  Energy coupling during relativistically intense laser-matter interactions is
encoded in the attosecond motion of strongly driven electrons at the pre-formed
plasma-vacuum boundary. Studying and controlling this motion can reveal details
about the microscopic processes that govern a vast array of light-matter
interaction physics and applications. These include research areas right at the
forefront of extreme laser-plasma science such as laser-driven ion
acceleration1, bright attosecond pulse generation2,3 and efficient energy
coupling for the generation and study of warm dense matter4. Here we
demonstrate attosecond control over the trajectories of relativistic electron
bunches formed during such interactions by studying the emission of extreme
ultraviolet (XUV) harmonic radiation. We describe how the precise addition of a
second laser beam operating at the second harmonic of the driving laser pulse
can significantly transform the interaction by modifying the accelerating
potential provided by the fundamental frequency to drive strong coherent
emission. Numerical particle-in-cell code simulations and experimental
observations demonstrate that this modification is extremely sensitive to the
relative phase of the two beams and can lead to significant enhancements in the
resulting harmonic yield. This work also reveals that the ability to control
these extreme interactions with attosecond precision is an essential
requirement for generation of ultra-bright, high temporal contrast attosecond
radiation for atomic and molecular pump-probe experiments5,6.
