Gamma-Ray Flash Generation in Irradiating Thin Foil Target by Single Cycle Tightly Focused Extreme Power Laser Pulse

We present a regime where an ultra-intense laser pulse interacting with a foil target results in high $\gamma$-photon conversion efficiency, obtained via three-dimensional quantum-electrodynamics particle-in-cell simulations. A single-cycle laser pulse is used under the tight-focusing condition for obtaining the $\mathrm{\lambda}^3$ regime. The simulations employ a radially polarized laser as it results in higher $\gamma$-photon conversion efficiency compared to both azimuthal and linear polarizations. A significant fraction of the laser energy is transferred to positrons, while a part of the electromagnetic wave escapes the target as attosecond single-cycle pulses.