  Precise knowledge of the behaviour of the phase of light in a focused beam is
fundamental to understanding and controlling laser-driven processes. More than
a hundred years ago an axial phase anomaly for focused monochromatic light
beams was discovered and is now commonly known as the Gouy phase. Recent
theoretical work has brought into question the validity of applying this
monochromatic phase formulation to the broadband pulses becoming ubiquitous
today. Based on electron back-scattering at sharp nanometre-scale metal tips, a
method is available to measure light fields with sub-wavelength spatial
resolution and sub-optical cycle time resolution. Here we report such a direct,
three-dimensional measurement of the spatial dependence of the optical phase of
a focused, 4-fs, near-infrared laser pulse. The observed optical phase deviates
substantially from the monochromatic Gouy phase - exhibiting a much more
complex spatial dependence, both along the propagation axis and in the radial
direction. In our measurements, these significant deviations are the rule and
not the exception for focused, broadband laser pulses. Therefore, we expect
wide ramifications for all broadband laser-matter interactions, such as in
high-harmonic and attosecond pulse generation, femtochemistry, ophthalmological
optical coherence tomography and light-wave electronics.
