Generation of 100-TW half-cycle zeptosecond x-ray pulses in the cascaded regime

The requirement for ever shorter laser pulses is endless in ultrafast science. Here we propose a mechanism to produce an ultrashort zeptosecond pulse (ZP) to break the attosecond pulse (AP) barrier, where a cascaded coherent synchrotron emission regime of high-order harmonics generation is used for the emission of a giant half-cycle ZP. As an ultraintense laser pulse interacts with the double-foil target, a relativistic electron sheet is formed by the first-foil electrons and accelerated in the transmission direction, by which a half-cycle AP is emitted. The generated half-cycle AP continues to interact with the second foil target, where another relativistic electron sheet is formed by the second-foil electrons and a giant half-cycle ZP is emitted. A theoretical model is proposed for the nanobunching mechanism of the relativistic electron sheet both in the reflection direction and in the transmission direction, and is supported by numerical simulation results. The intensity of the ZP can reach approximately 2.6 × 1021 W/cm2, with a duration of 380 zs, which corresponds to a power reaching the 100-TW level. This ultraintense ZP will open the door for nuclear excitation, intranuclear dynamics, or vacuum physics studies on the zeptosecond timescale.