Full melting of a two-dimensional complex plasma crystal triggered by localized pulsed laser heating

The full melting of a two-dimensional plasma crystal was induced in a principally stable monolayer by localized laser stimulation. Two distinct behaviors of the crystal after laser stimulation were observed depending on the amount of injected energy: (i) below a well-defined threshold, the laser melted area recrystallized; (ii) above the threshold, it expanded outwards in a similar fashion to mode-coupling instability-induced melting, rapidly destroying the crystalline order of the whole complex plasma monolayer. The reported experimental observations are due to the fluid mode-coupling instability, which can pump energy into the particle monolayer at a rate surpassing the heat transport and damping rates in the energetic localized melted spot, resulting in its further growth. This behavior exhibits remarkable similarities with impulsive spot heating in ordinary reactive matter.

Figure 1

Sketch of the experimental setup. The left inset shows an elementary cell of the hexagonal lattice and the frame of reference chosen in this article. The orientation of the wave vector $\mathbf{k}$ is measured in respect to the $x$ axis. The right inset is a snapshot of the crystal after 0.25 s of laser heating. The red ellipse shows where the laser light beam interacts with the monolayer.

Figure 2

Longitudinal and transverse current fluctuation spectra at an angle $\theta =0^{\circ }$. The dotted black curves represent the fitted dispersion relations.

Figure 3

State of the monolayer (red, melted or black, unmelted) at the end of the experiment as a function of $K_{\mathrm{t}}(t_{\mathrm{on}}+t_{\mathrm{th}})$. Each experiment of Table 1 is indicated by an arrow of the appropriate color. A threshold at $\sim 40$ keV is clearly identified. The inset shows the transition time before recrystalization sets in for injected energies below the melting threshold. The blue line is a guide for the eye.

Figure 4

$|{\mathrm{\Psi }}_6|$ maps (a, c) and energy maps (b, d) of the crystal during Expts. 19 (a, b) and 20 (c, d) (see Table 1). Each map is a square of $24.6\times 24.6{\mathrm{mm}}^2$.

Figure 5

(a) Evolution of the sum the kinetic energies of the particles in the field of view and in the melted zone as a function of time for Expts. 19 and 20 (see Table 1). (b) Evolution of the radius of the melted zone as a function of time.

Figure 6

Evolution of the temperature profile. (a) Experiment 19: recrystallization, (b) Experiment 20: melting. Solution of Eqs. (1) using the reaction model (4): (c) slightly below threshold, (d) slightly above threshold. For the numerical profiles, the following parameters were used: $\chi =9{\mathrm{mm}}^2$/s, ${\gamma }_{\mathrm{d}}=1.2{\mathrm{s}}^{-1}$, ${\gamma }_{\mathrm{MCI}}=0.9{\mathrm{s}}^{-1}$, $T_{\infty }=1800$ eV, $T_a=105$ eV, $n_{2\mathrm{D}}=6.75{\mathrm{mm}}^{-2}$, $C=2$, $w_{\mathrm{MZ}}=1.15$ mm and (c) $E_{\mathrm{l}}=40103$ eV ($\lambda =24.2497$, $\mathrm{\Gamma }=3.7722$) and (d) $E_{\mathrm{l}}=40103.7$ eV ($\lambda =24.2506$, $\mathrm{\Gamma }=3.7722$).