Diffraction-limited coherent wake emission

The plasma mirror is a relativistic optical element that under certain irradiation conditions generates intense attosecond extreme-ultraviolet light pulses, in a process known as coherent wake emission (CWE). CWE has been previously characterized by its high spatial divergence, originating from an intrinsic intensity-dependent phase accumulation. In this Letter, we show that the transverse variations of the plasma expansion can completely cancel the CWE intrinsic phase. Accordingly, we experimentally demonstrate nearly diffraction-limited CWE with unprecedented divergence, under 6 mrad. We validate our analytical model with particle in cell simulations. This understanding facilitates the development of plasma mirrors as applicable ultraviolet light sources.