Microplasma and the signal distribution [34
]. (a) A photo of the microplasma fluorescence obtained by focusing the laser beam with a high NA microscope objective. (b) A magnified image of the microplasma (unit is μ
m). However, the plasma spot’s contribution to the THz wave is about one micrometer or less. (c) Measured THz field as a function of the THz emission angle. The optimal angle is 80 degrees from the laser propagation direction.