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Shack-Hartmann Electronic Densitometer (SHED)

The Shack-Hartmann Electron Densitometer (SHED) is a novel method to diagnose ultrashort pulse laser–produced plasmas by measuring the phase change to a probe laser beam. Using the Shack-Hartmann method, the phasefront of a probe laser is measured through a lenslet array onto a camera. Small changes in the location of the individual focal points on the camera plane are translated into changes in the probe beam’s wavefront. These wavefront distortions arise from refractive index variations caused by the free electrons. This method allows for superior performance in measuring minute variations in the electron density in 2 dimensions, single-shot, with sub-picosecond time resolution. The data taken with SHED demonstrated the ability to diagnose plasmas with densities of approximately 5x1017 cm-3 and show the temporal evolution of the plasma long after the driving laser pulse has left. The method can be further improved by enclosing the probe beam and adding a second axis to allow for tomographic reconstruction of the electron density in 3 dimensions. By using SHED in a pump/probe setup, we are able to obtain temporally synchronized, ultrashort snap shots of the evolution of free electrons in laser-induced ionization of air and other transparent media.