Figure 1
(a) In the absence of relativistic transparency, an incoming, linearly polarised laser pulse, in the form of a fundamental TEM00 (or Gaussian mode, indicated by the red box in which the arrow represents the polarisation direction), produces a current of energetic electrons bunched at ωL and 2ωL and reflects from the overdense plasma. (b) In an ultrathin foil, relativistic self-induced transparency results in a plasma aperture in which the linearly polarised (along the Y axis) laser light is transmitted and produces a bipolar distribution of dense electron bunches at the aperture edge. The deceleration of these bunches in the electrostatic sheath field produces coherent light emission at ωL (blue boxes) and 2ωL (green boxes) in higher order TEM modes. The arrow heads show the local polarisation direction of these modes.
