Figure 4: Plasma evolution recorded by light-in-flight.

(a) In these frames, the femtosecond pulse is shown in red, and the plasma fluorescence in blue. The plasma is created as soon as the leading edge of the pulse passes through focus. The laser pulse is then scattered by the plasma, which can be seen in the second frame. As a large portion of the beam is scattered or absorbed at the focus by the plasma, the exiting beam is weaker, as we observe in the third to fifth frames. We also see the plasma being formed at focus when the laser pulse is focused, then evolving and decaying after the pulse has passed. A full video is provided as Supplementary Video 2. (b) The graph shows the temporal histogram of the plasma fluorescence (solid black curve, normalized photon counts, raw data) corresponding to the region indicated with a white square in the last frame. The dashed line is the detector impulse response function, the dotted red curve is an exponential decay fit with a 600-ps decay constant.