Attosecond bunches of gamma photons and positrons generated in nanorod array targets.

The sub-atomic experimental exploration of physical processes on extremely short time scales has become possible by the generation of high quality electron bunches and x-ray pulses with sub-femtosecond durations. Temporal coherence is crucial for achieving high resolution in diffraction-based diagnostics and if the transverse spatial coherence is also provided, then phenomena with very small cross sections could also be observed. Increasing the photon energy from x-ray to gamma-ray regime makes probing of extremely small space-time domains accessible. Here, a mechanism for generating attosecond gamma photon and positron bunches with small divergence using laser intensities below $10^{23}$ W/cm$^2$ is proposed. Numerical simulations are used to formulate the conditions for coherent radiation and to characterize the generated photon and positron bunches.