Laser-sustained plasma of high radiance in the ultraviolet spectral range based on the reservoir effect of the annular beam.

Laser-sustained plasma (LSP) featured by high radiance in the ultraviolet (UV) spectral range is found to be powerful in high resolution inspection and spectroscopy applications. In this paper, we demonstrate significant enhancement of UV radiance in LSP excited by a high power focused annular laser beam of small F-number. A two-dimensional refractive index distribution in the plasma region is established to obtain the propagation path of the annular beam. Subsequently, the spatial distribution of the annular beam power density is calculated to establish the quantitative relationship between laser power and plasma dimension. Theoretical analysis of a "reservoir" effect constructed by the annular beam is carried out. Abrupt plasma dimension change is observed when the laser power exceeds the "reservoir" boundary. The new model is then validated by experiments. LSP with the highest reported UV radiance to our knowledge has been achieved. This work paves what we believe is a novel way of generating high radiance UV sources by spatial manipulation of excitation laser beam.