Subcycle terahertz field waveforms clocked by attosecond high-harmonic pulses from relativistic laser plasmas

A high-intensity ultrashort laser pulse interacting with a thin plasma target is shown to couple to plasma electrons, driving electron oscillations within the plasma and making these electrons bounce back and forth between plasma boundaries. Each time these recirculating electrons traverse the plasma boundary, they emit bright subcycle terahertz (THz) field waveforms via laser-driven coherent transition radiation. As a concurrent process, laser-driven electrons near the front surface of the plasma target are accelerated to relativistic velocities to emit high-order harmonics (HHs), giving rise to attosecond pulses of vacuum-ultraviolet radiation. These attosecond pulses are shown to provide a high-precision clock for subcycle THz field waveforms. We demonstrate that the delay time between HH pulses and THz waveforms can be tuned with an attosecond precision by varying the thickness of the plasma target, thus opening an avenue toward HH-pump–THz-probe studies of ultrafast processes on the attosecond time scale with table-top laser sources.