Relation between Inner Structural Dynamics and Ion Dynamics of Laser-Heated Nanoparticles

When a nanoparticle is irradiated by an intense laser pulse, it turns into a nanoplasma, a transition that is accompanied by many interesting nonequilibrium dynamics. So far, most experiments on nanoplasmas use ion measurements, reflecting the outside dynamics in the nanoparticle. Recently, the direct observation of the ultrafast structural dynamics on the inside of the nanoparticle also became possible with the advent of x-ray free electron lasers (XFELs). Here, we report on combined measurements of structural dynamics and speeds of ions ejected from nanoplasmas produced by intense near-infrared laser irradiations, with the control of the initial plasma conditions accomplished by widely varying the laser intensity ($9\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ to $3\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$). The structural change of nanoplasmas is examined by time-resolved x-ray diffraction using an XFEL, while the kinetic energies of ejected ions are measured by an ion time-of-fight method under the same experimental conditions. We find that the timescale of crystalline disordering in nanoplasmas strongly depends on the laser intensity and scales with the inverse of the average speed of ions ejected from the nanoplasma. The observations support a recently suggested scenario for nanoplasma dynamics in the wide intensity range, in which crystalline disorder in nanoplasmas is caused by a rarefaction wave propagating at a speed comparable with the average ion speed from the surface toward the inner crystalline core. We demonstrate that the scenario is also applicable to nanoplasma dynamics in the hard x-ray regime. Our results connect the outside nanoplasma dynamics to the loss of structure inside the sample on the femtosecond timescale.

Popular Summary

When a nanoparticle is irradiated by an intense laser pulse, it turns into a nanometer-sized plasma, or “nanoplasma.” So far, the physics of nanoplasma—seen in a wide range of investigations wherever intense laser pulses are used—has been investigated by measuring ions ejected from nanoplasmas. A key outstanding question is how the structural dynamics inside the nanoplasma is related to the outer dynamics of ion ejections. In this work, we address this question by combining measurements of the structural changes of a nanoplasma with the speeds of its ejected ions.

We produce nanoplasmas by intense near-infrared laser pulses, measure the ejected ion speeds, and examine the structural change by time-resolved x-ray diffraction using an x-ray free-electron laser. With precise control of plasma conditions by varying the infrared laser intensity, we reveal a key relation between the structural dynamics and the ion speeds: The timescale of the crystalline disordering in the nanoplasma scales as the inverse of the ejected ion speeds.

Our findings should be general for any nanoplasmas, and therefore we believe that this achievement is of paramount importance in various fields, including laser machining and surgeries that employ laser ablation. Another prominent application is radiation damage caused by intense x-ray pulses. Using our findings, one can evaluate the timescale of structural damage by measuring the ejected ion speed in the single-shot imaging of nanoparticles using an x-ray free-electron laser.

Figure 1

(a) Radial distribution of the detected Bragg spots from pristine Xe clusters on the detector. The pattern exhibits peaks at positions corresponding to $\{111{\}}_{\mathrm{fcc}}$ and $\{200{\}}_{\mathrm{fcc}}$ reflections. The broad peak around $r\approx 38\mathrm{mm}$ originates from the rhcp structure [38]. In the subsequent analysis, we only focus on the Bragg spots located on the sharp peak at $r\approx 36\mathrm{mm}$ (filled with pink). (b) Representative example of a Bragg spot profile in a MPCCD image. (c) 2D Gaussian fit of panel (b).

Figure 2

(a) Delay-dependent radial profile of the diffraction intensities $I(r,t)$ around the $\{111{\}}_{\mathrm{fcc}}$ peak at $I_{\mathrm{peak}}^{\mathrm{NIR}}=9\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. The upper axis indicates the estimated NIR intensity at the corresponding particle positions. (b) Top view of the spatial intensity distribution of the NIR laser in the interaction region. (c) NIR laser intensity distribution along the XFEL beam. (d) Schematic geometry of the diffraction measurements.

Figure 3

Delay dependence of $\{111{\}}_{\mathrm{fcc}}$ diffraction intensities of Xe clusters $I(t)$ at (a) $I^{\mathrm{NIR}}=(2.2\pm 0.6)\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$, (b) $I^{\mathrm{NIR}}=(3.2\pm 0.5)\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$, and (c) $I^{\mathrm{NIR}}=(7.3\pm 0.8)\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. Solid curves show the fitting results with the nanoplasma model. For details of the model, see main text.

Figure 4

Red points: propagation speeds $v_{\mathrm{prop}}$ of the crystals deduced from the delay dependence of the diffraction intensities plotted against NIR intensity. The NIR intensities are evaluated by considering the spatial intensity profiles (see the main text and Appendix pp1). The vertical error bars indicate uncertainties originating from the statistical errors in the diffraction intensities. Blue points: plasma sound speeds deduced from the ion TOF spectra of the NIR-irradiated Xe clusters plotted against NIR peak intensity. The vertical error bars indicate uncertainties corresponding to the standard deviation of the ion kinetic energies in the fitting range.

Figure 5

Ion TOF spectra of NIR-irradiated Xe clusters at (a) $I_{\mathrm{peak}}^{\mathrm{NIR}}=3\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$, (b) $I_{\mathrm{peak}}^{\mathrm{NIR}}=4\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$, and (c) $I_{\mathrm{peak}}^{\mathrm{NIR}}=9\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. Ion TOFs at the highest charge states are simulated with SIMION and fitted with the experimental data (for details, see Appendix pp2).

Figure 6

(a,c) Delay-dependent radial profile of the diffraction intensities around the $\{111{\}}_{\mathrm{fcc}}$ peak at (a) $I_{\mathrm{peak}}^{\mathrm{NIR}}=3\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$ and (c) $I_{\mathrm{peak}}^{\mathrm{NIR}}=4\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$. The upper axis indicates the estimated NIR intensity at the interaction region. (b,d) Top view of the spatial intensity distribution of the NIR laser at (b) $I_{\mathrm{peak}}^{\mathrm{NIR}}=3\times {10}^{16}\mathrm{W}/{\mathrm{cm}}^2$ and (d) $I_{\mathrm{peak}}^{\mathrm{NIR}}=4\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 7

(a) Ion TOF spectrum of NIR-irradiated Xe clusters at $I_{\mathrm{peak}}^{\mathrm{NIR}}=4\times {10}^{15}\mathrm{W}/{\mathrm{cm}}^2$. The spectrum was fitted with a sum of Gaussian functions (dotted lines). Inset: correlation between the charge state and the ion kinetic energy. (b) Experimental spectrum fitted with the sum of the charge-state-resolved spectra simulated with an ion optics simulation software.

Figure 8

Delay-dependent Bragg diffraction intensity of Xe clusters irradiated by intense x-ray pulses. The data were reproduced from Ferguson et al. [27]. The diffraction intensity was fitted with the surface “melting” model (orange).