Laser absorption in microdroplet plasmas

Abstract

We present experimental measurements of the absorption of ultrashort laser pulses by 15 μ m diameter methanol microdroplets. The droplet absorbs upto 70% of the incidence laser energy in the presence of a prepulse at intensities of about 1.5×10¹⁶ W cm⁻². In the absence of a prepulse, the absorption is only about 20%. Simultaneous measurements of X-ray yield (12 keV to 350 keV) and the absorption in the droplet plasma, shows that our earlier measurements of efficient generation (Anand M. et al. Appl. Phys. Lett., 88 (2006) 181111) of hard X-rays from the droplet plasma is due to the increased absorption in the droplets in the presence of optimum prepulse. 1-D PIC simulations, mimicing the mass-limited droplet density profile, demonstrate the effectiveness of the large scale-length droplet plasma in providing optimal conditions for resonant laser absorption energy and generation of hot electrons.