Plasma-Formation Dynamics in Intense Laser-Droplet Interaction

Abstract

We study the ionization dynamics in intense laser-droplet interaction using three-dimensional, relativistic particle-in-cell simulations. Of particular interest is the laser intensity and frequency regime for which initially transparent, wavelength-sized targets are not homogeneously ionized. Instead, the charge distribution changes both in space and in time on a sub-cycle scale. One may call this the extreme nonlinear Mie-optics regime. We find that-despite the fact that the plasma created at the droplet surface is overdense-oscillating electric fields may penetrate into the droplet under a certain angle, ionize, and propagate in the just generated plasma. This effect can be attributed to the local field enhancements at the droplet surface predicted by standard Mie theory. The penetration of the fields into the droplet leads to the formation of a highly inhomogeneous charge density distribution in the droplet interior, concentrated mostly in the polarization plane. We present a selfsimilar, exponential fit of the fractional ionization degree which depends only on a dimensionless combination of electric field amplitude, droplet radius, and plasma frequency with only a weak dependence on the laser frequency in the overdense regime.PACS numbers: 52.35. Mw, 52.50.Jm, 32.80.Fb, 52.65.Rr Introduction -Spherical, wavelength-sized, homogeneous dielectric or metal objects in plane-wave electromagnetic radiation fall into the realm of Mie theory [1] and are of fundamental importance in optics. Standard Mie theory provides the electromagnetic field configuration inside and outside a homogeneous sphere of a given dielectric constant, assuming an incoming plane wave. However, nowadays available short and intense laser pulses interacting with matter create plasmas on a sub-laser period time scale [2,3]. These plasmas, in turn, modify the further propagation of the laser pulse. We call this the extreme nonlinear optics regime, and in the case of (initially) spherical targets nonlinear Mie optics.

The interaction of intense femtosecond laser pulses with argon microdroplets studied near the soft x-ray emission threshold

“…Particle-in-cell simulations show that an inhomogeneous plasma layer develops which extends deeper into the droplets than expected from classical predictions [25].…”

Time-resolved Thomson scattering on high-intensity laser-produced hot dense helium plasmas

“…We obtain an asymmetric ionization of the droplet which is caused by a focused penetration of the laser into the droplet. The effect may be understood using Mie theory [30].…”

Simulations of femtosecond laser pulse interaction with spray target

“…We have studied there the dependence of proton acceleration on the laser pulse intensity in the range from 5 × 10 17 to 5 × 10 19 W/ cm 2 , on the laser pulse duration, on the droplet diameter, on initial density profile, and the role of the ionization of submicron water droplet (charge of oxygen ions) was also investigated. A number of other theoretical and numerical studies (Murakami et al, 2006;Ter-Avetisyan et al, 2012;Liseykina & Bauer, 2013) related to the interaction with single droplet composed of one or multiple ion species have been published. However, to our knowledge, there are no relevant studies of the interaction between laser and multiple droplets of submicron diameter.…”