  Water-immersed gold nanoparticles irradiated by a laser can trigger the
nucleation of plasmonic bubbles after a delay time of a few microseconds [Wang
et al., Proc. Natl. Acad. Sci. USA 122, 9253,(2018)]. Here we systematically
investigated the light-vapor conversion efficiency, {\eta}, of these plasmonic
bubbles as a function of the ambient pressure. The efficiency of the formation
of these initial-phase and mainly water-vapor containing bubbles, which is
defined as the ratio of the energy that is required to form the vapor bubbles
and the total energy dumped in the gold nanoparticles before nucleation of the
bubble by the laser, can be as high as 25%. The amount of vaporized water first
scales linearly with the total laser energy dumped in the gold nanoparticles
before nucleation, but for larger energies the amount of vaporized water levels
off. The efficiency {\eta} decreases with increasing ambient pressure. The
experimental observations can be quantitatively understood within a theoretical
framework based on the thermal diffusion equation and the thermal dynamics of
the phase transition.
