.0 PHz ($\sim$ 500 as in optical cycle) and a pulse width of $\sim$ 86 as. Below the optical damage threshold of the field-confinement material, the single-pulse photon number can exceed $10^6$. 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.