The optimal phase in strong-field photoemission experiments at sharp needle tips

When metallic needle tips are illuminated with intense femtosecond laser pulses, the photoemitted electrons can be driven strongly in the optical field at the surface of the needle tip. In particular, electrons can elastically rescatter at the tip, leading to tell-tale features in the electron spectra, such as the plateau and the cut-off. In this work, we first review intense two-color fields driving the electron dynamics. We show how from the two-color phase-dependent spectra one can obtain the optimal phase, i.e., the phase as function of energy, for which the count rate is maximized. We demonstrate that rich information can be extracted from this optimal phase. This first, review part sets the stage for the second part of the paper, containing original data on carrier-envelope phase-dependent spectra. We show that with a similar analysis we can reveal similarly rich information from the optimal phase also in the CEP variation case. In particular, we show that the optimal phase is highly sensitive to the driving pulse duration. We expect this work to widen the tool park of strong-field physics, with direct ramifications to the fields of attosecond physics and future petahertz electronics.