Dopant-Induced Ignition of Helium Nanodroplets in Intense Few-Cycle Laser Pulses

We demonstrate ultrafast resonant energy absorption of rare-gas doped He nanodroplets from intense few-cycle ($\sim 10\mathrm{fs}$) laser pulses. We find that less than 10 dopant atoms “ignite” the droplet to generate a nonspherical electronic nanoplasma resulting ultimately in complete ionization and disintegration of all atoms, although the pristine He droplet is transparent for the laser intensities applied. Our calculations at those intensities reveal that the minimal pulse length required for ignition is about 9 fs.

Figure 1

(a) Yields of ${\mathrm{He}}^{++}$ and ${\mathrm{He}}^{+}$ ions as a function of the mean number of Xe dopants in a nanodroplet containing $1.5\times {10}^4$ He atoms at a peak laser intensity of $7\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. (b) Numerical calculation of the charge per He atom as a function of the number of Xe atoms in a droplet containing 4000 He atoms. All lines are to guide the eye. Inset: Time-of-flight mass spectra of ${\mathrm{He}}^{++}$ and ${\mathrm{He}}^{+}$ ions for different numbers of dopant Xe atoms ($K$) as shown in the legend.

Figure 2

Xe doping dependence of ${\mathrm{He}}^{++}$ yields at a peak laser intensity of $1.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ for droplets with a mean number of He atoms per droplet $N_{\mathrm{He}}=4500$ (red) and $N_{\mathrm{He}}=15000$ (blue). Horizontal bars present the error in estimating the number of doped Xe atoms. The dash-dotted curves present the fraction of droplets lost due to complete evaporation, which is significant only for numbers of Xe dopant atoms $K⪆20$ (gray shading).

Figure 3

The ${\mathrm{He}}^{++}$ yields for a droplet containing 15 000 He atoms as a function of the number of Xe dopant atoms (a) for different laser intensities $I$ and (b) for different dopant species at $I=7\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 4

Average charge per He atom $Y(K)$ as a function of the number of doped Xe atoms for a droplet with 4000 He atoms and intensity as in Fig. 1 from molecular dynamics calculations. The curves correspond to different laser pulse lengths (full width at half maximum): from 3 (bottom) to 15 fs (top) in steps of 2 fs, as shown in the labels. The lines are fits of the data points according to Eq. (2) (solid line) and with the power law $Y\propto K^{3/2}$ corresponding to ionization by a static field for $T\le 9\mathrm{fs}$ (dashed line). For details, see the text.

Figure 5

The number of dopants for which the average charge per He atom $Y$ in Fig. 4 has a turning point: $Y^{\prime \prime }(K_{\mathrm{cr}})=0$. The shaded area distinguishes the regime of DII from the field ionization regime (not shaded), at a peak intensity of $7\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$.