Beating spatio-temporal coupling: implications for pulse shaping and coherent control experiments.

Diffraction of finite sized laser beams imposes a limit on the control that can be exerted over ultrafast pulses. This limit manifests as spatio-temporal coupling induced in standard implementations of pulse shaping schemes. We demonstrate the influence this has on coherent control experiments that depend on finite excitation, sample, and detection volumes. Based on solutions used in pulse stretching experiments, we introduce a double-pass scheme that reduces the errors produced through spatio-temporal coupling by at least one order of magnitude. Finally, employing single molecules as nanoscale probes, we prove that such a double pass scheme is capable of artifact-free pulse shaping at dimensions two orders of magnitude smaller than the diffraction limit.