Relativistic free electrons in an intense laser field: Experimental observations of optically-induced deflection of an ultrashort electron beam

We present experimental evidence of the deflection of electrons via the transfer of longitudinal momentum from an intense laser beam. The electrons are in the form of a narrow divergence beam created through self-modulated laser-wakefield acceleration with energies up to 6 MeV and an expected temporal duration of about 1 ps. A second laser pulse intersects the electron beam at an angle of 135° causing part of the beam to be deflected. The deflection is detected by using the scintillating plastic LANEX that provides spatial information of the electron beam. By taking column-wise and row-wise summations of the signal from the LANEX we examine how the beam profile changes with a change in the delay between the electron pulse and the secondary laser pulse. By using a set of metrics, we show how the beam is deflected and distorted. By measuring the time elapsed through the change in the electron beam, an estimate of the electron beam duration is given as less than 2 picoseconds. Inside of the 2 ps window, we show that different periods of deflection based on electron beam temperature can be explained by the laser sampling portions of the electron beam with different temperatures. It is also demonstrated in both theory and experiment that this process has no dependence on the polarization direction of the laser field. This physical process can be altered by changing the angle of incidence and laser intensity to examine deflection of different ranges of electron energies. This provides an important tool for the temporal measurement of ultrafast electron beams that can provide electron energy information.